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Title: Water relations in grassland and desert ecosystems to elevated atmospheric CO2

Author
item Morgan, Jack
item PATAKI, DIANE - UNIVERSITY OF UTAH
item KORNER, CHRISTIAN - UNIVERSITY OF BASEL
item CLARK, HENRY - AGRESEARCH
item DEL GROSSO, STEVE - COLORADO STATE UNIVERSITY
item GRUNZWEIG, JOSE - WEIZMANN INST OF SCIENCE
item KNAPP, ALAN - KANSAS STATE UNIVERSITY
item Mosier, Arvin
item NEWTON, PAUL - AGRESEARCH
item NIKLAUS, PAUL - UNIVERSITY OF BASEL

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2004
Publication Date: 5/20/2004
Citation: Morgan, J.A., Pataki, D.E., Korner, C., Clark, H., Del Grosso, S., Grunzweig, J., Knapp, A., Mosier, A.R., Newton, P., Niklaus, P. 2004. Water relations in grassland and desert ecosystem responses to elevated atmosperhic CO2. Oecologia. 140:11-25.

Interpretive Summary: Atmospheric CO2 concentrations are arising and expected to increase from over 365 parts per million (ppm) today to over 600 ppm by the end of the 21st century. This paper examines how plant production in eight different grassland and desert ecosystems will be affected by these increases in ambient CO2 concentration. Results are described from mostly field experiments conducted in four humid temperate grasslands (Swiss calcareous grasslands; New Zealand pasture; Kansas tallgrass prairie; Texas C3/C4 grassland), a Mediterranean grassland (California annual grassland), two semi-arid system (Colorado shortgrass steppe; Mojave Desert), and one controlled environment experiment of semi-arid seasonal grassland assemblages from the northern Negev Desert. The results suggest that elevated CO2 can have a significant beneficial affect on plant water status by reducing transpiration water loss, thereby increasing the efficiency with which plants consume water during growth. In both dry and semi-natural grasslands, enhanced growth of plants under elevated CO2 appears driven primarily by this water relations benefit. However, this response is not universal to all grasslands, and responses of individual plant species to elevated CO2 is extremely variable. The results indicate that rising atmospheric concentrations of CO2 will lead to extensive changes in many world grasslands, with important consequences for their future management.

Technical Abstract: With atmospheric CO2 concentrations rising and expected to exceed 600 uL L-1 by the end of the 21st century, there is a critical need to understand implications for the world's ecosystems. Herein we describe the plant biomass and species responses of native or semi-natural grasslands and semi-arid systems to CO2, with an emphasis on water relations. Results are compiled mostly from seven field CO2 enrichment experiments conducted in four humid temperate grasslands (Swiss calcareous grasslands; New Zealand pasture; Kansas tallgrass prairie; Texas C3/C4 grassland), a Mediterranean grassland, (California annual grassland), two semi-arid systems (Colorado shortgrass steppe; Mojave Desert), and one controlled environment experiment of semi-arid seasonal grassland assemblages from the Northern Negev Desert. Increasing CO2 led to decreased leaf conductance, improved plant water status, reduced stand evapotranspiration, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. The interactive responses of aboveground biomass to CO2 and water varied greatly among sites. In analyses conducted both within and across grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were greater in dry years (negative correlation with precipitation; r2=0.47 across all three sites). In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 only in a relatively wet year. Aboveground biomass of Mediterranean grasslands was inconsistently stimulated by increased ambient CO2, and sometimes failed to respond to CO2-related increased late-season soil water, whereas enhanced late-season water in elevated CO2 treatments led to a 15% production in community biomass in grassland assemblages of the Negev. Biomass and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant water relations are an important aspect of aboveground biomass responses for most ecosystems evaluated. Since such moisture effects contribute strongly to experimentally induced CO2-effects, and they are tightly coupled to ambient temperature and humidity, any climatic feedback and climate change influences on the more direct, photosynthetic CO2 responses that involve moisture will need to be accounted for in landscape-scale predictions.