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Long-Term Studies (Non-Woody Plants) -- Summary
How do non-woody plants respond to elevated concentrations of atmospheric CO2 over periods of many years?  The only way to know for sure is to conduct long-term CO2 enrichment experiments; and we here briefly summarize the findings of a few such studies.

In Switzerland, Niklaus et al. (2001) exposed a species-rich, but nutrient-poor and water-limited, calcareous grassland dominated by Bromus erectus (which accounted for approximately half of the ecosystem's aboveground vegetative biomass) to atmospheric CO2 concentrations of approximately 360 and 600 ppm for a period of six years, using screen-aided CO2 control (SACC) technology.  CO2-induced increases in biomass production in years 1 through 6 of the experiment were, respectively, 5%, 20%, 22%, 27%, 31% and 18%, for an average of 23.6% over the last five years of the study (Niklaus and Korner, 2004), which ultimately increased carbon stocks in plant shoots and roots by 17 and 24%, respectively, while enhancing carbon stocks in vegetative litter by 34%.  The net effect of these increases was an initial air-to-soil carbon flux of 210 g C m-2 year-1.  After six years of treatment, however, the CO2-enriched soils only held about 44% of the carbon expected from this influx rate, due to the low soil residence time of the newly-input carbon.  Nevertheless, the study showed that plant growth and carbon sequestration in low-nutrient and water-limited soils can in fact be enhanced over a period of six years as a result of atmospheric CO2 enrichment.

In Italy, Bettarini et al. (1998) measured the stomatal densities and conductances of the leaves of 17 species of plants growing in the vicinity of a natural CO2-emitting spring that has produced twice-ambient atmospheric CO2 concentrations for at least two centuries, while making similar measurements on plants of the same species located further from the spring, where normal CO2 concentrations prevail.  The elevated CO2 decreased leaf stomatal conductances in all but one of the species by 19 to 73%.  These reductions, however, were not accompanied by decreases in stomatal density, which remained unaffected by long-term atmospheric CO2 enrichment in all but three species.  Consequently, life-long exposure to elevated CO2 reduced plant water usage primarily by controlling leaf stomatal function, and not by changing leaf anatomical features (i.e., the number of stomata per unit leaf area).

These findings are extremely encouraging, but many people believe they cannot persist indefinitely in all situations.  The productivity of earth's temperate grasslands, for example, is often limited by the availability of soil nitrogen (Vitousek and Howarth, 1991); and both empirical and modeling studies have suggested that the magnitude and duration of grassland growth responses to rising levels of atmospheric CO2 may be constrained by less than adequate supplies of soil nitrogen (Rastetter et al., 1997; Luo and Reynolds, 1999; Thornley and Cannell, 2000).

In light of this mix of real-world observations and theoretical calculations, it would seem only natural to believe, as Richter et al. (2003) hypothesize, "that increased below-ground translocation of photoassimilates at elevated pCO2 would lead to an increase in immobilization of N due to an excess supply of energy to the roots and rhizosphere," and that this phenomenon would ultimately lead to a reduction in the size of the growth-promoting effect of elevated atmospheric CO2 that is manifest in short-term CO2 enrichment experiments and at the start of long-term studies.

To test this hypothesis, Richter et al. (2003) measured gross rates of N mineralization, NH4+ consumption and N immobilization in soils upon which monocultures of Lolium perenne and Trifolium repens had been exposed to ambient (360 ppm) and elevated (600 ppm) concentrations of atmospheric CO2 at high and low rates of soil nitrogen addition for seven years in the Swiss FACE study conducted near Zurich.  After seven years of treatment, they found that "gross mineralization, NH4+ consumption and N immobilization in both the L. perenne and the T. repens swards did not show significant differences."  In addition, the size of the microbial N pool and immobilization of applied mineral 15N were not significantly affected by the elevated CO2.

Richter et al. note that the results of their study "did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated CO2.  They thus conclude "that differences in plant N demand and not changes in soil N mineralization/immobilization are the driving factors for N dynamics in these meadow grassland systems."  Hence, as in the woody plant studies of Finzi and Schlesinger (2003) and Schafer et al. (2003) conducted in the Duke Forest FACE experiment, Richter et al.'s work provides no evidence that the growth responses of earth's grasslands to atmospheric CO2 enrichment will ever be significantly reduced from what is suggested by moderate-term studies of a few to several years' duration.

