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United States Department of Agriculture

Agricultural Research Service

Title: Initial response of evapotranspiration from tallgrass prairie vegetation to CO2 at subambient to elevated concentrations

Authors
item Polley, Wayne
item Johnson, Hyrum
item Fay, Philip
item Sanabria, Joaquin - TEXAS AG EXPT STN

Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 20, 2007
Publication Date: December 10, 2007
Citation: Polley, H.W., Johnson, H.B., Fay, P.A., Sanabria, J. 2008. Initial response of evapotranspiration from tallgrass prairie vegetation to CO2 at subambient to elevated concentrations. Functional Ecology. 22:163-171.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in the atmosphere has increased by about 37% since the beginning of the Industrial Revolution and is expected to reach twice the pre-Industrial level within the century. Studies on single leaves have shown that CO2 enrichment usually reduces the rate of water loss per unit of leaf area, but our understanding of how plant and environmental variables, like total leaf area and air temperature, interact with CO2 to regulate water loss from plant stands is more limited. We installed scales or balances beneath a series of intact monoliths of soil in a field chamber used to regulate CO2 along a pre-Industrial to elevated gradient. Monoliths of three soil types were planted to species characteristic of tallgrass prairie. Weights of monoliths were recorded daily to determine how CO2 effects on water loss per unit of leaf area (ETla) varied with total leaf area (LAI), soil type, and environmental variables, including soil water deficit, light, and air temperature and humidity. Increasing CO2 reduced ETla, but the CO2 effect was greatest at relatively-low temperatures and low LAI for all soils combined. Dependence of the CO2 effect on LAI and environmental factors differed among soils, possibly because plant growth patterns and physiology differed among soils. Our results indicate that rising CO2 generally will slow water loss from grasslands, but water savings will be greatest early in the growing season when temperatures are mild and the plant canopy is re-establishing.

Technical Abstract: Effects of CO2 enrichment on leaf transpiration are well-documented, but our understanding of how CO2 interacts with other variables to regulate evapotranspiration is more limited. We installed weighing lysimeters planted to species characteristic of tallgrass prairie in a field chamber used to regulate CO2 along a subambient to elevated gradient. Lysimeters with three soils types were used to study how CO2 effects on evapotranspiration per unit of leaf area (ETla) varied with leaf area index (LAI), soil type, and environmental variables, including soil water deficit, photosynthetically-active radiation, air temperature, and air vapor pressure deficit. CO2 enrichment reduced ETla. The CO2–caused decrease in ETla was greatest at relatively-low temperatures and low LAI for all soils combined. Higher temperatures countered the CO2 effect by increasing ETla more at elevated than subambient CO2. Higher LAI also countered the CO2 effect by decreasing ETla more at subambient than elevated concentrations. Plant (LAI) and environmental effects on ETla differed among soils, possibly because plant growth patterns and physiology differed among soils. Our results imply that the CO2 effect on ETla will vary with seasonal change in temperature and LAI, independent of seasonal shifts in leaf age and physiological activity. The CO2–caused decrease in ETla should be greatest early in the growing season when temperatures are mild and the plant canopy is re-establishing.

Last Modified: 12/20/2014
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