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ARS Home » Pacific West Area » Tucson, Arizona » SWRC » Research » Publications at this Location » Publication #204428
Title: Sensitivity of mesquite shrubland CO2 exchange to precipitation in contrasting physiographic settings 1871

Author
item POTTS, D. - UC IRVINE
item Scott, Russell - Russ
item CABLE, J. - UNIVERSITY OF WYOMING
item HUXMAN, T. - UNIVERSITY OF ARIZONA
item WILLIAMS, D. - UNIVERSITY OF WYOMING

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2008
Publication Date: 10/20/2008
Citation: Potts, D.L., Scott, R.L., Cable, J.M., Huxman, T.E., Williams, D.G. 2008. Sensitivity of mesquite shrubland CO2 exchange to precipitation in contrasting physiographic settings. Ecology. 89(10):2900–2910.

Interpretive Summary: The ranges of woody plants have expanded in many semiarid areas around the globe. With this change in ecosystem structure, associated changes in how ecosystems cycle carbon and water are expected but largely unknown. This study examined the sensitivity of the carbon cycling in ecosystems invaded by the woody shrub, mesquite, to precipitation in southern Arizona. The sensitivity was examined for a range of shrub sizes and in a riparian and upland setting. For mesquite plants in the upland setting, water status and photosynthetic gas exchange were closely linked to the onset of the North American monsoon. In contrast, the smallest size classes of riparian mesquite were physiologically responsive to monsoonal rainfall while the larger size classes of riparian mesquite were physiologically insensitive to monsoonal rains suggesting they were accessing alluvial groundwater. Soil respiration was greatest beneath mesquite canopies and was coupled to shallow soil moisture associated with the monsoon. These results show that woody plant encroachment in riparian areas will have a bigger impact on ecosystem carbon cycling due to the deeper roots of shrubs which access the groundwater more readily.

Technical Abstract: Life-history characteristics and functional traits of dominant organisms may interact with abiotic factors to shape ecosystem processes such as net primary productivity. For example, in terrestrial environments, physiographic position may interact with plant ontogeny to constrain ecosystem exchanges of material and energy. To examine the linkage between ecosystem carbon exchange sensitivity, plant physiological sensitivity and soil respiration response to summer rainfall in contrasting physiographic settings, we compared 2005 growing season net ecosystem exchange of CO2 (NEE) with ecophysiological measurements of plant performance among Velvet mesquite (Prosopis velutina Woot.) in three size classes and soil respiration in an upland and a riparian mesquite shrubland. Flux-duration analysis of NEE revealed patterns of ecosystem sensitivity to seasonal precipitation consistent with plant- and soil-level observations at the two sites. For mesquite plants in the upland setting, water status and photosynthetic gas exchange were closely linked to the onset of the North American monsoon. In contrast, the smallest size classes of riparian mesquite were physiologically responsive to monsoonal rainfall while the larger size classes of riparian mesquite were physiologically insensitive to monsoonal rains suggesting they were accessing alluvial groundwater. Soil respiration was greatest beneath mesquite canopies and was coupled to shallow soil moisture associated with the monsoon. Physiographic setting, through its influence on the physiological sensitivity of plants to rainfall may interact with plant-mediated rates of soil respiration to affect the sensitivity of carbon exchange in semi-arid ecosystems to precipitation.