In a study of the same L. perenne and T. repens ecosystems that helps to explain some of these observations, Gamper et al. (2004) analyzed the effects of elevated CO2 and N fertilization (14 vs. 56 g N m-2) on arbuscular mycorrhizal fungi.  They report that "at elevated CO2 and under both N treatments, AMF root colonization of both host plant species was increased," and that "colonization levels of all three measured intraradical AMF structures (hyphae, arbuscules and vesicles) tended to be higher."  In addition, they say there was also an increase in non-AMF root colonization under elevated CO2.  As a result, in their words, they "hypothesize that AMF provide non-P-nutritional benefits under the phosphorus-rich soil conditions of our field experiment," and that these benefits "may include improved N nutrition and increased protection against pathogens and/or herbivores."

In another long-term study conducted in Switzerland, Ainsworth et al. (2003b) analyzed data obtained from what has become the longest-running FACE experiment ever to be conducted anywhere in the world.  The impetus for their analysis was the speculation that, in their words, "elevated CO2 may partition resources away from leaves and, through increased production, sequester nutrients into organic matter causing deficiencies which indirectly cause decreased photosynthetic capacity."  In this regard, they cite the theoretical study of these considerations conducted by Luo and Reynolds (1999), who "predicted that the initial stimulation of photosynthetic production in grasslands would be lost within nine years of a step increase in CO2, as imposed in FACE experiments."

Utilizing real-world data obtained over nearly a decade of experimentation with white clover (Trifolium repens), which was grown in monoculture in the Swiss FACE array, Ainsworth et al. (2003b) characterized the photosynthetic responses of the plants to the extra 240 ppm of CO2 delivered to them in the spring and autumn of the eighth year of the experiment.  They determined there was no acclimation or down regulation of photosynthetic capacity in the spring of the year.  In the autumn, however, there was a down regulation of approximately 20%; but it occurred "late in the growing season, when the 24-hour mean temperature had dropped below 10C, and nightly frosts were occurring," under which conditions "shoot growth is limited and the sink for carbohydrate is small, and acclimation of photosynthesis to elevated CO2 would be expected."  Yet in spite of that acclimation, and the stress of those cold conditions, the average photosynthetic rate of the CO2-enriched plants at that time of year was still 37% greater than that of the ambient-treatment plants.  Hence, the five scientists concluded that their results "do not support the prediction that the response of grassland species to elevated CO2 will be short-lived as the demand for nutrients increases," in clear contradiction of the claim of Luo and Reynolds, as well as the similar claims of others (see our Editorial of 10 Dec 2003); for as they reiterate in the concluding sentence of their paper, "contrary to the belief that the response of grassland species to elevated CO2 will be short-lived, stimulation of photosynthesis in T. repens remained after eight years of exposure to elevated CO2."

In another report on this longest of FACE studies ever to be conducted on a grassland species, Ainsworth et al. (2003a) note that "photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration, a physiological response that could significantly alter the future carbon cycle if it persists in the long term."  However, they also say that "an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2."  This negative perspective, in their words, suggests that "perennial systems will respond to elevated CO2 in the short term, but that the response for grasslands will be short-lived (Roumet et al., 2000)," and they cite Luo and Reynolds (1999) as suggesting an effective CO2-induced stimulatory period of less than ten years for both high- and low-productivity grasslands.

As was the case with trees [see our Editorial of 5 March 2003], where it was also long believed that the stimulatory effect of elevated CO2 on photosynthesis and growth would gradually waste away over time, the only way to resolve the issue with respect to grasslands is to conduct a long-term experiment - such as the sour orange tree study of Idso and Kimball (2001) - which is exactly what the eight-member Ainsworth et al. (2003a) team of American, British, Italian and Swiss scientists did in their ten-year study of perennial ryegrass (Lolium perenne).

The record-setting study was conducted in Switzerland within three replicate blocks of two 18-m-diameter FACE rings maintained at either 360 or 600 ppm CO2 throughout each growing season of the entire 10-year period.  The experimental plots, which were established in 1993 on a field of perennial ryegrass that had been planted in August of 1992, were further subdivided into low and high nitrogen fertilization treatments; and the plants grown within them were periodically harvested several times a year.  In addition, in the words of the authors, "more than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated CO2 across each growing season for the duration of the experiment."

