Biological Consequences of Increased Concentrations
of Atmospheric CO2
Sherwood B. Idso
For most of the past two millennia, the carbon-dioxide content of earth's atmosphere has
been amazingly constant, hovering at a mean concentration of approximately 280 parts per
million (ppm). With the dawning of the Industrial Revolution, however, this equilibrium
was disturbed: the CO2 content of the air began to rise as humanity embarked upon a course
of unprecedented economic development that was fueled by the burning of massive quantities
of coal, gas, and oil, which released huge amounts of carbon dioxide into the atmosphere.
The rise in the concentration of atmospheric CO2 that has accompanied this societal
transformation is currently viewed with considerable alarm, due to its purported ability
to induce what many believe will be catastrophic global warming. But the phenomenon is not
without its positive aspects, among which is its ability to stimulate vegetative
productivity and it is possible that this demonstrable biological consequence of increased
concentrations of atmospheric CO2 may ultimately prove to be of far greater significance
than its speculative climatic consequences. Hence, it is important to review what we know
about the little-publicized effects of increased concentrations of atmospheric CO2 on the
growth and development of plants; for these are the only effects of the rising CO2 content
of earth's atmosphere about which we can be truly confident.
Effects of CO2 upon plant growth
Carbon dioxide is the basis of almost all life on earth as it is the primary raw material
used by plants to produce organic matter. Rogers et al. (1992) have highlighted this fact
by noting that carbon dioxide is the first molecular link between the atmosphere and
biosphere, that it is essential for photosynthesis, which sustains plant life, the basis
of the entire food chain, and that no substance is more pivotal for ecosystems, either
natural or managed. The veracity of these claims is supported by literally hundreds of
experiments that have convincingly demonstrated that the more CO2 there is in the air, the
better plants perform their many vital functions. In the first major review of this
subject (Kimball 1983a), which was based upon 430 individual experimental results, Kimball
observed that the productivity of most herbaceous plants rose by about one-third in
response to a doubling of the air's CO2 content (330 to 660 ppm); and in a subsequent
analysis of 770 sets of measurements, he obtained essentially the same result (Kimball
1983b). Other reviews have revealed similar or even larger CO2-induced growth enhancements
(Lemon 1983; Cure and Acock 1986; Mortensen 1987; Lawlor and Mitchell 1991; Drake 1992a;
Poorter 1993; Ceulemans and Mousseau 1994; Strain and Cure 1994; Wullschleger et al. 1995,
1997); while in the most comprehensive study of all, which reviewed the results of over
1000 laboratory and field experiments conducted subsequent to the time of Kimball's
analyses, Idso (1992a) documented a mean productivity enhancement of 52 percent in
response to a 300 ppm increase in the air's CO2 content.
This positive response of plants to increases in the concentration of atmospheric CO2 cuts
across all botanical boundaries and is present, to a greater or lesser degree, in all
types of vegetation (Poorter 1993). Indeed, it is now realized to be one of the verities
of nature that, with more CO2 in the air, plants generally grow larger, have more branches
or tillers, more and thicker leaves, more extensive root systems, as well as more flowers
and fruit (Idso 1989a); and in consequence of these well established facts, Sylvan H.
Wittwer, the father of modern research in this area, stated recently that "it should
be considered good fortune that we are living in a world of gradually increasing levels of
atmospheric CO2" (1997: 13). As we shall shortly see, we have already reaped immense
benefits from this phenomenon and we can anticipate even greater positive consequences in
the years ahead.
Effects of CO2 upon the efficient use of water by plants
In addition to enhancing vegetative productivity, an increase in the concentration of
atmospheric CO2 tends to reduce the apertures of the small pores or stomates through which
water vapor escapes from plant leaves and is lost to the atmosphere (Pallas 1965; Morison
1985). The reduction in rate of evaporation from leaves produced by this phenomenon
averages about one-third for a doubling of the air's CO2 content (Kimball and Idso 1983;
Cure and Acock 1986); and combining this effect with the simultaneous CO2-induced increase
in plant productivity actually doubles the efficiency with which individual leaves utilize
water to produce organic matter.
The likely consequences of this phenomenon are truly impressive. As the concentration of
atmospheric carbon dioxide rises ever higher in the years ahead, plants should be able to
grow where it is presently too dry for them, enabling the most drought-resistant species
to reclaim great tracts of land previously lost to desertification (Idso and Quinn 1983;
Idso 1989b). Greater vegetative cover should also reduce the adverse effects of soil
erosion caused by the ravages of wind and rain (Idso 1991a). And, with greater plant
productivity both above-ground (as noted in the previous section) and below-ground (Idso
et al. 1988; Rogers et al. 1994; Jongen et al. 1995; Pregitzer et al. 1995; Tingey et al.
1996), there is typically an increase in soil organic matter (Lekkerkerk et al. 1990; Wood
et al. 1994; Sombroek 1995; Henning et al. 1996; Batjes and Sombroek 1997; King et al.
1997; Prior et al. 1997), which usually produces even further benefits.
Creatures such as earthworms that live in the soil are greatly stimulated by additions of
organic matter (Edwards 1988; Rogers et al. 1994); and an increase in their activity would
likely lead to the creation of much new soil (Graham et al. 1988; Johnson 1988) while at
the same time improving the fertility, structure, aeration, and drainage of existing soils
(Edwards 1997). These improvements, in turn, would likely boost plant productivity even
higher, putting still more organic matter into the soil, and so on (Idso 1991b), as the
several phenomena reinforce each other to lift the whole biosphere to a new level of
activity (Idso 1992b).
The challenge of inadequate resources
In spite of the impressive body of evidence that has established the reality of the many
biological benefits of an atmosphere enriched with increased concentrations of CO2, many
find it difficult to believe that a gaseous effluent of our industrial society might be
good for the biosphere (Gore 1992); and, from time to time, a few scientists have
suggested that the limited availability of resources characteristic of unmanaged
ecosystems will reduce or even totally negate the growth-enhancing effects of increased
concentrations of atmospheric CO2 upon natural (as opposed to managed, i.e. agricultural)
plant communities (Kramer 1981; Oechel and Strain 1985; Bazzaz and Fajer 1992).
Consequently, in an effort to resolve this issue, Idso and Idso (1994) reviewed the plant
science literature of the 10-year period following the original reviews of Kimball (1983a,
b) and analyzed the results of all paired sets of CO2-enrichment experiments that were
conducted simultaneously under growing conditions that were both ideal and less than
ideal. Results of their several findings are summarized in the following subsections.
Low levels of light
Decreasing light intensity had no significant effect upon plants' photosynthetic response
to increased concentrations of atmospheric CO2 until the lowest light intensity of the 37
experiments studied was encountered. At that level, contrary to the contentions of those
who view inadequate resources as impediments to the positive effects of increased
concentrations of CO2, there was a rise in CO2-induced benefits: the mean percentage
increase in photosynthesis due to a 300 ppm increase in atmospheric CO2 rose from 68
percent under normal light intensities to 80 percent at the lowest light intensity
studied, while the mean percentage increase in photosynthesis due to a 600 ppm increase in
CO2 rose from 111 percent to 194 percent.
Studies published subsequent to the review of Idso and Idso (1994) have continued to
demonstrate that low light intensities do not negate the beneficial effects of increased
concentrations of atmospheric CO2 upon plant growth and development (Maruyama and Huang
1996; Percival et al. 1996; Wang 1996; Kubiske and Pregitzer 1997). In fact, in a study of
plants in the forest understory, Osborne et al. (1997) found that elevated CO2
concentrations allowed for a positive net photosynthetic uptake of carbon on days and at
locations that typically experienced light intensities so low that they were generally
insufficient for positive net photosynthesis under current atmospheric CO2 concentrations,
i.e., they found that elevated CO2 concentrations allowed the plants to live where they
currently cannot due to a lack of sufficient light. Hence, they concluded that "the
potential range of habitats that such species could occupy will expand considerably with
rising atmospheric CO2" (1997: 343); similar conclusions are suggested by the work of
Caporn et al. (1994); Wang et al. (1995); Kubiske and Pregitzer (1996), and Liang et al.
When lack of water posed a limitation to vegetative growth and development, the results of
Idso and Idso's (1994) analyses of 55 experiments were even more dramatic than those
pertaining to low light levels. The growth enhancement caused by a 300 ppm increase in
atmospheric CO2 jumped from 31 percent when water supplies were optimal to 63 percent when
they were less than optimal while, for an increase of 600 ppm in the concentration of CO2,
the CO2-induced growth enhancement jumped from 51 percent to 219 percent when the
availability of water decreased from adequate to less than adequate. Here, too, subsequent
studies continue to support these general observations (Liang and Maruyama 1995; Roden and
Ball 1996; Goodfellow et al. 1997) and, in a model-driven review of experimentally
established principles derived from work on temperate forest species, Thornley and Cannell
have independently concluded that the on-going rise in the air's CO2 content "will
protect trees from debilitating water stress" (1996: 1343). Kellomaki and Wang (1996)
have come to the same conclusion as a result of their own observations and, in much the
same vein, Polley et al. (1996) have observed that a doubling of the concentration of
atmospheric CO2 can significantly enhance the percentage of seedlings surviving when water
Insufficient soil nutrients
In Idso and Idso's (1994) analysis of the effects of limitations of soil nutrients, the
growth enhancement due to a 300 ppm rise in the air's CO2 content exhibited a slight
decline, dropping from 51 percent to 45 percent when, in a group of 70 experiments,
nutrients went from a level that did not limit growth to a level that did. But when the
concentration of atmospheric CO2 increased to 600 ppm, this slight negative trend reversed
itself, going from a 43 percent CO2-induced growth stimulation when nutrients were present
in abundance to a 52 percent enhancement when their supply was sub-optimal. And, with a
1200 ppm increase in atmospheric CO2, growth enhancement jumped from 60 percent when the
soil nutrient supply was adequate to 207 percent when it was less than adequate.
Examples from specific ecosystems
Detailed investigations of several managed and unmanaged ecosystems have provided striking
examples of how increased concentrations of atmospheric CO2 can endow plants with the
capacities they need to overcome the restrictions upon growth that result from limited
resources (Koch and Mooney 1996a). In years when the productivities of these plant
communities have been unusually high, for example, the effects of elevated concentrations
of CO2 have been decidedly moderate; but when environmental factors have combined to
curtail their growth and development severely, the effects of increased concentrations of
atmospheric CO2 have typically been much more dramatic (Koch and Mooney 1996b).
In the case of a Kansas tallgrass prairie, doubling the atmospheric CO2 concentration
enhanced vegetative productivity by only 5 to 10 percent in several years of high
productivity but, in a year of intermediate productivity, it increased growth by
approximately 40 percent and, in a year of very low productivity, it boosted production by
nearly 80 percent (Owensby et al. 1996). Another place where this phenomenon has been
observed is in the world's most comprehensive set of Free-Air CO2 Enrichment (FACE)
experiments (Hendrey et al. 1993; Dugas and Pinter 1994). In a study where CO2 was
injected directly into the air over a wheat crop growing in a field devoid of any
alterations to the natural environment, the imposition of a yield-reducing water stress
raised the productivity enhancement created by a 180 ppm increase in atmospheric CO2 from
10 percent to 23 percent in two different years (Pinter et al. 1996). Further, in a cotton
crop where the wetter of two irrigation regimes reduced the yield resulting from the
ambient-air treatment, it was the over-watered plants with lower yield that experienced
the greater CO2-induced stimulation to growth in two different years (Pinter et al. 1996).
