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Title: SENSITIVITY OF RIPARIAN ECOSYSTEMS TO MOISTURE PULSES IN SEMIARID ENVIRONMENTS 1754

Author
item WILLIAMS, D. - UNIVERSITY OF WYOMING
item Scott, Russell - Russ
item HUXMAN, T. - UNIVERSITY OF ARIZONA
item Goodrich, David - Dave
item LIN, G. - CHINESE ACAD OF SCIENCES

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2006
Publication Date: 9/25/2006
Citation: Williams, D.G., Scott, R.L., Huxman, T.E., Goodrich, D.C., Lin, G. 2006. Sensitivity of riparian ecosystems to moisture pulses in semiarid environments. Hydrological Processes. Special Issue on Emerging Issues of Ecohydrology in Semiarid Areas, eds. Wilcox , B. and Scanlon, B. 20:3191-3205.

Interpretive Summary: How the structure and functioning of riparian vegetation in arid and semiarid basins depends on different hydrological processes is not well understood. In order to begin to better understand these dependencies, this paper brings together the findings of various studies conducted along the San Pedro River in southeastern Arizona to show that rooting depth and access to groundwater are key factors that control the vegetations water use and carbon dioxide exchange. Depth to groundwater, which varies substantially across the riparian landscape, is a key factor controlling the sensitivity of cottonwood (Populus fremontii) transpiration, leaf photosynthetic metabolism, and plant water sources to pulsed inputs of growing season moisture. Because mesquite (Prosopis velutina) accesses groundwater in these habitats, ET and gross ecosystem production are decoupled from precipitation. But ecosystem respiration remains highly coupled to rainfall due to the dominant contribution of litter and bulk soil organic matter decomposition. Responses of net ecosystem exchange of carbon dioxide to rainfall variability in riparian floodplain is therefore not simple, but depends on vegetation structure and the connection of dominant plants to the water table. The complex vegetation patterns, hydrologic setting and disturbance dynamics in the riparian landscape offer unparalleled opportunities to investigate fundamental processes linking water and carbon exchange.

Technical Abstract: Structural and functional dynamics of riparian vegetation in arid and semiarid basins are controlled by hydrological processes operating at local, landscape and catchment scales. However, the importance of climate variation as a control on evapotranspiration (ET) and carbon exchange in these ecosystems varies considerably across the riparian landscape depending largely on interactions between vegetation rooting depth and access to the near-surface water table. Here we highlight ongoing eco-hydrological studies being conducted along the Upper San Pedro River in semiarid, southeastern Arizona, USA. Depth to groundwater, which varies substantially across the riparian landscape, is a key factor controlling the sensitivity of cottonwood (Populus fremontii) transpiration, leaf photosynthetic metabolism, and plant water sources to pulsed inputs of growing season moisture. Landscape-scale functional heterogeneity in cottonwood forest has been incorporated into spatially explicit ET models for estimating water balance components of the whole riparian corridor. However, of greater importance for understanding controls on water and carbon exchange processes of the riparian corridor is the widespread conversion of floodplain grassland to communities dominated by the deep-rooted woody plant mesquite (Prosopis velutina). Mesquite is now the dominant cover in the riparian corridor and has increased by more than 300% across the whole watershed. The physiognomic shift from grassland to mesquite woodland on upper floodplain terraces alters the sensitivity of ET and CO2 exchange fluxes to inputs of growing season precipitation. Because mesquite accesses groundwater in these habitats, ET and gross ecosystem production are decoupled from precipitation. But ecosystem respiration remains highly coupled to rainfall due to the dominant contribution of litter and bulk soil organic matter decomposition. Responses of net ecosystem exchange of CO2 (NEE) to rainfall variability in riparian floodplain is therefore not simple, but depends on vegetation structure and the connection of dominant plants to the water table. The complex vegetation patterns, hydrologic setting and disturbance dynamics in the riparian landscape offer unparalleled opportunities to investigate fundamental ecohydrological processes linking water and carbon biogeochemistry and flux.