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Elevated CO2: What Can It Do for Semi-Arid Shortgrass Steppe Vegetation?
Volume 7, Number 25: 23 June 2004

How will earth's natural ecosystems respond to the ongoing rise in the air's CO2 content? This question is of paramount importance to the current debate over the nature of CO2-induced global change. Will plants and the animals that depend upon them for both sustenance and shelter be helped or hurt by the consequences of carbon dioxide's direct plant physiological impacts? And what of possible warming, CO2-induced or otherwise? Will it exacerbate deleterious effects or enhance good ones?

Nelson et al. (2004) broached these fundamental questions in a five-year study (1997-2001) of the semi-arid shortgrass steppe (SGS) of Colorado, USA. Working at the USDA-ARS Central Plains Experimental Range in the northern portion of the SGS about 60 km northeast of Fort Collins, Colorado, they used large (15.5 m2) open-top chambers to examine the effects of elevated CO2 (720 vs. 360 ppm) on plant water relations, ecosystem water use efficiency, soil moisture dynamics and root distributions of the ecosystem's dominant C3 (Pascopyrum smithii and Stipa comata) and C4 (Bouteloua gracilis) grasses. So what did they find?

The five Agricultural Research Service scientists and their collaborator from Colorado State University report that "seasonal average soil moisture throughout the soil profile (0-15, 15-45, 45-75, 75-105 cm) was increased under elevated CO2 compared to ambient CO2 for much of the study period," with the greatest relative increase (16.4%) occurring in the 75-105 cm depth increment. They remark that this finding of "increased soil moisture under elevated CO2 at the deepest soil depth suggests that water percolated deeper into the soil profile and that less moisture was lost to evapotranspiration under elevated CO2." Noting that "this phenomenon enhances water storage in the deep fine sandy loam soils underlying large portions of the SGS," they go on to say that "this increase in soil moisture has been shown to be the major controlling factor in improved carbon assimilation rates and increased total aboveground biomass in this system (LeCain et al., 2003) and will likely decrease the susceptibility of the SGS to drought."

Another important finding of the group of Colorado researchers was, in their words, that when averaged over the study period, "leaf water potential was enhanced 24-30% under elevated CO2 in the major warm- and cool-season grass species of the SGS (Bouteloua gracilis, C4, 28.5%; Pascopyrum smithii, C3, 24.7%; Stipa comata, C3, 30.4%)." They say these results are similar to those of "studies involving other C3 and C4 grass species (Owensby et al., 1993; Jackson et al., 1994)," and that the enhanced leaf water potential - "which reflects improved plant water status and increased drought tolerance (Tyree and Alexander, 1993)" - may lead to increased leaf turgor and allow the grasses "to continue growth further into periods of drought." Hence, it is not surprising that, averaged over the five years of the study, Nelson et al. found that "water-use efficiency (g aboveground biomass harvested / kg water consumed) was 43% higher in elevated than ambient CO2 plots."

In discussing the broader implications of their findings, the scientists say their results "suggest that a future, elevated CO2 environment may result not only in increased plant productivity due to improved water use efficiency, but also lead to increased water drainage and deep soil moisture storage in this semi-arid grassland ecosystem." And they say that "this, along with the ability of the major grass species to maintain a favorable water status under elevated CO2, should result in the SGS being less susceptible to prolonged periods of drought."

That Nelson et al.'s findings are the norm and not the exception is confirmed by their noting that "previous studies have reported increased soil moisture under elevated CO2 in semi-arid C3 annual grasslands in California (Fredeen et al., 1997), mesic C3/C4 perennial tallgrass prairie in Kansas (Owensby et al., 1993, 1999; Ham et al., 1995; Bremer et al., 1996), and mesic C3 perennial grasslands in Switzerland (Niklaus et al., 1998) and Sweden (Sindhoj et al., 2000)." Hence, we can validly expect the beneficent effects of atmospheric CO2 enrichment revealed in this impressive study to be found in grasslands throughout the world as the air's CO2 content continues to rise to double-and-beyond its current concentration.

But what if air temperature rises concurrently? Actually, things could get even better under that scenario. Nelson et al. note, for example, that "air temperature was on average 2.6C higher inside the chambers than outside," and they say that this warming "was implicated in the 36% enhanced biomass production observed in chambered-ambient compared to non-chambered plots." Consequently, since this already-enhanced biomass production was the starting point from which the 41% increase in biomass elicited by the doubling of the air's CO2 content was calculated, the increase in biomass caused by the concurrent actions of both factors (increasing air temperature and CO2 concentration) could well be something on the order of 90%.

So bring on the climate alarmists' "twin evils" of elevated CO2 and temperature ? and let the (ecological) good times roll.

Sherwood, Keith and Craig Idso

References
Bremer, D.J., Ham, J.M. and Owensby C.E. 1996. Effect of elevated atmospheric carbon dioxide and open-top chambers on transpiration in a tallgrass prairie. Journal of Environmental Quality 25: 691-701.

Freden, A.L., Randerson, J.T., Holbrook, N.M. and Field, C.B. 1997. Elevated atmospheric CO2 increases water availability in a water-limited grassland ecosystem. Journal of the American Water Resources Association 33: 1033-1039.

Ham, J.M., Owensby, C.E., Coyne, P.I. and Bremer, D.J. 1995. Fluxes of CO2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO2. Agricultural and Forest Meteorology 77: 73-93.

Jackson, R.B., Sala, O.E., Field, C.B. and Mooney, H.A. 1994. CO2 alters water use, carbon gain, and yield for the dominant species in a natural grassland. Oecologia 98: 257-262.

LeCain, D.R., Morgan, J.A., Mosier, A.R. and Nelson, J.A. 2003. Soil and plant water relations determine photosynthetic responses of C3 and C4 grasses in a semi-arid ecosystem under elevated CO2. Annals of Botany 92: 41-52.

Nelson, J.A., Morgan, J.A., LeCain, D.R., Mosier, A.R., Milchunas, D.G. and Parton, B.A. 2004. Elevated CO2 increases soil moisture and enhances plant water relations in a long-term field study in semi-arid shortgrass steppe of Colorado. Plant and Soil 259: 169-179.

Niklaus, P.A., Spinnler, D. and Korner, C. 1998. Soil moisture dynamics of calcareous grassland under elevated CO2. Oecologia 117: 201-208.

Owensby, C.E., Coyne, P.I., Ham, J.H., Auen, L.M. and Knapp, A.K. 1993. Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated CO2. Ecological Applications 3: 644-653.

Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M. 1999. Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2. Global Change Biology 5: 497-506.

Sindhoj, E., Hansson, A.C., Andren, O., Katterer, T., Marissink, M. and Pettersson, R. 2000. Root dynamics in a semi-natural grassland in relation to atmospheric carbon dioxide enrichment, soil water and shoot biomass. Plant and Soil 223: 253-263.

Tyree, M.T. and Alexander, J.D. 1993. Plant water relations and the effects of elevated CO2: A review and suggestions for future research. Vegetatio 104/105: 47-62.