How high CO2 levels help plants overcome resource deficiencies
One reason that plants are able to respond positively to increased levels of atmospheric
CO2 when limited resources significantly curtail their growth is that plants grown at
elevated concentrations of CO2 typically have more extensive and active root systems than
control plants growing in normal air (Curtis et al. 1990, 1994; Idso and Kimball 1991a,
1992b; Norby 1994; Prior et al. 1995; Gebauer et al. 1996; King et al. 1996; Kubiske et
al. 1997), which allows them to explore more thoroughly larger volumes of soil in search
of the things they need (Norby et al. 1992; Rogers et al. 1992; Stulen and den Hertog
1993). When more nutrients are encountered in the course of this activity, plants can also
acquire them more effectively (Luxmoore et al. 1986; Norby et al. 1986) because the uptake
of many essential elements requires the expenditure of metabolic energy (Pitman 1977;
Jackson et al. 1980), and the enhanced availability of carbohydrates typically provided by
the enrichment of the air with CO2 tends to promote this process (Clement et al. 1978;
Rufty et al. 1989; Rogers et al. 1994). A low level of nitrogen in the soil, in
particular, is not an impediment to CO2-induced growth enhancement because plants exposed
to elevated concentrations of atmospheric CO2 do not need to invest as much nitrogen in
their photosynthetic apparatus (Stitt 1991) as it operates so much more efficiently at
higher CO2 levels (Nie et al. 1995). Hence, even in the face of severe nutrient
deficiencies, plants' photosynthetic rates may be significantly stimulated by increased
concentrations of atmospheric CO2 (Norby et al. 1992; Wullschleger et al. 1992), setting
in motion still other beneficial phenomena.
One of the most important of these secondary or indirect consequences of CO2-induced
growth stimulation in situations where resources are limited is the enhancement of the
activity of soil microorganisms (Lamborg et al. 1983; Pregitzer et al. 1995; Tingey et al.
1996) that is provided by enhanced root exudation of organic substances (Norby 1997;
Hungate et al. 1997). This enhanced activity of microorganisms typically stimulates a
multiplicity of growth-promoting effects beneath the soil surface (Zak et al. 1993). As
the "better-fed" hyphae (filamentous structures) (Smith and Read 1996) of more
numerous and robust symbiotic fungi extend outward from their CO2-enriched hosts (Ineichen
et al. 1995), for example, they lengthen the life of absorptive root hairs (Fogel 1983)
and increase the area of root surface available for water and nutrient uptake (Tinker
1984; Clarkson 1985; Smith and Read 1996). The microscopic organisms that live in the
plant's root zone or rhizosphere also secrete a number of organic acids that hasten the
chemical weathering of soil minerals (Boyle and Voigt 1973; Boyle et al. 1974) and they
are especially adept at making phosphorus available to plants by this means (Ortuno et al.
1978; Molla et al. 1984; Babenko et al. 1985). In addition, these microscopic organisms
produce a variety of hormones that stimulate root growth (Simmons and Pope 1987, 1988),
enhancing the production of lateral roots and root hairs (Umali-Garcia et al. 1980;
Kapulnik et al. 1983).
Most important of all, perhaps, is the ability of elevated levels of atmospheric CO2 to
stimulate the activity of nitrogen-fixing bacteria directly (Burk 1961; Lowe and Evans
1962). The capacity of these bacteria to remove nitrogen from the atmosphere and make it
available to plants appears to be limited by their host plants' rates of carbohydrate
production (Sinclair and de Wit 1975; Hardy et al. 1976; Finn and Brun 1982).
Consequently, anything that stimulates vegetative productivity, including increased
concentrations of atmospheric CO2, generally stimulates bacterial nodule growth and
activity (Quebedeaux et al. 1975; Murphy 1986) and several-fold increases in the air's CO2
content have indeed been found to produce several-fold increases in nitrogen fixation in a
number of experiments (Hardy and Havelka 1973, 1975; Havelka and Hardy 1976; Phillips et
al. 1976; Hardy et al. 1978; MacDowall 1983).
Acting in concert, these phenomena typically allow the growth-enhancing effects of
increased concentrations of atmospheric CO2 to be expressed in the face of severe
nutritional deficiencies, suggesting that carbon starvation (Svedang 1992;
Cizkova-Koncalova et al. 1992; Robinson 1994) may well be a more significant impediment to
the growth of the planet's vegetation than is a lack of soil nutrients. In the words of
Gunderson and Wullschleger, who have reviewed the subject in depth, "hypotheses that
either nutrient or water limitations would limit photosynthetic responses in natural
environments have not been adequately supported by the currently available data"
(1994: 384). Quite to the contrary, the data demonstrate, as noted by Gifford, that
"high CO2 increases light-use efficiency, water-use efficiency and nitrogen-use
efficiency," so that "the Ôlaw of limiting factor' concept that such vegetation
cannot respond to increased CO2 concentration does not apply to C3 species [C3 plant
species are distinguished from C4 plant species in that they use the process of
photosynthesis first identified by science, a process using an enzyme that reacts with
both CO2 and O2 and that is, thus, less efficient in the presence of relatively higher
levels of O2 than the process used by C4 plant species, which depends upon a different
enzyme that does not interact with O2. Increased concentrations of atmospheric CO2 are,
therefore, of much greater benefit to C3 species of plants than to C4 species. The names
come from the three-carbon (C3) sugar that is the first product of the photosynthetic
process in C3 species and from the four-carbon (C4) organic acid that is the first product
in C4 species.]"
(1994: 33) which include fully 95 percent of all of earth's plants (Drake 1992b; Bowes
The challenge of environmental stresses
In addition to limited resources, there are a number of environmental stresses that are
typically encountered in natural and agricultural ecosystems and it has been claimed that
their debilitating influences reduce or negate the benefits that can accrue to plants from
increased concentrations of atmospheric CO2. The review of Idso and Idso (1994)
demonstrates once again, however, that such effects are generally not observed in
laboratory and field experiments and that, if anything, just the opposite is more likely
Soil salinity and air pollution
In an analysis of 42 sets of measurements of the reponse of plant growth to increased
concentrations of atmospheric CO2 at different values of soil salinity, Idso and Idso
(1994) found that the growth-promoting effects of elevated levels of CO2 in the air were
found to about the same degree in plants stessed by increased salinity of the soil as in
plants in soil of normal salinity, demonstrating that, on average, soil salinity does not
reduce the biological benefits of increased concentrations of atmospheric CO2. What is
more, in another 20 sets of measurements, when air pollutants were the source of stress,
the percentage enhancement in growth brought about by a 300 ppm increase in atmospheric
CO2 actually rose in the face of this adversity, from 38 percent when air pollutants were
absent to 54 percent when they were present in noxious quantities. And, as was the case
where there were limited resources, research conducted subsequent to the review of Idso
and Idso (1994) has continued to confirm their basic finding about the interaction between
CO2 and air pollution: a doubling of the air's CO2 content was often found to compensate
totally for the debilitating effects of air pollutants such as ozone (Rudorff et al. 1996;
Volin and Reich 1996; McKee et al. 1997; Mulholland et al. 1997; Fiscus et al. 1997).
The environmental stress about which certain scientists-and, therefore, the Press and the
public-seem to worry most is that produced by higher air temperatures (Peters and Darling
1985; Paine 1988; Davis 1989; Woodwell 1989; Gear and Huntley 1991; Dobson 1992). They are
concerned that future global warming may be so great that plants will need to migrate
towards the poles in order to remain within the climatic regimes to which they are
currently best suited (Overpeck et al. 1991; Dyer 1995) and, because the warming is
predicted to be so rapid, they fear that many plants may not be able to migrate fast
enough to avoid extinction (Possingham 1993; Root and Schneider 1993; Pitelka et al.
Although this scenario may sound reasonable, it is largely contradicted by basic research
into plant physiology. In an analysis of 42 different data sets, Idso and Idso (1994), for
example, found that the growth enhancement due to a 300 ppm increase in atmospheric CO2
actually rose with increasing air temperature, climbing from nearly 0 percent at 10șC to
a full 100 percent at 38șC; for greater increases in the air's CO2 content, the
percentage of growth enhancement was greater still, something that ongoing research
continues to confirm (Nijs and Impens 1996; Vu et al. 1997).
One reason for this counter-intuitive response is that the optimum temperature for plant
growth generally rises as the air's CO2 content rises (Berry and Bjorkman 1980; Taiz and
Zeiger 1991; McMurtrie et al. 1992; McMurtrie and Wang 1993). Long (1991), for example,
has calculated from basic principles of plant physiology that a 300 ppm increase in
atmospheric CO2 should cause the optimum temperatures of most C3 plants to rise by about
5șC; an analysis of the results of 7 studies that have experimentally evaluated this
phenomenon (Bjorkman et al. 1978; Nilsen et al. 1983; Jurik et al. 1984; Seemann et al.
1984; Harley et al. 1986; Stuhlfauth and Fock 1990; McMurtrie et al. 1992) reveals a mean
optimum temperature rise of 5.9șC with a 300 ppm increase in atmospheric CO2 (Idso and
Idso 1994). And, as this rise in optimum temperature is even larger than the rise in air
temperature predicted to result from the greenhouse warming caused by such a CO2 increase
(IPCC 1990, IPCC I 1996), a CO2-induced warming would not adversely affect the vast
majority of earth's plants, 95 percent of which are of the C3 variety (Drake 1992b; Bowes
1993). In addition, the remainder of the planet's species-which may not experience quite
as large a rise in optimum temperature (Chen et al. 1994)-are already adapted to earth's
warmer environments (De Jong et al. 1982; Drake 1989; Johnson et al. 1993), which are
expected to warm much less than the other portions of the globe (IPCC 1990, IPCC I 1996).
In view of these facts, it is clear that a CO2-induced global warming would not produce
massive poleward migrations of plants seeking cooler weather, for the temperatures at
which nearly all plants perform at their optimum would rise at the same rate as (or faster
than) and to the same degree as (or higher than) the temperatures of their respective
environments. What is more, the photosynthetic rates of the 7 plants for which these
evaluations have been experimentally derived were found to be nearly twice as great at
their CO2-enriched optimum temperatures as they were at their optimum temperatures under
present CO2 conditions (Idso and Idso 1994), suggesting not only that typically predicted
increases in atmospheric CO2 and global air temperatures would not hurt earth's
vegetation, but also that they might actually help it, as subsequent investigations have
revealed as well (Wang 1996).
Finally, at the highest air temperatures encountered by plants, increased concentrations
of atmospheric CO2 have been demonstrated to be especially valuable for they often mean
the difference between life and death (Rowland-Bamford et al. 1996; Idso 1997), as they
typically enable plants to maintain positive carbon exchange rates in situations where
plants growing under present CO2 concentrations exhibit negative rates that ultimately
lead to their demise (Idso et al. 1989, 1995). Faria et al. (1996) have studied this
phenomenon in detail in seedling oak trees and believe that the life-sustaining function
of increased concentrations of atmospheric CO2 at high air temperatures may also be partly
due to a stabilization of enzymes susceptible to heat through the increased concentration
of sugars generally found in CO2-enriched leaves.
Long-term exposure to increased concentrations of CO2
Most studies of plants' responses to increased concentrations of atmospheric CO2 have been
of rather short duration, ranging from hours to days to weeks. Some agricultural
investigations have lasted longer, encompassing entire growing seasons; even in the case
of long-lived plants, such as trees, however, few experiments have had their durations
measured in years. Consequently, there has always been a concern that results derived from
short-term experiments may not apply to plants over the longer term; and, in fact, there
is an experimental basis for this worry.
Acclimation to high CO2
According to a concept variously referred to as "acclimation" or "down
regulation," it is believed by some biologists that long-term exposure to elevated
levels of carbon dioxide will result in a reduction of the photosynthetic capacity of
plants. This contention is based on the results of a number of experiments that have
indeed revealed such behaviour in several plants (Kramer 1981; Pearcy and Bjorkman 1983;
Delucia et al. 1985; Cure and Acock 1986; Tissue and Oechel 1987) and, although it is
possible that such a phenomenon may regularly occur in certain species (Sicher and Bunce
1997), and that it may occur more generally under certain circumstances (Lewis et al.
1994; Luo et al. 1994; Reining 1994; Ziska et al. 1995; Marek and Kalina 1996; Micallef et
al. 1996), there are reasons to believe that it is not expressed-or not as consistently
expressed (Sicher and Kremer 1994; Van Oosten and Besford 1996)-in the majority of plants
growing in natural situations.
Many of the experiments that have exhibited long-term declines in CO2-induced benefits to
plants, for example, have been conducted in controlled environments or greenhouses (Drake
and Leadley 1991), where the plants that were studied were grown in pots or some other
type of root-restricting container. As a number of studies have demonstrated (Masle et al.
