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Growth Response to CO2 with Other Variables (Temperature: Grassland Species) -- Summary
As the atmosphere's CO2 concentration continues to rise, most plants tend to exhibit increased rates of photosynthesis and biomass production, including those of various grassland ecosystems. This increase in productivity should increase the amount of forage available for grazing animals and possibly reduce the land area occupied by bare soil in certain environments. However, some people claim that global warming will negate the growth-promoting effects of atmospheric CO2 enrichment and actually stimulate the opposite process of desertification. This summary thus seeks to develop an answer to this important question by reviewing published scientific studies of the photosynthetic and growth responses of grassland plants to atmospheric CO2 enrichment when exposed to higher-than-normal temperatures.

The first thing to note in this regard is that the optimum growth temperatures of many plants have been demonstrated to rise substantially with increasing concentrations of atmospheric CO2 (Berry and Bjorkman, 1980; Stuhlfauth and Fock, 1990; McMurtrie et al., 1992; McMurtrie and Wang, 1993), as has been described in more detail by Long (1991), Idso and Idso (1994) and Cowling and Sykes (1999). Hence, we here proceed to see if these previously-determined positive CO2 x temperature interactions have continued to appear in more recent studies of the subject, focusing specifically on grassland species.

In the study of Lilley et al. (2001), swards of subterranean clover (Trifolium subterraneum) were grown at 380 and 690 ppm CO2 in combination with simultaneous exposure to ambient and elevated (ambient plus 3.4°C) air temperatures; and after one year of these treatments, they report that elevated CO2 increased foliage growth by 19% at ambient air temperatures. At elevated air temperatures, however, the CO2-enriched plants only displayed a growth enhancement of 8%; but the plants grown at ambient CO2 exhibited a 28% reduction in foliage growth. Similarly, Morgan et al. (2001) determined that twice-ambient levels of atmospheric CO2 increased aboveground biomass in native shortgrass steppe ecosystems by an average of 38%, in spite of an average air temperature increase of 2.6°C. And when bahiagrass was grown across a temperature gradient of 4.5°C, Fritschi et al. (1999) found that a 275 ppm increase in the air's CO2 content boosted photosynthesis and aboveground biomass by 22 and 17%, respectively, independent of air temperature.

Other studies have reported similar results. Greer et al. (2000), for example, grew five pasture species at 18 and 28°C and reported that plants of the five species concomitantly exposed to 700 ppm CO2 displayed average photosynthetic rates that were 36 and 70% greater, respectively, than average rates exhibited by control plants grown in air of ambient CO2 concentration. What is more, the average CO2-induced biomass increase for the five species rose dramatically with increasing air temperature: from only 8% at 18°C to 95% at 28°C.

Stirling et al. (1998) had earlier found much the same thing. They had nurtured five fast-growing native annual species in glasshouses maintained at two different combinations of CO2 (ambient and ambient plus 340 ppm) and temperature (ambient and ambient plus 3°C) for eight weeks, in order to assess their growth responses to elevated CO2 and temperature. This work revealed that the elevated CO2 significantly increased photosynthetic rates by 18-36% for all species, independent of growth temperature, for the entire eight weeks of the experiment. And the persistence of this photosynthetic enhancement led to total plant biomass increases for CO2-enriched plants that were, on average, 25% greater than those of control plants grown in ambien-CO2 air. And although elevated CO2 and elevated temperature, in combination, had but few significant interactive effects on the various metrics of growth, the overall CO2 growth response was generally just a bit larger at elevated than at ambient temperatures.

