Sensitivity of riparian ecosystems in arid and semiarid environments to moisture pulses.
Authors:
Williams, D.G. Scott, R.L. Huxman, T.E. Goodrich, D.C. Lin, G. USDA, ARS
Source:
Hydrological processes. 2006 Oct. 15, v. 20, issue 15, p. 3191-3205.
NALT Subjects:
riparian areas riparian forests arid zones semiarid zones ecosystems watershed hydrology rivers water table drainage basins carbon ecological invasion Populus fremontii transpiration photosynthesis monsoon season growing season hydrologic models spatial distribution estimation water balance biogeochemical cycles Sporobolus grasslands floodplains Prosopis velutina vegetation structure woodlands carbon dioxide precipitation shrubs soil organic matter plant litter Arizona
Other Subjects:
Salix gooddingii
Issue Date:
15-Oct-2006
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 growing-season precipitation as a control on evapotranspiration (ET) and carbon cycling in these ecosystems varies considerably across the riparian landscape, depending largely on access to the near-surface water table. Here we describe key findings from ongoing ecohydrological studies along the Upper San Pedro River (USPR) in semiarid, south-eastern Arizona, USA. Depth to the water table varies substantially across the riparian landscape along the USPR drainage, and is a key factor controlling the sensitivity of cottonwood (Populus fremontii) water-source use, transpiration rate and photosynthetic metabolism to inputs of monsoonal moisture during the growing season. Landscape-scale functional variation in cottonwood forests has been incorporated into spatially explicit ET models for estimating water balance components along the USPR. However, of greater importance for understanding controls on water and carbon exchange processes in the riparian corridor is the conversion of sacaton (Sporobolus spp.) grasslands on pre-entrenchment floodplain terraces to communities dominated by the deep-rooted woody legume, mesquite (Prosopis velutina). Mesquite is now the dominant cover in the riparian corridor and has increased by more than 300% in the USPR basin since 1972. The physiognomic shift from grassland to mesquite woodland on pre-entrenchment floodplain terraces alters the sensitivity of ET and CO2 exchange to inputs of growing-season precipitation. Because mature mesquite shrubs and trees have greater access to groundwater in these habitats than sacaton, ET and gross ecosystem production (GEP) are relatively decoupled from variation in monsoonal precipitation. However, decomposition of litter and soil organic matter in floodplain ecosystems remains highly coupled to monsoonal moisture inputs after mesquite encroachment. Responses of net ecosystem exchange of CO2 (NEE) to inputs of monsoonal rainfall are therefore not simple, but depend on vegetation composition and the connection of dominant plants to the water table. The heterogeneous vegetation patterns and groundwater depths in the riparian landscape offer unique opportunities for understanding fundamental ecohydrological processes linking carbon and water cycles in both riparian and upland ecosystems.