1990; Arp 1991; Hogan et al. 1991; Thomas and Strain 1991; Samuelson and Seiler 1992),
root restrictions may sometimes prevent the full expression of the effects of increased
concentrations of atmospheric CO2, particularly when the plants grow larger, a natural
consequence of longer experiments.
In contrast to the results of studies in controlled environments, long-term field
experiments often show no reductions in the photosynthetic capacities of plants exposed to
elevated concentrations of CO2 (Radin et al. 1987; Sage et al. 1989; Idso and Kimball
1991b; Norby and O'Neill 1991; Gunderson et al. 1993; Dufrene et al. 1993; Jones et al.
1995; Teskey 1995, 1997; Wang and Kellomaki 1997), or they show an actual increase or
"up regulation" in photosynthesis and growth over extended periods of time
(Campbell et al. 1988; Conroy 1989; Chen and Sung 1990; Ziska et al. 1990; Arp and Drake
1991; Long and Drake 1991; Drake 1992c; Barton et al. 1993; Vogel and Curtis 1995; Liu and
Teskey 1995; Jacob et al. 1995). There has been one field experiment that produced a
contrary result (Oechel and Strain 1985; Tissue and Oechel 1987; Grulke et al. 1990) but
the Arctic tussock tundra of that study was growing close to the temperature (10șC) at
which its response to increased concentrations of atmospheric CO2 truly should have been
near zero, according to the analysis of temperature effects in the review of Idso and Idso
(1994) and in harmony with the published opinions of several scientists who have analyzed
this particular experiment in some detail (Drake and Leadley 1991; Long 1991; Drake 1992c;
Kirschbaum 1994; Webber et al. 1994). What is more, in a subsequent three-year study of
this same ecosystem, an artificially imposed warming of only 4șC totally eliminated the
down regulation observed at current air temperatures (Field 1994; Oechel et al. 1994).
Consequently, although not to be ruled out in all instances, down regulation of
photosynthesis and growth does not appear to be a major impediment to the long-term
effectiveness of the aerial fertilization effect of increased concentrations of
atmospheric CO2 in stimulating the productivity of earth's vegetation for, as Woodrow has
concluded after reviewing the subject in considerable depth, "C3 plants probably
possess the genetic feedback mechanisms required to efficiently Ôsmooth out' any
imbalance within the photosynthetic system caused by a rise in atmospheric CO2"
(1994: 401) so that, in the words of Amthor, "acclimation . . . of photosynthesis to
increasing CO2 concentration is unlikely to be complete" (1995: 243).
The world's longest experiment studying
increased concentration of CO2
In the longest study of its kind ever to be conducted-and where down-regulation would be
expected to appear if it were a truly ubiquitous consequence of long-term exposure to
increased concentrations of CO2-my colleagues and I planted 8 seedling sour-orange trees
directly into the ground at Phoenix, Arizona, in July of 1987 and enclosed them in pairs
within 4 open-topped chambers made of clear polyethylene film. Then, in November of that
year, we began to pump air with a CO2 concentration of 700 ppm continuously into two of
the enclosures through perforated plastic tubes that lay upon the ground while we pumped
ambient air with a CO2 concentration of 400 ppm into the other two enclosures (Idso et al.
1991). This protocol we have faithfully maintained to the present day, documenting the
course of the effects of increased concentrations of atmospheric CO2 upon the trees as
they have progressed from tiny seedlings through the juvenile stage of development and
into full maturity, which they appear to have reached about two years ago.
The results of this decade-long experiment have been truly remarkable. Over the entire
course of the study, the CO2-enriched trees, which receive 75 percent more CO2 than the
control trees-have continually produced over twice as much biomass as the trees growing in
normal air (Idso and Kimball 1997); at our last harvest, we picked over three times as
much fruit from the trees exposed to the extra CO2. Hence, although we intend to continue
the experiment for several more years and do not yet have the final word on the subject,
we feel confident that the beneficial effects of increased concentrations of atmospheric
CO2 observed to date will likely persist over the entire life span of the trees.
Historical trends in forest productivity
In addition to the experiments of scientists, the biosphere itself is providing evidence
for the reality and long-term sustainability of the aerial-fertilization effects of
increased concentrations of atmospheric CO2. In the southwestern United States, for
example, there is considerable evidence that the woody plants that were present in
pre-industrial times, when the air's CO2 content was much lower than now, were somewhat
stunted compared to their present-day descendants (Johnston 1963; Scifres 1980). This
phenomenon was not fully appreciated, however, until LaMarche et al. published a paper
(1984) describing an analysis of annual growth rings obtained from pine trees growing near
the timberline in California, Colorado, Nevada, and New Mexico. All of the trees that they
studied exhibited large increases in growth rate between 1859 and 1983 and some of them
doubled their productivity during this time. The researchers noted that the increased
growth rates exceeded those expected from climatic trends but were consistent in magnitude
with what would be expected from the historical trend of increasing concentrations of
atmospheric CO2, a trend particularly pronounced in recent decades.
Reports of a similar nature followed in quick succession. Along the western coast of North
America, conifers growing in the high altitudes of the Cascade Mountains of Washington
were found to have increased their growth rates by approximately 60 percent since 1890
(Graumlich et al. 1989). In British Columbia, ring-width measurements of Douglas firs also
revealed a marked increase in growth in recent decades and Parker noted that
"environmental influences other than increased CO2 have not been found that would
explain [the phenomenon]" (1987: 511).
In New England, a study of a 320-year-old Pinus rigida rock-outcrop community showed its
trees to have experienced a dramatic increase in growth since 1970 (Abrams and Orwig
1995). Another study of 10 tree species revealed a mean growth enhancement of 24 percent
from 1950 to 1980 (Hornbeck et al. 1988). In Georgia, annual growth increments of
long-leaf pines began to rise dramatically about 1920, increasing by approximately 30
percent by the mid-1980s (West 1988). In this study, too, it was reported that the
increased growth could not be explained by trends in either precipitation or temperature,
leaving the rising CO2 content of the air as the most likely cause of the increase in
In between these regions, the growth rates of several hardwoods in the Great Smokey
Mountains also increased over the last half-century (Busing 1989), as did those of other
conifers in other regions of both the United States (Jacoby 1986; Graybill 1987; Hornbeck
1987; Kienast and Luxmoore 1988) and Canada (Jozsa and Powell 1987). In one such study, a
colleague and I analyzed data collected from long-lived bristlecone, foxtail, and limber
pines at high-altitude sites in Arizona, California, Colorado, and Nevada (Graybill and
Idso 1993). In almost all of the chronologies we developed, a sharp upward growth trend
began about the middle of the 1850s and has continued to the present. No comparable
variations were observed anywhere else in the records, which continued back in time almost
two millennia, suggesting that the accelerating growth observed in the last
century-and-a-half of the several chronologies were truly unique and the result of some
major regional or global factor. Furthermore, comparisons of the chronologies with records
of changes in temperature and precipitation ruled out the possibility that either of these
climatic variables played a significant role in enhancing the trees' growth rates,
strongly implicating the rise in the air's CO2 content as the factor responsible for the
trees' increasing productivities over the past 150 years.
In Europe, a number of field studies have told much the same story. In forests of silver
firs in the mountains of northeastern France, radial growth measurements have revealed a
productivity increase of nearly 70 percent from 1830 to 1935 (Becker 1989) and the last
decades of the century have seen significant increases in the growth of beech and spruce
stands in southern Sweden (Bjorkdahl and Eriksson 1989; Falkengren-Grerup and Eriksson
1990). Similarly, stands of Scots pine in northern Finland have experienced growth
increases ranging from 15 to 43 percent between 1950 and 1983 (Hari et al. 1984; Hari and
Arovaara 1988). Since CO2 is the only environmental factor that has changed systematically
during this century in the remote area of this study, it was thus to this factor that Hari
and Aravaara attributed the increased growth of the trees.
Germany presents an even more intriguing situation. In the late 1970s and early 1980s,
reports of a large-scale decline in forest productivity were commonplace. When detailed
studies based on extensive measurements finally appeared, however, just the opposite
proved to be true (Eichkorn 1986; Glatzel et al. 1987; Spelsberg 1987). From the middle of
the 1950s at the latest, Norway spruce, silver fir, and beech have all showed remarkable
increases in growth rate (Kenk 1989; Spieker 1990), not following the normal age trend
(Kenk and Fischer 1988); the growth of pine likewise increased (Pretzsch 1985a, 1985b). By
the end of the 1980s, yields of most European forests were clearly above normal (Innes and
Cook 1989). In early 1992, for example, scientists from the Finnish Forest Research
Institute reported a general increase in forest growth in Austria, Finland, France,
Germany, Sweden, and Switzerland (Kauppi et al. 1992) and Weidenbach (1992) reported that
annual growth increments in the German state of Baden-Wrttemberg had risen
approximately 20 percent in just the past 20 years.
Possibly the most revealing observations of all are those that come from tropical forests.
Noting that the turnover rates of mature tropical woodlands correlate well with measures
of their net productivity (Weaver and Murphy 1990), Phillips and Gentry (1994) assessed
the turnover rates of forty tropical forests from around the world in order to test the
hypothesis that forest productivity is increasing globally. Not surprisingly in view of
the material just reviewed, they found that the turnover rates of these highly productive
tropical woodlands have indeed been rising ever higher since 1960, if not earlier, and
that there seems to have been a worldwide acceleration of this trend since about 1980.
They also noted that "the accelerating increase in turnover coincides with an
accelerating buildup of CO2" (Phillips and Gentry 1994: 957). And as Pimm and Sugden
stated in a companion article, it was "the consistency and simultaneity of the
changes on several continents that [led] Phillips and Gentry to their conclusion that
enhanced productivity induced by increased CO2 is the most plausible candidate for the
cause of the increased turnover" (1994: 933-34).
The breath of the biosphere
Compelling evidence for increasing stimulation of earth's plant life by the ongoing rise
in the atmosphere's CO2 concentration is to be found in the air itself. Each spring, when
the northern hemisphere's thin veneer of vegetation awakens from the dormancy of winter
and begins a new season of growth, it draws enough carbon dioxide out of the air to reduce
the concentraton of atmospheric CO2 by several parts per million. In the fall, when much
of this vegetation dies, it releases huge quantities of carbon dioxide back to the air,
causing the CO2 content of the atmosphere to rise by a small amount. Meticulous
measurements obtained over the past four decades have clearly demonstrated that the
difference between the high and low points of this seasonal oscillation-this "breath
of the biosphere" (Idso 1995)-is growing ever larger. Several groups of scientists
have studied the phenomenon in detail and nearly all of them have concluded that it
implies that the photosynthetic activity of the earth's plant life is growing greater and
greater each year; many suggest that the ever-increasing aerial fertilization effect of
the steadily rising CO2 content of the atmosphere is its primary cause.
Working with data for atmospheric CO2 from Mauna Loa Observatory in Hawaii, Point Barrow
in Alaska, and Weather Station P in the North Pacific Ocean, Pearman and Hyson (1981)
concluded that "it is most probable that there has been an increase in the summer net
ecosystem production of the Northern Hemisphere of 8.6 percent over the period
1958-1978," stating that "the results are consistent with the concept of
enhanced activity due to increased levels of CO2" (1981: 9842). Two years later,
Cleveland et al. (1983) confirmed this finding with additional data from Mauna Loa and a
companion record from the South Pole, stating: "we believe it most likely that the
CO2 seasonal behaviour reflects an increase in either the seasonally varying biomass or in
global photosynthetic activity resulting from the increasing concentration of atmospheric
carbon dioxide" (1983: 10,945). Two years later, Keeling et al. (1985) confirmed the
earlier results for Ocean Weather Station P, stating that the increase in the amplitude of
its annual CO2 cycle "reflects an increase in activity of terrestrial plants"
(1985: 10,522). Contemporaneously, after two more years of data were obtained from Mauna
Loa, Bacastow et al. (1985) reported that the increase in the seasonal amplitude since
1958 was "approximately 1 ppm, a sizeable fraction of the average amplitude of 6
ppm," adding that "it seems likely that the increase mainly reflects enhanced
metabolic activity of the land biota" and that "one obvious factor that might
produce this enhancement is the CO2 concentration itself " (1985: 10,529).