In a similar study with similar findings, Newman et al. (2001) grew two perennial grassland species (rhizoma peanut-Arachis glabrata and bahiagrass-Paspalum notatum) native to South America and common to Florida, USA, in greenhouses fumigated with air containing either 360 or 700 ppm CO2 for three full growing seasons, while the C3 and C4 grasses were simultaneously exposed to air temperatures that ranged from ambient to 4.5°C above ambient. And averaged across the three growing seasons, they found that the elevated CO2 increased dry matter production in rhizoma peanut and bahiagrass by 25 and 15%, respectively. Once again, however, there were no significant interactive effects of elevated CO2 and temperature on dry mass production in these species; and on their own, air temperatures 4.5°C above ambient increased dry matter production in both species by an average of 13% across all three years.

In a contemporary study, Niklaus et al. (2001) established experimental plots in a nutrient-poor calcareous grassland in northwestern Switzerland that contained either 31, 12 or 5 species by removing selected species from some of the plots so that the proportion of plant functional types in each of the plots remained unchanged (55% graminoids, 15% legumes, and 30% non-legume forbs), after which they fumigated the plots with air containing either 360 or 600 ppm CO2 for a period of four years, in order to determine the ecological effects of elevated CO2 across a biodiversity gradient in this grassland community. This effort ultimately revealed that as plant community diversity decreased at ambient CO2, soil nitrate concentrations increased. Elevated CO2, on the other hand, acted to reduce soil nitrate concentrations at all of the studied levels of plant diversity. In addition, they found that nitrification - a biological process that yields nitrate - increased with decreasing species diversity at ambient CO2, while at elevated CO2, rates of nitrification were 25% lower than those observed at ambient CO2 at all levels of community diversity, which suggests that in a CO2-enriched world of the future there would be less risk of nitrate pollution of groundwater.

A few years earlier, Hakala and Mela (1996) had grown field-sown meadow fescue (Festuca pratensis cv. Kalevi) in open-top chambers and glasshouses that were maintained at atmospheric CO2 concentrations of 350 and 700 ppm in combination with ambient and elevated (ambient plus 3°C) air temperatures for four consecutive years to determine the effects of these parameters on aboveground biomass production in this important forage crop. This study revealed that elevated CO2 significantly increased aboveground biomass by an average of 18% in each of the four study years, but that the effect was only present when plants were concomitantly exposed to elevated air temperatures.

In a study of a second species of fescue (Festuca arundinacea Schreb), Sinclair et al. (2007) began the report of their findings by noting that for plants that appear to be adapted to cool temperatures, and which typically exhibit reduced growth rates in warmer environments, it had long been believed (as only seems logical) that global warming would be bad for them. However, as with many things, appearances can be deceiving; and so they were in this case.

Using climate-controlled mini-greenhouses, the five researchers examined the interacting effects of air temperature and vapor pressure deficit (VPD) on the growth of tall fescue (Festuca arundinacea Schreb), a cool-season grass that from past studies was expected to show declining growth with warmer temperatures over the range of 18.5 to 27°C. This they did by growing well watered and fertilized plants in two sets of six-week-long experiments, one in which air VPD was held constant at 1.2 kPa while air temperature was maintained at either 18.5, 21, 24 or 27°C, and one in which air temperature was held constant at 22°C while air VPD was maintained at either 0.9, 1.2, 1.4 or 1.7 kPa. And in the experiment where the air VPD was held constant, they found that "in direct contrast to the anticipated results, the weekly growth of the tall fescue was substantially increased with increased temperature," reporting that "growth at 24 and 27°C was about 2.3 times that at 18.5°C and 1.4 times that at 21°C," while in the experiment where air temperature was held constant, they say "there was a strong, negative influence of increasing VPD on plant growth." In addition, they observed that "transpiration rates were similar across treatments," indicating that "water movement through the plants did not increase in response to increasing VPD," which led them to conclude that limitation of water movement through the plant "is likely a result of stomatal closure in response to elevated VPD (Bunce, 2006)." This phenomenon, of course, would also restrict the CO2 diffusion pathway into the plants and result in a decrease in photosynthesis, which is likely what caused the decreased growth at increased VPD. Be that as it may, their results indicate that as long as the air VPD does not rise concurrently, increasing temperatures do not lead to growth reductions in this cool-season plant. In fact, they found that just the opposite was true -- that warming dramatically increased tall fescue growth.

