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Title: VIEWPOINT: ATMOSPHERIC CO2, SOIL WATER, AND SHRUB/GRASS RATIOS ON RANGELANDS

Author
item Polley, Wayne
item Mayeux Jr, Herman
item Johnson, Hyrum
item Tischler, Charles

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Grasses primarily use water from upper soil layers, while many trees and shrubs depend on rainfall that drains into deeper soil layers, beyond the reach of the shallowly-rooting grasses. On many rangelands, therefore, abundance of trees and shrubs increases and grasses decrease when rainfall increases or more of rainfall drains deeply into soil where it can be used by woody plants alone. Currently rising concentrations of carbon dioxide gas (CO2) in the atmosphere will reduce the rate at which all plants use soil water, leading to wetter soil and a greater chance that rainfall will drain deeply into soil. On rangelands, this increase in the amount of water at depth in soil should favor an increase in the number of trees and shrubs at the expense of grasses and thereby reduce the economic value of rangelands for livestock production.

Technical Abstract: In many ecosystems where trees or other deeply-rooting species coexist with grasses, the productivity and tree/grass ratio of vegetation depend strongly on soil water availability and its vertical distribution. Studies support a "two-layer" model of competition for soil moisture in these systems wherein the abundance of deeply-rooting species depends on the amount of water that reaches soil below most grass roots. We utilize this two-layer model of competition to explore implications of changes in transpiration caused by rising atmospheric CO2 concentration for soil water availability and the balance between grasses and trees. Rising CO2 modifies the coupling between vegetation and water availability by increasing plant water use efficiency. In many grasslands, this may derive largely from a decrease in transpiration per unit leaf area. Where feedbacks that raise leaf temperature and increase leaf area do not completely offset lower transpiration/leaf area, rising CO2 should increase soil water availability, increase percolation to depth, and favor taller and more-deeply rooting species. Predicted effects of CO2 are supported by results from CO2-enrichment studies in the field and are compatible with recent models of interactions between soil resources and vegetation.