Reaffirmations of the reality of this CO2-induced phenomenon continue to appear as more
and more data are collected and analyzed. Keeling (1994), for example, observed that the
amplitude of the seasonal CO2 cycle increased by as much as 20 percent at high latitudes
in both hemispheres between 1988 and 1993, noting that "a preliminary investigation
suggests that an increase in the net primary production of plants . . . caused the
amplitude increase" (1994: 110). Fourteen months later, Keeling et al. (1995)
reported that the amplitude of the seasonal CO2 cycle at Mauna Loa had also risen
significantly over this time period, while Okamoto et al. (1995), after analyzing seasonal
CO2 amplitude trends at 17 stations stretching from the south pole to Alaska, concluded
that "CO2 fertilization exists on the global scale" (1995: 206).
Shortly thereafter, Keeling et al. (1996), working with an ever-expanding data base,
reported that the annual amplitude of the seasonal CO2 cycle had increased by fully 20
percent in the latitudinal vicinity of Hawaii and by 40 percent in the Arctic since the
early 1960s, noting that "the amplitude increases reflect increasing assimilation of
CO2 by land plants" (1996: 146). They also observed shifts in the time of occurrence
of different portions of the seasonal CO2 cycle, which suggested that the vegetative
growing season had lengthened by about a week during the latter part of the same time
period. Then, in an analysis of reflectance data obtained from satellites deployed to
monitor various processes on the earth's surface, Myneni et al. (1997) found that
terrestrial vegetation between 45șN and 70șN latitudes had grown steadily more
productive from 1981 to 1991 and that, simultaneously, the region's active growing season
had lengthened by approximately 12 days, a development that Fung has called "the
first direct observation of the biosphere that photosynthesis has increased on such a
broad scale for such a long time" (1997: 659).
Productivity of plants and biodiversity
in the earth's ecosystems
The evidence for an ongoing CO2-induced rejuvenation of the biosphere that has been
reviewed in the preceding pages is irrefutable: the totality of earth's plant life is
growing ever more vigorously as we flood the air with ever more carbon dioxide. This
development is fortunate indeed, for many of man's activities have consequences that are
not nearly so benevolent and that truly do degrade earth's ecosystems, leading to massive
reductions in biodiversity (Pimm et al. 1995; Vitousek et al. 1997). This one beneficent
side-effect of our industrial activities, however, tends to mitigate the adverse
consequences of many facets of industrialization. In some cases, the increase in the
concentration of atmospheric CO2 more than compensates: it has clearly resulted in a net
increase in the planet's vegetative biomass over the past several decades, as is readily
evident from long- term measurements of the air's CO2 concentration and contemporary
measurements of the planet's surface reflectance. Even measurements of atmospheric oxygen
support this scenario (Keeling and Shertz 1992; Bender et al. 1996; Keeling et al. 1996)
for, as Bender (1996) has noted, they suggest "the existence of a large oxygen
source, which can only be from photosynthesis associated with the net growth of the land
biosphere" (1996: 195).
This CO2-induced increase in vegetative productivity may well be one of the best allies we
will ever have in our battle to preserve the planet's biodiversity; in a landmark study of
the vascular plant components of 94 terrestrial ecosystems from every continent of the
globe except Antarctica, it was found that biodiversity in an ecosystem is more positively
correlated with its productivity than it is with anything else (Scheiner and Rey-Benayas
1994). What is more, a major review of interactions between plants and animals in 51
terrestrial ecosystems found that the biomass of plant-eating animals or herbivores was
also an increasing function of above-ground primary production (McNaughton et al. 1989). A
review of 22 aquatic ecosystems found that the herbivore biomass of watery habitats
increased in response to a rise in underwater vegetative productivity (Cyr and Pace 1993)
and a number of experiments have demonstrated that the growth of aquatic plants is also
enhanced as the air's CO2 content rises (Titus et al. 1990; Raven 1991, 1993; Sand-Jensen
et al. 1992; Titus 1992; Madsen 1993; Riebesell et al. 1993; Madsen and Sand-Jensen 1994;
Hein and Sand-Jensen 1997; Shapiro 1997). Consequently, it is abundantly clear that
earth's animal life-both terrestrial and aquatic- will respond to rising levels of
atmospheric CO2 with increases in population that will parallel the increases in the plant
kingdom, for "the greater the food base, the greater the superstructure of life that
can be supported" (Idso 1995a: 13). Greater populations of individual organisms are
clearly required for greater biodiversity, as all species must maintain a certain
"critical biomass" to sustain their identities and ensure their long-term
An increase in the concentration of atmospheric CO2 significantly stimulates the growth
and development of plants while dramatically enhancing their efficient use of water. These
desirable manifestations of carbon dioxide's aerial fertilization effect are rooted in
fundamental properties of plants that express themselves almost universally, even in the
face of significant resource deficiencies and environmental stresses. Consequently, as the
carbon dioxide content of the atmosphere has risen because of the large-scale utilization
of fossil fuels, so too has the productivity of the planet's vegetation risen because of
the magnitude of mankind's CO2 emissions.
The striking consequences of this mutually beneficial phenomenon are evident in numerous
tree-ring records, historical trends in global forest productivity, and the increasing
amplitude of the atmosphere's seasonal CO2 cycle. Indeed, the on-going rise in the air's
CO2 content is enhancing agricultural productivity the world over (Wittwer 1995) at the
same time as it helps to sustain the biodiversity of the planet's natural ecosystems.
Thus, Wittwer has written truly of this exceptional consequence of humanity's industrial
activity that, "the rising level of atmospheric CO2 is a universally free premium,
gaining in magnitude with time, on which we can all reckon for the future" (1997:
Abrams, M.D., and D.A. Orwig (1995). Structure, radial growth dynamics and recent climatic
variations of a 320-year-old Pinus rigida rock outcrop community. Oecologia 101: 353-360.
Amthor, J.S. (1995). Terrestrial higher-plant response to increasing atmospheric [CO2] in
relation to the global carbon cycle. Global Change Biol 1: 243-274.
Arp, W.J. (1991). Effects of source-sink relations on photosynthetic acclimation to
elevated CO2. Plant Cell Environ 14: 869-75.
Arp, W.J., and B.G. Drake (1991). Increased photosynthetic capacity of Scirpus olneyi
after 4 years of exposure to elevated CO2. Plant Cell Environ 14: 1003-06.
Babenko, Y.S., G.I. Tyrugina, E.F. Origoreev, L.M. Dalgikh, and T.I. Borisova (1985).
Biological activity and physiological biochemical properties of phosphate dissolving
bacteria. Microbiol 53: 427-33.
Bacastow, R.B., C.D. Keeling, and T.P. Whorf (1985). Seasonal amplitude increase in
atmospheric CO2 concentration at Mauna Loa, Hawaii, 1959-1982. J Geophys Res 90:
Barton, C.V.M., H.S.J. Lee, and P.J. Jarvis (1993). A branch bag and CO2 control system
for long-term CO2 enrichment of mature Sitka spruce (Picea sitchensis (Bong. Carr.). Plant
Cell Environ 16: 1139-48.
Batjes, N.H., and W.G. Sombroek (1997). Possibilities for carbon sequestration in tropical
and subtropical soils. Global Change Biol 3: 161-73.
Bazzaz, F.A., and E.D. Fajer (1992). Plant life in a CO2-rich world. Sci Amer (Jan):
Becker, M. (1989). The role of climate on present and past vitality of silver fir forests
in the Vosges mountains of northeastern France. Can J For Res 19: 1110-17.
Bender, M. (1996). A quickening on the uptake? Nature 381: 195-96.
Bender, M., T. Ellis, P. Tans, R. Francey, and D. Lowe (1996). Variability in the O2/N2
ratio of southern hemispheric air, 1991-1994: implications for the carbon cycle. Global
Biogeochem Cycles 10: 9-21.
Berry, J., and O. Bjorkman (1980). Photosynthetic response and adaptation to temperature
in higher plants. Ann Rev Plant Physiol 31: 491-543.
Bjorkdahl, G., and H. Eriksson (1989). Effects of forest decline on increment in Norway
spruce (Picea abies (L.) Karst) in southern Sweden. In K. Bjor and B. Halvorsen (eds), Air
pollution as stress factor in Nordic forests. Medd Norsk Inst Skogforsk 42: 19-36.
Bjorkman O., M. Badger, and P.A. Armond (1978). Thermal acclimation of photosynthesis:
Effect of growth temperature on photosynthetic characteristics and components of the
photosynthetic apparatus in Nerium oleander. Carnegie Inst Wash Yearbook 77: 262-76.
Bowes, G. (1993). Facing the inevitable: Plants and increasing atmospheric CO2. Ann Rev
Plant Physiol Plant Mol Biol 44: 309-32.
Boyle, J.R., and G.K. Voigt (1973). Biological weathering of silicate minerals:
implications for tree nutrition and soil genesis. Plant Soil 38: 191-201.
Boyle, J.R., G.K. Voigt, and B.L. Sawhney (1974). Chemical weathering of biotite by
organic acids. Soil Sci 117: 42-45.
Burk, D. (1961). On the use of carbonic anhydrase in carbonate and amine buffers for CO2
exchange in manometric vessels, atomic submarines, and industrial CO2 scrubbers. Ann New
York Acad Sci 92: 372-400.
Busing, R.T. (1989). A half century of change in a Great Smokey Mountain cove forest. Bull
Torrey Bot Club 116: 282-88.
Campbell, W.J., L.H. Allen Jr., and G. Bowes (1988). Effects of CO2 concentration on
rubisco activity, amount, and photosynthesis in soybean leaves. Plant Physiol 88: 1310-16.
Caporn, S.J.M., D.W. Hand, T.A. Mansfield, and A.R. Wellburn (1994). Canopy photosynthesis
of CO2-enriched lettuce (Lactuca sativa L.). Response to short-term changes in CO2,
temperature and oxides of nitrogen. New Phytol 126: 45-52.
Ceulemans, R., and M. Mousseau (1994). Effects of elevated atmospheric CO2 on woody
plants. New Phytol 127: 425-46.
Chen, D.-X., M.B. Coughenour, A.K. Knapp, C.E. Owensby (1994). Mathematical simulation of
C4 grass photosynthesis in ambient and elevated CO2. Ecol Modelling 73: 63-80.
Chen, J.J., and J.M. Sung (1990). Gas exchange rate and yield responses of Virginia-type
peanut to carbon dioxide enrichment. Crop Sci 30: 1085-89.
Cizkova-Koncalova, H., J. Kvet, and K. Thompson (1992). Carbon starvation: a key to reed
decline in eutrophic lakes. Aquatic Bot 43: 105-13.
Clarkson, D.T. (1985). Factors affecting mineral nutrient acquisition by plants. Ann Rev
Plant Physiol 36: 77-115.
Clement, C.R., M.J. Hopper, L.H.P. Jones, and E.L. Leafe (1978). The uptake of nitrate by
Lolium perenne from flowering nutrient solution. II. Effect of light, defoliation, and
relationship to CO2 flux. J Exp Bot 29: 1173-83.
Cleveland, W.S., A.E. Frenny, and T.E. Graedel (1983). The seasonal component of
atmospheric CO2: information from new approaches to the decomposition of seasonal
time-series. J Geophys Res 88: 10,934-40.
Conroy, J. (1989). Influence of high CO2 on Pinus radiata. PhD thesis, Macquarie
Cure, J.D., and B. Acock (1986). Crop responses to carbon dioxide doubling: a literature
survey. Agric For Meteorol 8: 127-45.
Curtis, P.S., L.M. Balduman, B.G. Drake, and D.F. Whigham (1990). Elevated atmospheric CO2
effects on below ground processes in C3 and C4 estuarine marsh communities. Ecology 71:
Curtis, P.S., D.R. Zak, K.S. Pregitzer, and J.A. Terri (1994). Above- and below-ground
response of Populus grandidentata to elevated atmospheric CO2 and soil N availability.
Plant Soil 165: 45-51.
Cyr, H., and M.L. Pace (1993). Magnitude and patterns of herbivory in aquatic and
terrestrial ecosystems. 361: 148-50.
