PARENT SESSION
Oral Session # 56: Plant Ecology IV: Plant - Water Relations II.
Presiding: D Rosenthal
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.

Grassland productivity responses to rising atmospheric carbon dioxide are driven primarily by water relations.

Morgan, Jack^*,1, Pataki, Diane ², Gruenzweig, Jose³, Koerner, Christian⁴, Niklaus, Pascal⁴, Polley, H.⁵, Nowak, Robert⁶, Newton, Paul⁷, ¹ USDA-ARS, Fort Collins, CO, USA² University of Utah, Salt Lake City, UT, USA³ Weizmann Institute of Science, Rehovot, Israel⁴ University of Basel, Basel, Switzerland⁵ USDA-ARS, Temple, TX, USA⁶ University of Nevada, Reno, NV, USA⁷ AgResearch, Palmerston North, New Zealand

ABSTRACT- Increases in CO₂ and other trace gases are implicated in global warming that occurred in the past century, and are predicted to cause substantial increases in temperature and alter precipitation patterns in this century. However, the direct effect of CO₂ on plant and soil responses may be just as important, and in some cases, more important than their predicted responses to climatic change. Thus, an understanding of how ecosystems respond directly to rising CO₂ concentrations is key to predicting their future functioning and impacts in a CO₂ enriched world. A number of field and microcosm CO₂ enrichment studies have now been completed in natural and semi-natural grassland ecosystems, and a survey of the pertinent literature revels a wide range of responses to increases in CO₂, from no production responses in alpine grasslands or for wet years in the sub-humid tallgrass prairie to consistent and substantial production responses every year in semi-arid shortgrass steppe. We propose that most of the variation in grassland production to CO₂ may be accounted for by variability in precipitation and soil/plant water dynamics, with production being enhanced relatively more by increased CO₂ in drier than in wet systems (or years). To evaluate this notion, experimental results from several CO₂ enrichment studies from contrasting natural or semi-natural grasslands (temperate and Mediterranean grasslands, tallgrass prairie, shortgrass steppe) and one desert ecosystem were compared. We found that relative aboveground plant biomass enhancements due to increasing CO₂ concentrations were generally stronger as annual precipitation declined, and that these biomass enhancements were associated with increases in soil and/or plant water status. The results suggest that CO₂-induced production responses in natural and semi-natural grasslands may be predicted moderately well by considering only the effects on plant/soil water relations.

Key words: grassland, global change, desert