So what was learned in this measurement-intensive study?  Ainsworth et al. (2003a) report that "over the 10 years as a whole, growth at elevated CO2 resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake."  Interestingly, the 36% increase in daily CO2 uptake was, in their words, "almost identical to the 38% increase seen on the first day of measurements in August 1993 and the 39% stimulation on the last day of measurements in May 2002."

There was also a seasonal trend in the CO2-induced increase in the daily integral of CO2 fixation, which ranged from 25% in the spring to 41% in the summer and 48% in the fall.  The scientists say this finding "is consistent with theoretical expectation, where because of the differing sensitivities of Rubisco oxygenase and carboxylase activity, the proportionate stimulation of photosynthesis by a given increase in CO2 will rise with temperature (Long, 1991)."  This phenomenon has also been observed in a number of other plants, as described in our major report The Specter of Species Extinction.

Ainsworth et al. (2003a) additionally note that "the percentage increase in photosynthetic carbon uptake in the first 20 days following a harvest (45%) was nearly double the percentage increase later in the regrowth cycle (23%)."  This finding is indicative of the fact that CO2-induced growth stimulation is greatest when plant source:sink ratio is small, i.e., when there are few photosynthesizing leaves and many photosynthate-storing roots, so that the CO2-induced enhancement of photosynthesis need not immediately decline for lack of a sufficiently large repository to deposit the fruits of its labors, so to speak.

Summing up, the international team of scientists said the CO2-induced photosynthetic stimulation "was maximal following harvest, at the warmest times of year and with a high supply of nitrogen."  Most important of all, however, was their ultimate conclusion: "this open-air field experiment provides no support for the prediction that stimulation of photosynthesis under elevated CO2 is a transient phenomenon," or as they phrased it in the abstract of their paper, "in contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no [evidence of] significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation ... in the latter years in either nitrogen treatment."

The ultimate plant response, of course, is biomass production, which was studied in the same experiment by Schneider et al. (2004), who found that "in 1993, the CO2 response of harvested biomass was 7.2%, increasing to 32% in 2002."  At low N, however, they report that the CO2 response "varied annually."  Nevertheless, it too exhibited a slowly increasing (though non-significant) trend, suggesting that given enough time, it might have gained statistical significance as well.  In addition, Schneider et al. report that "at high N supply, more N was mobilized from the soil after long-term exposure to elevated CO2 than after ambient CO2," in contrast to the suggestion of Hungate et al. (2003) that just the opposite would likely occur.  At low N, however, the Swiss team writes that "the reduced availability of N constantly limited the harvestable biomass to elevated CO2 throughout the experiment," more in harmony with Hungate et al.'s suggestion.  Nevertheless, as noted above, there is a tantalizing indication that this limitation may have been slightly reduced over the course of the 10-year study, and that a still longer experiment may be needed to resolve the issue in the case of low-N soils; for as we demonstrate in our Editorial of 5 Mar 2003, two or more decades may well be required to resolve some of these knotty issues.

It was thus somewhat sad to see this long-term study terminated, without knowing how high the biomass response may have ultimately risen, or whether the biomass response of the ryegrass in the low-nitrogen treatment might have also been increasing with time, but at a much reduced rate, support for which possibility is provided by an even longer study of a grasslike plant.

Rasse et al. (2005) evaluated the long-term effects of atmospheric CO2 enrichment on the net CO2 exchange, shoot density and shoot biomass of the wetland sedge, Scirpus olneyi, in a long-term in situ elevated CO2 experiment at the Smithsonian Environmental Research Center on the USA's Chesapeake Bay.  They found that in every one of the 17 years of the experiment's duration to the time of their analysis, the net CO2 exchange rate and shoot biomass and density of the plants growing in the CO2-enriched (ambient +340 ppm) air were all greater than they were among the plants growing in ambient air.  In the case of the net CO2 exchange rate, the extra CO2 boosted it by 80% in the first year of the study, but the enhancement declined to about 35% by the end of the third year and remained relatively constant at that value over the following 15 years.  Shoot biomass and density also increased, but whereas the CO2-induced stimulation of the net CO2 exchange rate remained essentially constant over the past 15 years, the CO2-induced stimulations of shoot biomass and density increased over time.  After 5 years of a nearly constant stimulation of 16%, for example, shoot density increased in near linear fashion to a value 128% above the ambient-air value at the end of year 17.  The response of shoot biomass to CO2 enrichment was also nearly linear, reaching a value approximately 70% above ambient at year 17.  What is more, the trends in shoot density and biomass do not appear to be leveling off, leading one to wonder just how high the CO2-induced stimulations will ultimately rise.