Davis, M.B. (1989). Lags in vegetation response to greenhouse warming. Climatic Change 15:
De Jong, T.M., B.G. Drake, and R.W. Pearcy (1982). Gas exchange responses of Chesapeake
Bay tidal marsh species under field and laboratory conditions. Oecologia 52: 5-11.
Delucia, E.H., T.W. Sasek, and B.R. Strain (1985). Photosynthetic inhibition after
long-term exposure to elevated levels of atmospheric carbon dioxide. Photosyn Res 7:
Dobson, A. (1992). Withering heats: global warming will exact heavy toll on earth's
biodiversity. Nat Hist 101, 9: 2-8.
Drake, B.G. (1989). Photosynthesis of salt marsh species. Aquatic Bot 34: 167-80.
--- (1992a). The impact of rising CO2 on ecosystem production. Water Air Soil Poll 64:
--- (1992b). Global warming: the positive impact of rising carbon dioxide levels.
Eco-Logic 1, 3: 20-22.
--- (1992c). A field study of the effects of elevated CO2 on ecosystem processes in a
Chesapeake Bay wetland. Aust J Bot 40: 579-95.
Drake B.G., and P.W. Leadley (1991). Canopy photosynthesis of crops and native plant
communities exposed to long-term elevated CO2. Plant Cell Environ 14: 853-60.
Dufrene, E., J.-Y. Pontailler, and B. Saugier (1993). A branch bag technique for
simultaneous CO2 enrichment and assimilation measurements on beech (Fagus sylvatica L.).
Plant Cell Environ 16: 1131-38.
Dugas, W.A., and P.J. Pinter, Jr. (1994). The free-air carbon dioxide enrichment (FACE)
cotton project: a new field approach to assess the biological consequences of global
change. Agric For Meteorol 70: 1-342.
Dyer, J.M. (1995). Assessment of climatic warming using a model of forest species
migration. Ecol Modelling 79: 199-219.
Edwards, C.A. (1988). Earthworms and agriculture. Agron Abstr 80: 274.
--- (1997). Earthworm Ecology. Boca Raton, FL: St. Lucie Press.
Eichkorn, T. (1986). Wachstumanalysen an Fichten in Sudwestdeutschland. Allg Forst Jagdztg
Falkengren-Grerup, U., and H. Eriksson (1990). Changes in soil, vegetation and forest
yield between 1947 and 1988 in beech and oak sites of southern Sweden. For Ecol Manage 38:
Field, C.B. (1994). Arctic chill for CO2 uptake. Nature 371: 472-73.
Fiscus, E.L., C.D. Reid, J.E. Miller, and A.S. Heagle (1997). Elevated CO2 reduces O3 flux
and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies. J
Exp Bot 48: 307-13.
Faria, T., D. Wilkins, R.T. Besford, M. Vaz, J.S. Pereira, and M.M. Chaves (1996). Growth
at elevated CO2 leads to down-regulation of photosynthesis and altered response to high
temperature in Quercus suber L. seedlings. J Exp Bot 47: 1755-61.
Finn, G.A., and W.A. Brun (1982). Effect of atmospheric CO2 enrichment on growth,
nonstructural carbohydrate content, and root nodule activity in soybean. Plant Physiol 69:
Fogel, R. (1983). Root turnover and productivity of forests. Plant Soil 71: 75-85.
Fung, I. (1997). A greener north. Nature 386: 659-60.
Gear, A.J.,and B. Huntley (1991). Rapid changes in the range limits of Scots pine 4000
years ago. Science 251: 544-47.
Gebauer, R.L.E., J.F. Reynolds, B.R. Strain (1996). Allometric relations and growth in
Pinus taeda: the effect of elevated CO2 and changing N availability. New Phytol 134:
Gifford, R.M. (1992). Interaction of carbon dioxide with growth-limiting environmental
factors in vegetative productivity: implications for the global carbon cycle. Adv Bioclim
Glatzel, G., M. Kazda, D. Grill, G. Halbwachs, and K. Katzensteiner (1987).
Ernahrungsstorungen bei Fichte als Komplexwirkung von Nadelschaden und erhohter
Stickstoffdeposition-ein Wirkungsmechanismus des Waldsterbens? Allg Forst Jagdztg 158:
Goodfellow, J., D. Eamus, and G. Duff (1997). Diurnal and seasonal changes in the impact
of CO2 enrichment on assimilation, stomatal conductance and growth in a long-term study of
Mangifera indica in the wet-dry tropics of Australia. Tree Physiol 17: 291-99.
Gore, A. (1992). Earth in the balance: ecology and the human spirit. Boston: Houghton
Graham, R.C., H.B. Wood, and M.A. Lueking (1988). Soil morphologic development in a
40-year-old chaparral biosequence. Agron Abstr 80: 258.
Graumlich, L.J., L.B. Brubaker, and C.C. Grier (1989). Long-term trends in forest net
primary productivity: Cascade Mountains, Washington. Ecology 70: 405-10.
Graybill, D.A. (1987). A network of high elevation conifers in the western US for
detection of tree-ring growth response to increasing atmospheric carbon dioxide. In G.C.
Jacoby Jr., and J. W. Hornbeck (eds), Proceedings of the international symposium on
ecological aspects of tree-ring analysis (Washington, DC: US Dept. of Energy): 463-74.
Graybill, D.A., and S.B. Idso (1993). Detecting the aerial fertilization effect of
atmospheric CO2 enrichment in tree-ring chronologies. Global Biogeochem Cycles 7: 81-95.
Grulke, N.E., G.H. Riechers, W.C. Oechel, U. Hjelm, and C. Jaeger (1990). Carbon balance
in tussock tundra under ambient and elevated atmospheric CO2. Oecologia 83: 485-94.
Gunderson, C.A., R.J. Norby, and S.D. Wullschleger (1993). Foliar gas exchange responses
of two deciduous hardwoods during three years of growth in elevated CO2: no loss of
photosynthetic enhancement. Plant Cell Environ 16: 797-807.
Gunderson, C.A.,and S.D. Wullschleger (1994). Photosynthetic acclimation in trees to
rising atmospheric CO2: a broader perspective. Photosyn Res 39: 369-88.
Hardy, R.W.F., and U.D. Havelka (1973). Symbiotic N2 fixation: multifold enhancement by
CO2-enrichment of field-grown soybeans. Plant Physiol Supplement 48: 35.
--- (1975). Photosynthate as a major factor limiting nitrogen fixation by field-grown
legumes with emphasis on soybeans. In P.S. Nutman (ed), Symbiotic Nitrogen Fixation in
Plants (Cambridge: Cambridge University Press: 421-39.
Hardy, R.W.F., U.D. Havelka, and B. Quebedeaux (1976). Opportunities for improved seed
yield and protein production: N2 fixation, CO2 fixation, and O2 control of reproductive
growth. In National Research Council (eds), Genetic Improvement of Seed Protein
(Washington, DC: National Academy Press): 196-228.
--- (1978). The opportunity for and significance of alteration of ribulose
1,5-bisphosphate carboxylase activities in crop production. In H.W. Siegelman and G. Hind
(eds), Photosynthetic Carbon Assimilation (New York: Plenum): 165-78.
Hari, P., and H. Arovaara (1988). Detecting CO2 induced enhancement in the radial
increment of trees. Evidence from the northern timberline. Scand J For Res 3: 67-74.
Hari, P., H. Arovaara, T. Raunemaa, and A. Hautojarvi (1984). Forest growth and the
effects of energy production: a method for detecting trends in the growth potential of
trees. Can J For Res 14: 437-40.
Harley, P.C., J.D. Tenhunen, O.L. Lange (1986). Use of an analytical model to study the
limitations on net photosynthesis in Arbutus unedo under field conditions. Oecologia 70:
Havelka, U.D., R.W.F. Hardy (1976). Legume N2 fixation as a problem in carbon nutrition.
In W.E. Newton and C.J. Nyman (eds), Proceedings of the 1st Symposium on Nitrogen
Fixation, Vol. 2. (Pullman, WA: Washington State University Press): 456-75.
Hein, M., and S, Sand-Jensen (1997). CO2 increases oceanic primary production. Nature 388:
Hendrey, G.R., K.F. Lewin, and J. Nagy (1993). Free air carbon dioxide enrichment:
Development, progress, results. Vegetatio 104/105: 17-31.
Henning, F.P., C.W. Wood, H.H. Rogers, G.B. Runion, and S.A. Prior (1996). Composition and
decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon
dioxide. J Environ Qual 25: 822-27.
Hogan, K.P., A.P. Smith, L.H. Ziska (1991). Potential effects of elevated CO2 and changes
in temperature on tropical plants. Plant Cell Environ 14: 763-78.
Hornbeck, J.W. (1987). Growth patterns of red oak and red and sugar maple relative to
atmospheric decomposition. In Proceedings of the 16th Central Hardwood Forest Conference
(Knoxville, TN: University of Tennessee): 277-82.
Hornbeck, J.W., R.B. Smith, and C.A. Federer (1988). Growth trends in 10 species of trees
in New England, 1950-1980. Can J For Res 18: 1337-40.
Hungate, B.A., E.A. Holland, R.B. Jackson, F.S. Chapin III, H.A. Mooney, and C.B. Field
(1997). The fate of carbon in grasslands under carbon dioxide enrichment. Nature 388:
Intergovernmental Panel on Climate Change (IPCC) (1990). Climate Change: The IPCC
Scientific Assessment. Report prepared for IPCC by Working Group I. John T. Houghton et
al. (eds). Cambridge: Cambridge University Press.
Intergovernmental Panel on Climate Change, Working Group I (IPCC I) (1996). Climate Change
1995: The Science of Climate Change. Contribution of Working Group I to the Second
Assessment Report of the Intergovernmental Panel on Climate Change. John T. Houghton et
al. (eds). Cambridge: Cambridge University Press.
Idso, K.E. (1992a). Plant responses to rising levels of atmospheric carbon dioxide: A
compilation and analysis of the results of a decade of international research into the
direct biological effects of atmospheric CO2 enrichment. Tempe, AZ: Office of Climatology,
Arizona State University.
--- (1995a). Rising CO2: a breath of new life for the biosphere. World Climate Rev 3(3):
Idso, K.E., and S.B. Idso (1994). Plant responses to atmospheric CO2 enrichment in the
face of environmental constraints: A review of the past 10 years' research. Agric For
Meteorol 69: 153-203.
Idso, S.B. (1989a). Carbon Dioxide: Friend or Foe? Tempe, AZ: IBR Press.
--- (1989b). Carbon dioxide, soil moisture, and future crop production. Soil Sci 147:
--- (1991a). Cooling the global greenhouse? The World and I 6, 3: 276-83.
--- (1991b). Carbon dioxide and the fate of Earth. Global Environ Change 1: 178-82.
--- (1992b). Carbon dioxide and global change: end of nature or rebirth of the biosphere?
In J.H. Lehr (ed), Rational Readings on Environmental Concerns (New York: Van Nostrand
--- (1995b). CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.
St. Paul, MN: Department of Soil, Water and Climate, University of Minnesota.
--- (1997). The poor man's biosphere, including simple techniques for conducting CO2
enrichment and depletion experiments on aquatic and terrestrial plants. Environ Exp Bot:
Idso, S.B., S.G. Allen, M.G. Anderson, and B.A. Kimball (1989). Atmospheric CO2 enrichment
enhances survival of Azolla at high temperatures. Environ Exp Bot 29: 337-41.
Idso, S.B., K.E. Idso, R.L. Garcia, B.A. Kimball, and J.K. Hoober (1995). Effects of
atmospheric CO2 enrichment and foliar methanol application on net photosynthesis of sour
orange tree (Citrus aurantium; Rutacea) leaves. Amer J Bot 82: 26-30.
Idso, S.B., and B.A. Kimball (1991a). Effects of two and a half years of atmospheric CO2
enrichment on the root density distribution of three-year-old sour orange trees. Agric For
Meteorol 55: 345-49.
--- (1991b). Downward regulation of photosynthesis and growth at high CO2 levels: no
evidence for either phenomenon in three-year study of sour orange trees. Plant Physiol 96:
--- (1992b). Seasonal fine-root biomass development of sour orange trees grown in
atmospheres of ambient and elevated CO2 concentration. Plant Cell Environ 15: 337-41.