In further commenting on their findings, Sinclair et al. write that "during the past 50 years, VPD has remained virtually constant (Szilagyi et al., 2001) due to an increase in atmospheric dew point temperature (Gaffen and Ross, 1999; Robinson, 2000)," even in the face of what climate alarmists describe as unprecedented global warming; and because of this fact, they conclude that, in a future warmer world, "tall fescue, and perhaps other cool season species, could experience a substantial benefit with temperature increases expected in temperate zones if VPD were to remain unchanged," which indeed appears to be what will happen in light of the real-world behavior of the air's VPD over the past half-century of warming.

In the introduction to their study of the subject, Wolfe-Bellin et al. (2006) wrote that "nocturnal temperatures are predicted to increase more than diurnal temperatures," as has also been observed in the real world over much of the 20th century, and that it might be expected that "increased nocturnal temperature would increase dark respiration rate" and thereby "diminish the positive effects of elevated CO2 on whole-plant growth, as measured by total biomass." And, therefore, in an experiment designed to explore this hypothesis, they grew the C3 forb Phytolacca americana L. from the four-leaf stage to maturity under well watered and fertilized conditions in 6.2-L containers filled with a general purpose growing medium within controlled-environment glass chambers maintained at either 370 or 740 ppm CO2 at diurnal/nocturnal temperatures of either 26°/20°C or 26°/24°C, during which time they periodically measured their light-saturated photosynthetic rates and whole-plant biomass. This study revealed, in their words, that "plant photosynthetic rate was greater under elevated CO2 [+69% during the first part of the growing season], while dark respiration rate, predicted to increase under higher nocturnal temperatures, exhibited no response to the nocturnal temperature treatment." Hence, they stated that in contrast to their prediction, the forb they studied "exhibited no diminishment of total plant size in response to elevated nocturnal temperature," and that "time to flowering decreased and biomass allocation to reproduction increased under conditions of elevated nocturnal temperatures." And so they concluded that "elevated CO2 and high nocturnal temperatures of the future could have a neutral or even positive effect on the growth of northern P. americana populations," even to the extent of "increasing population sizes, at least for plants growing at the northern edge of the species' range."

More recently, and taking a similar tack on the issue, Niu et al. (2010) wrote that "most modeling studies predict ecosystem carbon storage will decrease as respiration is stimulated more than photosynthesis by rising temperature, with a consequent positive feedback to climate warming," which thus ends up enhancing what the world's climate alarmists generally consider to be one of the greatest threats currently facing the planet. And, therefore, working in a tallgrass prairie of the U.S. Great Plains located in McClain County, Oklahoma - which was dominated by C4 grasses and C3 forbs that had not been grazed for the prior forty years - they conducted a warming experiment, where infrared heaters were used to elevate soil temperature at a depth of 2.5 cm by an average of 1.96°C from 2000 to 2008, and where they say that "yearly biomass clipping mimicked hay or biofuel feedstock harvest."

This study clearly demonstrated, as they describe it, that the experimental warming "significantly stimulated carbon storage in aboveground plant, root, and litter pools by 17%, 38%, and 29%, respectively, averaged over the nine years ," but that it "did not change soil carbon content or nitrogen content in any pool." Thus, they concluded that increased plant nitrogen use efficiency played a more important role than soil nitrogen availability in regulating carbon cycling in this particular ecosystem, since the tallgrass prairie experienced a significant increase in productivity that was caused solely by the warming of its soil and not promoted by any addition of nitrogen to it. And they explained this result by stating that "increased inputs of more recalcitrant [higher carbon:nitrogen ratio] material into soil counterbalanced any direct warming stimulation of carbon release, leading to little change in soil carbon stock and no apparent feedback to climate warming."