Net CO2 exchange, shoot density and shoot biomass were closely correlated with bay water salinity in this study, such that the higher the salinity, the more detrimental were its effects on these variables.  Nevertheless, even at the highest levels of salinity reported, atmospheric CO2 enrichment was able to produce a positive, albeit reduced, stimulatory effect on net CO2 exchange.  For shoot biomass and density, the responses were better still: not only did atmospheric CO2 enrichment essentially eradicate the detrimental effects of salinity, there was, in the words of Rasse et al., "circumstantial evidence suggesting that salinity stress increased the stimulation of shoot density by elevated atmospheric CO2 concentration."

Several important things are demonstrated by this experiment.  First, as the researchers state, their results "leave no doubt as to the sustained response of the salt march sedge to elevated atmospheric CO2 concentration."  Second, given the fact that the initial responses of the three growth variables declined or remained low during the first few years of the study, but leveled out or increased thereafter, it is clear that much more long-term research needs to be carried out if we are to ascertain the full and correct impacts of atmospheric CO2 enrichment on plants.  In the case of the wetland sedge of this study, for example, it took ten long years before an increasing trend in the shoot density could clearly be recognized.  Last of all, there is the authors "most important finding," i.e., "that a species response to elevated atmospheric CO2 concentration can continually increase when [it] is under stress and declining in its natural environment."  This result is highly significant and once again bears witness to the fact that earth's rising atmospheric CO2 concentration is not a catastrophic disaster, as climate alarmists would have one believe, but actually a boon to the biosphere for which we will all someday be extremely grateful.

Last of all, Gifford (2004) describes the findings of an international FACE workshop on Short- and Long-Term Effects of Elevated Atmospheric CO2 on Managed Ecosystems, concentrating on a few key aspects of the aerial fertilization effect of atmospheric CO2 enrichment and how it likely will be expressed in the real world of nature as the air's CO2 content continues to rise.  He begins by noting that Kimball et al. (2002) compared what was learned about elevated CO2 effects on eleven different crops from recent FACE experiments with what had been learned from prior chamber studies, including open-top chambers.  He reports that Kimball et al. determined that the FACE experiments confirmed, under longer-term field conditions and with but a couple of exceptions, "all the prior quantitative chamber findings on crops grown and measured in elevated CO2 concentration compared with ambient CO2 concentration."

Next, Gifford notes that the subsequent study of Long et al. (2004) confirmed, "with greater statistical rigour and for a much wider range of species including crops, pasture species and trees, most of the conclusions of the evaluation by Kimball et al. (2002)."  In this regard he reports that Long presented an elegant exposition of how plants optimize "the deployment of N from photosynthetic machinery to growth organs such that a balance between C-source and C-sinks is maintained in the plant under elevated CO2 concentration - a response that generally increases nitrogen use efficiency (Wolfe et al., 1998)."  What is more, he reports that several FACE studies demonstrate an increased abundance of legumes in CO2-enriched plots, and that this observation "is supportive of the notion that, in the long run, elevated CO2 concentration may cause N-fixation to entrain more atmospheric N2 into the ecosystem, leading ultimately to fuller expression of the increased growth and standing biomass potential that the elevated CO2 provides (Gifford, 1992)."

Last of all, in an update of the analysis of Hendry et al. (1997), which focused on the effects of the rapidly fluctuating atmospheric CO2 concentrations that are characteristic of FACE experiments, the technique's primary developer (George Hendry) concluded, according to Gifford, that plant photosynthesis rates "can be decreased by 17% or more for the mean concentration reported when that mean is of large CO2 fluctuations on the order of half the mean, and the deviations from the mean occur over a minute or longer."  In light of this finding, both of them suggest, in Gifford's words, that "FACE technology might be systematically understating [our italics] the effect of globally elevated CO2 on ecosystem productivity."

In concluding his review, Gifford sums up the consensus of the participants at the FACE workshop with respect to "the CO2 fertilizing effect," stating that "the evidence for its existence in the real world continues to consolidate."  Yes, the aerial fertilization effect of atmospheric CO2 enrichment is both real and enduring, enhancing the growth of both woody and non-woody plants alike.  It has operated over the course of the Industrial Revolution, it operates now, and it will continue to operate as far into the future as eye can see and mind can comprehend, the negative and unfounded claims of the world's climate alarmists notwithstanding.

References
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Last updated 27 July 2005