--- (1997). Effects of long-term atmospheric CO2 enrichment on the growth and fruit
production of sour orange trees. Global Change Biol 3: 89-96.
Idso, S.B., B.A. Kimball, and S.G. Allen (1991). CO2 enrichment of sour orange trees:
two-and-a-half years into a long-term experiment. Plant Cell Environ 14: 351-53.
Idso, S.B., B.A. Kimball, and J.R. Mauney (1988). Effects of atmospheric CO2 enrichment on
root:shoot ratios of carrot, radish, cotton and soybean. Agric Ecosys Environ 22: 293-99.
Idso, S.B., and J.A. Quinn (1983). Vegetational Redistribution in Arizona and New Mexico
in Response to a Doubling of the Atmospheric CO2 Concentration. Tempe, AZ: Climatology
Laboratory , Arizona State University.
Ineichen, K., V. Wiemken, and A. Wiemken (1995). Shoots, roots, and ectomycorrhiza of pine
seedlings at elevated atmospheric carbon dioxide. Plant Cell Environ 18: 703-07.
Innes, J.L., and E.R. Cook (1989). Tree-ring analysis as an aid to evaluating the effects
of pollution on tree growth. Can J For Res 19: 1174-89.
Jackson, W.A., R.J. Volk, and D.W. Israel (1980). Energy supply and nitrate assimilation
in root systems. In A. Tanaka (ed), Carbon-nitrogen Interaction in Crop Production (Tokyo:
Japanese Society for Promotion of Science): 25-40.
Jacob, J., C. Greitner, and B.G. Drake (1995). Acclimation of photosynthesis in relation
to Rubisco and non-structural carbohydrate contents and in situ carboxylase activity in
Scirpus olneyi grown at elevated CO2 in the field. Plant Cell Environ 18: 875-84.
Jacoby, G.C. (1986). Long-term temperature trends and a positive departure from the
climate- growth response since the 1950s in high elevation lodgepole pine from California.
In C. Rosenzweig and R. Dickinson (eds), Proceedings of the NASA Conference on
Climate-Vegetation Interactions (Boulder, CO: University Corporation for Atmospheric
Johnson, D.L. (1988). Biomantle evolution and the redistribution of earth materials and
artifacts. Agron Abstr 80: 259.
Johnson, H.B., H.W. Polley, and H.S. Mayeux (1993). Increasing CO2 and plant-plant
interactions: effects on natural vegetation. Vegetatio 104/105: 157-70.
Johnston, M.C. (1963). Past and present grasslands of southern Texas and northeastern
Mexico. Ecology 44: 456-66.
Jones, M.B., J.C. Brown, A. Raschi, and F. Miglietta (1995). The effects on Arbutus unedo
L. of long- term exposure to elevated CO2. Global Change Biol 1: 295-302.
Jongen, M., M.B. Jones, T. Hebeisen, H. Blum, and G. Hendrey (1995). The effects of
elevated CO2 concentrations on the root growth of Lolium perenne and Trifolium repens
grown in a FACE system. Global Change Biol 1: 361-71.
Jozsa, L.A., and J. M. Powell (1987). Some climatic aspects of biomass productivity of
white spruce stem wood. Can J For Res 17: 1075-79.
Jurik, T.W., J.A. Weber, and D.M. Gates (1984). Short-term effects of CO2 on gas exchange
of leaves of bigtooth aspen (Populus grandidentata) in the field. Plant Physiol 75:
Kapulnik, Y., R. Gafny, and Y. Okon (1983). Effect of Azospirillum spp. inoculation
development and NO3- uptake in wheat (Triticum aestivum cv. Miriam) in hydroponic system.
Can J Bot 63: 627-31.
Kauppi, P.E., K. Mielikainen, K. Kuusela (1992). Biomass and carbon budget of European
forests, 1971-1990. Science 256: 70-74.
Keeling, C.D. (1994). A study of the abundance and 13C/12C ratio of atmospheric carbon
dioxide and oceanic carbon in relation to the global carbon cycle. In M.R. Riches (ed),
Global Change Research: Summaries of Research in FY 1994. (Washington, DC: US Department
of Energy): 109-10.
Keeling, C.D., J.F.S. Chin, and T.P. Whorf (1996). Increased activity of northern
hemispheric vegetation inferred from atmospheric CO2 measurements. Nature 382: 146-49.
Keeling, C.D., T.P. Whorf, M. Wahlen, and J. van der Pilcht (1995). Interannual extremes
in the rate of rise of atmospheric carbon dioxide since 1980. Nature 375: 666-70.
Keeling, C.D., T.P. Whorf, C.S. Wong, R.D. Bellagay (1985). The concentration of carbon
dioxide at ocean weather station P from 1969-1981. J Geophys Res 90: 10,511-28
Keeling, R., S.C. Piper, M. Heimann (1996). Global and hemispheric CO2 sinks deduced from
changes in atmospheric O2 concentration. Nature 381: 218-21.
Keeling, R., amd S. Shertz (1992). Seasonal and interannual variations in atmospheric
oxygen and implications for the global carbon cycle. Nature 358: 723-27.
Kellomaki, S., and K.-Y. Wang (1996). Photosynthetic responses to needle water potentials
in Scots pine after a four-year exposure to elevated CO2 and temperature. Tree Physiol 16:
Kenk, G. (1989). Zuwachsuntersuchungen im zusammenhang mit den gegenwartigen waldschaden
in Baden-Wrttemberg. In J.B. Bucher and W.I. Bucher (eds), Air pollution and forest
decline (Brimensdorf, Switzerland: Eidgenossische Anstalt fr das forstliche
Kenk, G, and H. Fischer (1988). Evidence from nitrogen fertilization in the forests of
Germany. Environ Poll 54: 199-218.
Kienast, F.,and R.J. Luxmoore (1988). Tree-ring analysis and conifer growth responses to
increased atmospheric CO2 levels. Oecologia 76: 487-95.
Kimball, B.A. (1983a). Carbon dioxide and agricultural yield: an assemblage and analysis
of 430 prior observations. Agron J 75: 779-88.
--- (1983b). Carbon Dioxide and Agricultural Yield: An Assemblage and Analysis of 770
Prior Observations. Phoenix, AZ: US Water Conservation Laboratory.
Kimball, B.A., and S.B. Idso (1983). Increasing atmospheric CO2: effects on crop yield,
water use and climate. Agric Water Management 7: 55-72.
King, A.W., W.M. Post, and S.D. Wullschleger (1997). The potential response of terrestrial
carbon storage to changes in climate and atmospheric CO2. Climatic Change 35: 199-227.
King, J.S., R.B. Thomas, and B.R. Strain (1996). Growth and carbon accumulation in root
systems of Pinus taeda and Pinus ponderosa seedlings as affected by varying CO2,
temperature and nitrogen. Tree Physiol 16: 635-42.
Kirschbaum, M.U.F. (1994). The sensitivity of C3 photosynthesis to increasing CO2
concentration: a theoretical analysis of its dependence on temperature and background CO2
concentration. Plant Cell Environ 17: 747-54.
Koch, G.W., and H.A. Mooney (1996a). Carbon Dioxide and Terrestrial Ecosystems. San Diego,
CA: Academic Press.
--- (1996b). Response of terrestrial ecosystems to elevated CO2: a synthesis and summary.
In Koch and Mooney 1996a: 415-29.
Kramer, P.J. (1981). Carbon dioxide concentration, photosynthesis and dry matter
production. BioSci 31: 29-34.
Kubiske, M.E., and K.S. Pregitzer (1996). Effects of elevated CO2 and light availability
on the photosynthetic light response of trees of contrasting shade tolerance. Tree Physiol
Kubiske, M.E., and K.S. Pregitzer (1997). Ecophysiological responses to simulated canopy
gaps of two tree species of contrasting shade tolerance in elevated CO2. Functional Ecol
Kubiske, M.E., K.S. Pregitzer, C.J. Mikan, D.R. Zak, J.L. Maziasz, and J.A. Teeri (1997).
Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and
soil N availability. Oecologia 110: 328-36.
LaMarche, V.C., Jr., D.A. Graybill, H.C. Fritts, and M.R. Rose (1984). Increasing
atmospheric carbon dioxide: tree ring evidence for growth enhancement in natural
vegetation. Science 223: 1019-21.
Lamborg, M.R., R.W. Hardy, and E.A. Paul (1983). Microbial effects. In E.R. Lemon (ed).
CO2 and Plants: The Response of Plants to Rising Levels of Atmospheric Carbon Dioxide
(Boulder, CO: Westview Press): 131-76.
Lawlor, D.W., R.A.C. Mitchell (1991). The effects of increasing CO2 on crop photosynthesis
and productivity: a review of field studies. Plant Cell Environ 14: 807-18.
Lekkerkerk, L.J.A., J.A. Van Veen, S.C. Van de Geijn (1990). Influence of climatic change
on soil quality; consequences of increased atmospheric CO2-concentration on carbon input
and turnover in agro-ecosystems. In J. Goudriaan, H. Van Keulen, and H.H. Van Laar (eds).
The Greenhouse Effect and Primary Productivity in European Agro-ecosystems (Wageningen,
NL: Pudoc): 46-47.
Lemon, E.R. (1983). CO2 and Plants: The Response of Plants to Rising Levels of Atmospheric
Carbon Dioxide. Boulder, CO: Westview Press.
Lewis, J.D., K.L. Griffin, R.B. Thomas, and B.R. Strain (1994). Phosphorus supply affects
the photosynthetic capacity of loblolly pine grown in elevated carbon dioxide. Tree
Physiol 14: 1229-44.
Liang, N., and K. Maruyama (1995). Interactive effects of CO2 enrichment and drought
stress on gas exchange and water-use efficiency in Alnus firma. Environ Exp Bot 35:
Liang, N., K. Maruyama, and Y. Huang (1996). Effects of CO2 concentration on the
photosynthetic and carboxylation efficiencies of Fagus crenata and Quercus crispula.
Photosynthetica 32: 355-65.
Liu, S., and R.O. Teskey (1995). Responses of foliar gas exchange to long-term elevated
CO2 concentrations in mature loblolly pine trees. Tree Physiol 15: 351-59.
Long, S.P. (1991). Modification of the response of photosynthetic productivity to rising
temperature by atmospheric CO2 concentrations: has its importance been underestimated?
Plant Cell Environ 14: 729-39.
Long, S.P., B.G. Drake (1991). Effect of the long-term elevation of CO2 concentration in
the field on the quantum yield of photosynthesis of the C3 sedge, Scirpus olneyi. Plant
Physiol 96: 221-26.
Lowe, R.H., and H.J. Evans (1962). Carbon dioxide requirement for growth of legume nodule
bacteria. Soil Sci 94: 351-56.
Luo, Y., C.B. Field, and H.A. Mooney (1994). Predicting responses of photosynthesis and
root fraction to elevated [CO2]a: interactions among carbon, nitrogen, and growth. Plant
Cell Environ 17: 1195-204.
Luxmoore, R.J., E.G. O'Neill, J.M. Ells, H.H. Rogers (1986). Nutrient-uptake and growth
responses of Virginia pine to elevated atmospheric CO2. J Environ Qual 15: 244-51.
MacDowall, F.D.H. (1983). Effects of light intensity and CO2 concentration on the kinetics
of 1st month growth and nitrogen fixation of alfalfa. Can J Bot 61: 731-40.
Madsen, T.V. (1993). Growth and photosynthetic acclimation by Ranunculus aquatilis L. in
response to inorganic carbon availability. New Phytol 125: 707-15.
Madsen, T.V., and K. Sand-Jensen (1994). The interactive effects of light and inorganic
carbon on aquatic plant growth. Plant Cell Environ 17: 955-62.
Marek, M.V., and J. Kalina (1996). Comparison of two experimental approaches used in the
investigations of the long-term effects of elevated CO2 concentration. Photosynthetica 32:
Maruyama, L.K., and Y. Huang (1966). Effects of CO2 concentration on the photosynthetic
and carboxylation efficiencies of Fagus crenata and Quercus crispula. Photosynthetica 32:
Masle, J., G.D. Farquhar, and R.M. Gifford (1990). Growth and carbon economy of wheat
seedlings as affected by soil resistance to penetration and ambient partial pressure of
CO2. Aust J Plant Physiol 17: 465-87.