Hard on the heels of the report of this study came that of the study of Morgan et al. (2011), who wrote that "global warming is predicted to induce desiccation in many world regions through increases in evaporative demand," but who also said that "rising CO2 may counter that trend by improving plant water-use efficiency." However, they were very forthright in noting that "it is not clear how important this CO2-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO2 also leads to higher plant biomass, and therefore greater transpirational surface." Therefore, to explore these issues in a real-world setting, Morgan et al. conducted a Prairie Heating and CO2 Enrichment (PHACE) experiment in which they evaluated the productivity of native mixed-grass prairie west of Cheyenne, Wyoming (USA) to two levels of atmospheric CO2 concentration (385 and 600 ppm, supplied via standard FACE technology) and two temperature regimes - ambient and elevated (ambient plus 1.5/3.0°C warmer day/night temperatures) that were maintained for three full growing seasons (2007-2009) by means of T-FACE technology (Kimball et al., 2008) - after first having measured grassland productivity under unmodified conditions for one growing season (2005) and with CO2 enrichment alone for a second season (2006). So what did they learn?

The ten researchers report that their warming treatment reduced annual soil water content by 13.1%, but that their elevated CO2 treatment increased annual soil water content by 17.3%, demonstrating that "the water conservation effects of elevated CO2 can completely cancel the desiccating effects of moderately warmer temperatures." In addition, they found that "exposure of the prairie to 600 ppm CO2 increased peak total above-ground biomass by an average 33% in the first 3 years of the experiment when annual precipitation amounts were within 7% of the site's 132-year average of 388 mm," but that "CO2 enrichment had no effect on above-ground biomass in 2009," when "annual precipitation was 17% higher than the long-term mean." And they speculate, in this regard, that the "higher soil water content in 2009 minimized the potential water-relations benefit of CO2 enrichment on plant productivity."

In further discussing their findings, Morgan et al. write that "many believe that CO2-induced reductions in transpiration at the leaf level will be largely offset at the canopy level by increases in leaf area," citing the studies of McNaughton and Jarvis (1991), Piao et al. (2007), Frelich and Reich (2010) and Seager and Vecchi (2010). However, they are fully justified in stating, in the final sentence of the body of their paper, that their results "clearly illustrate the importance of compensating CO2 and warming effects in semi-arid ecosystems," and declaring in the final sentence of the abstract of their paper that their results "indicate that in a warmer, CO2-enriched world, both soil water content and productivity in semi-arid grasslands may be higher than previously expected." In addition, in an accompanying commentary on their paper, Baldocchi (2011) writes that "Morgan and colleagues provide one of the first and best views of how a mixed-grass ecosystem growing in a semi-arid climate will respond to future CO2 and climatic conditions." And, last of all, Morgan et al.'s findings help to explain the great CO2-induced greening of the earth phenomenon, especially as it pertains to semi-arid regions of the planet.

Finally, even if the air's CO2 content were to cease rising or to have no effect on plants, it is still possible that temperature increases alone would promote plant growth and development in some situations. This was the case in the study of Norton et al. (1999), for example, where elevated CO2 had essentially no effect on the growth of the perennial grass Agrostis curtisii after two years of fumigation, but where a 3°C increase in air temperature increased the growth of the species considerably.

In conclusion, the recent scientific literature continues to indicate that as the air's CO2 content rises, grassland plants will likely exhibit enhanced rates of photosynthesis and biomass production that will not be diminished by any global warming that might occur concurrently. In fact, if the ambient air temperature does rise, the growth-promoting effects of atmospheric CO2 enrichment will likely rise right along with it, becoming more and more robust in agreement with the experimental observations reviewed by Idso and Idso (1994). The future ability of grasslands to produce increasingly greater amounts of forage, and perhaps reclaim areas of barren ground in certain environments, thus looks promising indeed, as long as the air's CO2 content continues to rise.

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Last updated 7 August 2013