McKee, I.F., J.F. Bullimore, and S.P. Long (1997). Will elevated CO2 concentrations
protect the yield of wheat from O3 damage? Plant Cell Environ 20: 77-84.
McMurtrie, R.E., H.N. Comins, M.U.F. Kirschbaum, and Y.-P. Wang (1992). Modifying existing
forest growth models to take account of effects of elevated CO2. Aust J Bot 40: 657-77.
McMurtrie, R.E., and Y.-P. Wang (1993). Mathematical models of the photosynthetic response
of tree stands to rising CO2 concentrations and temperatures. Plant Cell Environ 16: 1-13.
McNaughton, S.J., M. Oesterheld, D.A. Frank, and K.J. Williams (1989). Ecosystem-level
patterns of primary productivity and herbivory in terrestrial habitats. Nature 341:
Micallef, B.J., P.J. Vanderveer, snf T.D. Sharkey (1996). Responses to elevated CO2 of
Flaveria linearis plants having a reduced activity of cytosolic
fructose-1,6-bisphosphatase. Plant Cell Environ 19: 10-16.
Molla, M.A.Z., A.A. Chowdhary, and A.H. Islam (1984). Microbial mineralization of organic
phosphate in soil. Plant Soil 78: 393-99.
Morison, J.I.L. (1985). Sensitivity of stomata and water use efficiency to high CO2. Plant
Cell Environ 8: 467-74.
Mortensen, L.M. (1987). Review: CO2 enrichment in greenhouses. Crop responses. Sci Hort
Mulholland, B.J., J. Craigon, C.R. Black, J.J. Colls, J. Atherton, and G. Landon (1997).
Effects of elevated carbon dioxide and ozone on the growth and yield of spring wheat
(Triticum aestivum L.) J Exp Bot 48: 113-22.
Murphy, P.M. (1986). Effect of light and atmospheric carbon dioxide concentration on
nitrogen fixation by herbage legumes. Plant Soil 95: 399-409.
Myneni, R.B., C.D. Keeling, C.J. Tucker, G. Asrar, and R.R. Nemani (1997). Increased plant
growth in the northern high latitudes from 1981 to 1991. Nature 386: 698-702.
Nie, G.Y., S.P. Long, R.L. Garcia, B.A. Kimball, R.L. LaMorte, P.J. Pinter, Jr., G.W.
Wall, and A.N. Weber (1995). Effects of free-air CO2 enrichment on the development of the
photosynthetic apparatus in wheat, as indicated by changes in leaf proteins. Plant Cell
Environ 18: 855-64.
Nijs, I., I. Impens (1996). Effects of elevated CO2 concentration and climate-warming on
photosynthesis during winter in Lolium perenne. J Exp Bot 47: 915-24.
Nilsen, S., K. Hovland, C. Dons, and S.P. Sletten (1983). Effect of CO2 enrichment on
photosynthesis, growth and yield of tomato. Sci Hort 20: 1-14.
Norby, R.J. (1994). Issues and perspectives for investigating root responses to elevated
atmospheric carbon dioxide. Plant Soil 165: 9-20.
Norby, R.J. (1997). Inside the black box. Nature 388: 522-23.
Norby, R.J., C.A. Gunderson, S.D. Wullschleger, E.G. O'Neill, and M.K. McCracken (1992).
Productivity and compensatory responses of yellow-poplar trees in elevated CO2. Nature
Norby, R.J., and E.G. O'Neill (1991). Leaf area compensation and nutrient interactions in
CO2-enriched seedlings of yellow-poplar (Liriodendron tulipifera L.). New Phytol 117:
Norby, R.J., E.G. O'Neill, and R.J. Luxmoore (1986). Effects of atmospheric CO2 enrichment
on the growth and mineral nutrition of Quercus alba seedlings in nutrient-poor soil. Plant
Physiol 82: 83-89.
Oechel, W.C., S. Cowles, N. Grulke, S.J. Hastings, B. Lawrence, T. Prudhomme, G. Riechers,
B. Strain, D. Tissue, and G. Vourlitis (1994). Transient nature of CO2 fertilization in
Arctic tundra. Nature 371: 500-03.
Oechel, W.C., and B.R. Strain (1985). Native species responses to increased atmospheric
carbon dioxide concentration. In B.R. Strain, and J.D. Cure (eds), Direct Effects of
Increasing Carbon Dioxide on Vegetation (Washington, DC: US Department of Energy): 117-54.
Okamoto, K., S. Tanimoto, and K. Okano (1995). Statistical analysis of the increase in
atmospheric CO2 concentrations and its relation to the possible existence of CO2
fertilization of a global scale. Tellus 47B: 206-11.
Ortuno, A., A. Hermansarz, J. Noguera, V. Morales, and T. Armero (1978). Phosphorus
solubilizing effect of A. niger and Pseudomonas fluorescens. Microbiol Esp 30: 113-20.
Osborne, C.P., B.G. Drake, J. LaRoche, S.P. Long (1997). Does long-term elevation of CO2
concentration increase photosynthesis in forest floor vegetation? Plant Physiol 114:
Overpeck, J.T., P.J. Bartlein, and T. Webb III (1991). Potential magnitude of future
vegetation change in eastern North America: comparisons with the past. Science 254:
Owensby, C.E., J.M. Ham, A.K. Knapp, C.W. Rice, P.I. Coyne, and L.M. Auen (1996).
Ecosystem-level responses of tallgrass prairie to elevated CO2. In G.W. Koch and H.A.
Mooney (eds), Carbon Dioxide and Terrestrial Ecosystems (San Diego, CA: Academic Press):
Paine, S. (1988). No escape from the global greenhouse. New Sci 1638: 38-43.
Pallas, J.E. (1965). Transpiration and stomatal opening with changes in carbon dioxide
content of the air. Science 147: 171-73.
Parker, M.L. (1987). Recent abnormal increase in tree-ring widths: a possible effect of
elevated atmospheric carbon dioxide. In G.C. Jacoby, Jr. and J.W. Hornbeck (eds),
Proceedings of the International Symposium on Ecological Aspects of Tree-ring Analysis
(Washington, DC: US Department of Energy): 511-21.
Pearcy, R.W., and O. Bjorkman (1983). Physiological effects. In E.R. Lemon (ed), CO2 and
Plants: The Response of Plants to Rising Levels of Atmospheric Carbon Dioxide (Boulder,
CO: Westview Press): 65-105.
Pearman, G.I., and P. Hyson (1981). The annual variation of atmospheric CO2 concentration
observed in the northern hemisphere. J Geophys Res 86: 9839-43.
Percival, D.C., J.T.A. Proctor, and M.J. Tsujita (1996). Whole-plant net CO2 exchange of
raspberry as influenced by air and root-zone temperature, CO2 concentration, irradiation,
and humidity. J Amer Soc Hort Sci 121: 838-45.
Peters, R.L., and J.D.S. Darling (1985). The greenhouse effect and nature reserves. BioSci
Phillips, D.A., K.D. Newell, S.A. Hassel, and C.E. Felling (1976). The effect of CO2
enrichment on root nodule development and symbiotic N2 reduction in Pisum sativum L. Amer
J Bot 63: 356-62.
Phillips, O.L., and A.H. Gentry (1994). Increasing turnover through time in tropical
forests. Science 263: 954-58.
Pimm, S.L., G.J. Russell, J.L. Gittleman, and T.M. Brooks (1995). The future of
biodiversity. Science 269: 347-50.
Pimm, S.L., and A.M. Sugden (1994). Tropical diversity and global change. Science 263:
Pinter, P.J. Jr., B.A. Kimball, R.L. Garcia, G.W. Wall, D.J. Hunsaker, and R.L. LaMorte
(1996). Free-air CO2 enrichment: responses of cotton and wheat crops. In G.W. Koch, and
H.A. Mooney (eds), Carbon Dioxide and Terrestrial Ecosystems (San Diego, CA: Academic
Pitelka, L.F., R.H. Gardner, J. Ash, S. Berry, H. Gitay, I.R. Noble, A. Saunders, R.H.W.
Bradshaw, L. Brubaker, J.S. Clark, M.B. Davis, S. Sugita, J.M. Dyer, R. Hengeveld, G.
Hope, B. Huntley, G.A. King, S. Lavorel, R.N. Mack, G.P. Malanson, M. McGlone, I.C.
Prentice, and M. Rejmanek (1997). Plant migration and climate change. Amer Sci 85: 464-73.
Pitman, M. (1977). Ion transport into the xylem. Ann Rev Plant Physiol 28: 71-88.
Polley, H.W., H.B. Johnson, H.S. Mayeux, C.R. Tischler, and D.A. Brown (1996). Carbon
dioxide enrichment improves growth, water relations and survival of droughted honey
mesquite (Prosopis glandulosa) seedlings. Tree Physiol 16: 817-23.
Poorter, H. (1993). Interspecific variation in the growth response of plants to an
elevated ambient CO2 concentration. Vegetatio 104/105: 77-97.
Possingham, H.P. (1993). Impact of elevated atmospheric CO2 on biodiversity: mechanistic
population-dynamic perspective. Aust J Bot 41: 11-21.
Pregitzer, K.S., D.R. Zak, P.S. Curtis, M.E. Kubiske, J.A. Teeri, C.S. Vogel (1995).
Atmospheric CO2, soil nitrogen and turnover of fine roots. New Phytol 129: 579-85.
Pretzsch, H. (1985a). Wachstumsmerkmale suddeutscher Kiefernbestande in den letzen 25
Jahren. Forschungsbericht der Forstlichen Forschungs-Anstalt Munchen 65.
--- (1985b). Wachstumsmerkmale oberpfalzischer Kiefernbestande in den letzten 30 Jahren.
Vitalitatszustand-Strukturverhaltnisse-Zuwachsgang. Allg Forstzeitschr 42: 1122-26.
Prior, S.A., H.H. Rogers, G.B. Runion, B.A. Kimball, J.R. Mauney, K.F. Lewin, J. Nagy, and
G.R. Hendrey (1995). Free-air carbon dioxide enrichment of cotton: Root morphological
characteristics. J Environ Qual 24: 678-83.
Prior, S.A., H.A. Torbert, G.B. Runion, H.H. Rogers, C.W. Wood, B.A. Kimball, R.L.
LaMorte, P.J. Pinter, and G.W. Wall (1997). Free-air carbon dioxide enrichment of wheat:
soil carbon and nitrogen dynamics. J Environ Qual 26: 1161-66.
Quebedeaux, B., U.D. Havelka, K.L. Livak, R.W.F. Hardy (1975). Effect of altered pO2 on
the aerial part of soybean on symbiotic N2 fixation. Plant Physiol 56: 761-64.
Radin, J.W., B.A. Kimball, D.L. Hendrix , and J.R. Mauney (1987). Photosynthesis of cotton
plants exposed to elevated levels of carbon dioxide in the field. Photosyn Res 12:
Raven, J.A. (1991). Physiology of inorganic C acquisition and implications for resource
use efficiency by marine phytoplankton: relation to increased CO2 and temperature. Plant
Cell Environ 14: 779-94.
--- (1993). Phytoplankton: limits on growth rates. Nature 361: 209-10.
Reining, E. (1994). Acclimation of C3 photosynthesis to elevated CO2: hypotheses and
experimental evidence. Photosynthetica 30: 519-25.
Riebesell, U., D.A. Wolf-Gladrow, and V. Smetacek (1993). Carbon dioxide limitation of
marine phytoplankton growth rates. Nature 361: 249-51.
Robinson, J.M. (1994). Speculations on carbon dioxide starvation, Late Tertiary evolution
of stomatal regulation and floristic modernization. Plant Cell Environ 17: 345-54.
Roden, J.S., and M.C. Ball (1996). The effect of elevated [CO2] on growth and
photosynthesis of two eucalyptus species exposed to high temperatures and water deficits.
Plant Physiol 111: 909-19.
Rogers, H.H., L.H. Allen, Jr, B.A. Kimball, S.B. Idso, J.E. Miller, S.L. Rawlins, and R.C.
Dahlman (1992). Potential impacts of climate change on agricultural production. Testimony
before public hearing of the Committee of Enquiry on Protecting the Earth's Environment
(17-18 February). Enquete Kommission, Deutcher Bundestag, Bundeshaus, Bonn, D.
Rogers, H.H., C.M. Peterson, J.N. McCrimmon, and J.D. Cure (1992). Response of plant roots
to elevated atmospheric carbon dioxide. Plant Cell Environ 15: 749-52.
Rogers, H.H., G.B. Runion, and S.V. Krupa (1994). Plant responses to atmospheric CO2
enrichment with emphasis on roots and the rhizosphere. Environ Poll 83: 155-89.
Root, T.L., and S.H. Schneider (1993). Can large-scale climatic models be linked with
multiscale ecological studies? Conserv Biol 7: 256-70.
Rowland, A.J., J.T. Baker, L.H. Allen, Jr., and G. Bowes (1996). Interactions of CO2
enrichment and temperature on carbohydrate accumulation and partitioning in rice. Environ
Exp Bot 36: 111-24.
Rudorff, B.F.T., C.L. Mulchi, E. Lee, R. Rowland, and R. Pausch (1996). Photosynthetic
characteristics in wheat exposed to elevated O3 and CO2. Crop Sci 36: 1247-51.
Rufty, T.W., Jr., C.T. MacKown, and R.M. Volk (1989). Effects of altered carbohydrate
availability on whole-plant assimilation of 15NO3-. Plant Physiol 89: 457-63.
Sage, R.F., T.D. Sharkey, and J.R. Seemann (1989). Acclimation of photosynthesis to
elevated CO2 in five C3 species. Plant Physiol 89: 590-96.
Scheiner, S.M., and J.M. Rey-Benayas (1994). Global patterns of plant diversity. Evol Ecol
Samuelson, L.J., and J.R. Seiler (1992). Fraser fir seedling gas exchange and growth in
response to elevated CO2. Environ Exp Bot 32: 351-56.
Sand-Jensen, K., M.F. Pedersen, and S. Laurentius (1992). Photosynthetic use of inorganic
carbon among primary and secondary water plants in streams. Freshwater Biol 27: 283-93.
Scifres, C.J. (1980). Brush Management: Principles and Practices for Texas and the
Southwest. College Station, TX: Texas A and M University Press.
Seemann, J.R., J.A. Berry, and J.S. Downton (1984). Photosynthetic response and adaptation
to high temperature in desert plants. A comparison of gas exchange and fluorescence
methods for studies of thermal tolerance. Plant Physiol 75: 364-68.
Shapiro, J. (1997). The role of carbon dioxide in the initiation and maintenance of
blue-green dominance in lakes. Freshwater Biol 37: 307-23.
Sicher, R.C., and J.A. Bunce (1997). Relationship of photosynthetic acclimation to changes
of Rubisco activity in field-grown winter wheat and barley during growth in elevated
carbon dioxide. Photosyn Res 52: 27-38.
Sicher, R.C., and D.F. Kremer (1994). Responses of Nicotiana tabacum to CO2 enrichment at
low-photon flux density. Physiol Plant 92: 383-88.
Simmons, G.L., and P.E. Pope (1987). Influence of soil compaction and vesicular-arbuscular
mycorrhizae on root growth of yellow poplar and sweet gum seedlings. Can J For Res 17:
--- (1988). Influence of soil water potential and mycorrhizal colonization on root growth
of yellow-poplar and sweet gum seedlings grown in compacted soil. Can J For Res 18:
Sinclair, T.R., C.T. deWit (1975). Photosynthate and nitrogen requirements for seed
productivity by various crops. Science 189: 565-67.
Smith, S.E., and D.J. Read (1996). Mycorrhizal symbiosis. London: Academic Press.
Sombroek, W.G. (1995). Aspects of soil organic matter and nutrient cycling in relation to
climate change and agricultural sustainability. In International Symposium on Nuclear and
Related Techniques in Soil-Plant Studies on Sustainable Agriculture and Environmental
Preservation, Nuclear Techniques in Soil-Plant Studies for Sustainable Agriculture and
Environmental Preservation (Vienna, AT: International Atomic Energy Agency): 15-26.
Spelsberg, G. (1987). Zum Problem der Beurteilung des Zuwachses in geschadigten Bestanden.
Allg Forst Jagdztg 158: 205-10.
Spieker, M. (1990). Growth variation and environmental stresses: long-term observations on
permanent research plots in southwestern Germany. Water Air Soil Poll 54: 247-56.
Stitt, M. (1991). Rising CO2 levels and their potential significance for carbon flow in
photosynthetic cells. Plant Cell Environ 14: 741-62.
Strain, B.R., and J.D. Cure (1994). Direct effects of atmospheric CO2 enrichment on plants
and ecosystems: an updated bibliographic data base. Oak Ridge, TN: Oak Ridge National
Stuhlfauth, T., and H.P. Fock (1990). Effect of whole season CO2 enrichment on the
cultivation of a medicinal plant, Digitalis lanata. J Agron Crop Sci 164: 168-73.
Stulen, I., and J. den Hertog (1993). Root growth and functioning under atmospheric CO2
enrichment. Vegetatio 104/105: 99-115.
Svedang, M.U. (1992). Carbon dioxide as a factor regulating the growth dynamics of Juncus
bulbosus. Aquatic Bot 42: 231-40.
Taiz, L.,and E. Zeiger (1991). Plant Physiology. Redwood City, CA: Benjamin-Cummings.
Teskey, R.O. (1995). A field study of the effects of elevated CO2 on carbon assimilation,
stomatal conductance and leaf and branch growth of Pinus taeda trees. Plant Cell Environ
--- (1997). Combined effects of elevated CO2 and air temperature on carbon assimilation of
Pinus taeda trees. Plant Cell Environ 20: 373-80.
Thomas, R.B., and B.R. Strain (1991). Root restriction as a factor in photosynthetic
acclimation of cotton seedlings grown in elevated carbon dioxide. Plant Physiol 96:
Thornley, J.H.M., and M.G.R. Cannell (1996). Temperate forest responses to carbon dioxide,
temperature and nitrogen: a model analysis. Plant Cell Environ 19: 1331-48.
Tingey, D.T., M.G. Johnson, D.L. Phillips, D.W. Johnson, and J.T. Ball (1996). Effects of
elevated CO2 and nitrogen on the synchrony of shoot and root growth in ponderosa pine.
Tree Physiol 16: 905-14.
Tinker, P.B. (1984). The role of microorganisms in mediating and facilitating the uptake
of plant nutrients from the soil. Plant Soil 76: 77-91.
Tissue, D.T., and W.C. Oechel (1987). Response of Eriophorum vaginatum to elevated CO2 and
temperature in the Alaskan tussock tundra. Ecology 68: 401-10.
Titus, J.E. (1992). Submersed macrophyte growth at low pH. II. CO2 sediment interactions.
Oecologia 92: 391-98.
Titus, J.E., R.S. Feldman, and D. Grise (1990). Submersed macrophyte growth at low pH. I.
CO2 enrichment effects with fertile sediment. Oecologia 84: 307-313
Umali-Garcia, M., D.H. Hubbel, M.H. Gaskin, F.B. Dazzo (1980). Association of Azospirillum
with grass roots. Appl Environ Microbiol 39: 219-26.
Van Oosten, J.-J., and R.T. Besford (1996). Acclimation of photosynthesis to elevated CO2
through feedback regulation of gene expression: climate of opinion. Photosyn Res 48:
Vitousek, P.M., H.A. Mooney, J. Lubchenco, and J.M. Melillo (1997). Human domination of
Earth's ecosystems. Science 277: 494-99.
Vogel, C.S., and P.S. Curtis (1995). Leaf gas exchange and nitrogen dynamics of N2-fixing,
field-grown Alnus glutinosa under elevated atmospheric CO2. Global Change Biol 1: 55-61.
Volin, J.C., P.B. Reich (1996). Interaction of elevated CO2 and O3 on growth,
photosynthesis and respiration of three perennial species grown in low and high nitrogen.
Physiol Plant 97: 674-84.
Vu, J.C.V., L.H. Allen Jr., K.J. Boote, and G. Bowes (1997). Effects of elevated CO2 and
temperature on photosynthesis and Rubisco in rice and soybean. Plant Cell Environ 20:
Wang, K.-Y. (1996a). Apparent quantum yield in Scots pine after four years of exposure to
elevated temperature and CO2. Photosynthetica 32: 339-53.
--- (1996b). Canopy CO2 exchange of Scots pine and its seasonal variation after four-year
exposure to elevated CO2 and temperature. Agric For Meteorol 82: 1-27.
Wang K.-Y., and S. Kellomaki (1997). Effects of elevated CO2 and soil-nitrogen supply on
chlorophyll fluorescence and gas exchange in Scots pine, based on a branch-in-bag
experiment. New Phytol 136: 277-86.
Wang, K.-Y., S. Kellomaki, and K. Laitinen (1995). Effects of needle age, long-term
temperature and CO2 treatments on the photosynthesis of Scots pine. Tree Physiol 15:
Weaver, P.L., and P.G. Murphy (1990). Forest structure and productivity in Puerto Rico's
Luquillo Mountains. Biotropica 22: 69--82.
Webber, A.N., G.-Y. Nie, S.P. Long (1994). Acclimation of photosynthetic proteins to
rising atmospheric CO2. Photosyn Res 39: 413-25.
Weidenbach, P. (1992). Waldbauliche ziele und ergebnisse. Allg Forstzeitschr 13: 711-17.
West, D.C. (1988). Detection of forest response to increased atmospheric carbon dioxide.
In F.A. Koomanoff (ed), Carbon Dioxide and Climate: Summaries of Research in FY 1988
(Wasington, DC: US Department of Energy): 57.
Wittwer SH (1995). Food, Climate, and Carbon Dioxide: The Global Environment and World
Food Production. Boca Raton, FL: CRC Press/Lewis Publishers.
--- (1997). The global environment: it's good for food production. In P.J. Michaels (ed),
State of the Climate Report: Essays on Global Climate Change (New Hope, VA: New Hope
Environmental Services): 8-13.
Wood, C.W., H.A. Torbert, H.H. Rogers, G.B. Runion, and S.A. Prior (1994). Free-air CO2
enrichment effects on soil carbon and nitrogen. Agric For Meteorol 70: 103-16.
Woodrow, I.E. (1994). Optimal acclimation of the C3 photosynthetic system under enhanced
CO2. Photosyn Res 39: 401-12.
Woodwell, G.M. (1989). The warming of the industrialized middle latitudes 1985-2050:
causes and consequences. Climatic Change 15: 31-50.
Wullschleger, S.D., R.J. Norby, and C.A. Gunderson (1997). Forest trees and their response
to atmospheric CO2 enrichment: a compilation of results. In L.H. Allen Jr., M.B. Kirkham,
D.M. Olszyk, and C.E. Whitman (eds)., Advances in Carbon Dioxide Effects Research
(Madiosn, WI: American Society of Agronomy): 79-100.
Wullschleger, S.D., R.J. Norby, and D.L. Hendrix (1992). Carbon exchange rates,
chlorophyll content, and carbohydrate status of two forest tree species exposed to carbon
dioxide enrichment. Tree Physiol 10: 21-31.
Wullschleger, S.D., W.M. Post, and A.W. King (1995). On the potential for a CO2
fertilization effect in forests: estimates of the biotic growth factor based on 58
controlled-exposure studies. In G.M. Woodwel, and F.T. Mackenzie (eds), Biotic Feedbacks
in the Global Climatic System (Oxford: Oxford University Press): 85-107.
Zak, D.R., K.S. Pregitzer, P.S. Curtis, J.A. Teeri, R. Fogel, D.L. Randlett (1993).
Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles. Plant Soil 151:
Ziska, L.H., B.G. Drake, and S. Chamberlain (1990). Long-term photosynthetic response in
single leaves of a C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ.
Oecologia 83: 469-72.
Ziska, L.H., R.C. Sicher, and D.F. Kremer (1995). Reversibility of photosynthetic
acclimation of swiss chard and sugarbeet grown at elevated concentrations of CO2. Physiol
Plant 95: 355-64.
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