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Williams, David G

Physiological activity and structural dynamics in arid and semi-arid ecosystems are driven by discrete inputs or "pulses" of growing season precipitation. Here we describe the short-term dynamics of ecosystem physiology in experimental stands of native (Heteropogon contortus) and invasive (Eragrostis lehmanniana) grasses to an irrigation pulse across two geomorphic surfaces with distinctly different soils: a Pleistocene-aged surface with high clay content in a strongly horizonated soil, and a Holocene-aged surface with low clay content in homogenously structured soils. We evaluated whole-ecosystem and leaf-level CO2 and H2O exchange, soil CO2 efflux, along with plant and soil water status to understand potential...
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Leaf carbonisotope discrimination (?) was measured for three dominant, semi-arid woodland species along a summer monsoon gradient inthe southwestern United States over a 2-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf ? values. Sites of similar elevation along the transect were selected and the range in monsoon contribution to overall annual precipitation varied from 18 to 58%, while total annual precipitation differed by a maximum of only 25% across this gradient. Leaf ? values in Quercus gambelii were negatively correlated with ?, a seasonally-weighted estimate of the evaporative humidity gradient, suggesting that stomatal...
1. Hydraulic redistribution may have important consequences for ecosystem water balance where plant root systems span large gradients in soil water potential. To assess seasonal patterns of hydraulic redistribution, we measured the direction and rate of sap flow in tap-roots, lateral roots and main stems of three mature Prosopis velutina Woot. trees occurring on a floodplain terrace in semiarid south-eastern Arizona, USA. Sap-flow measurements on two of the trees were initiated before the end of the winter dormancy period, prior to leaf flush. 2 Despite the absence of crown transpiration during the dormant season, sap flow was detected in lateral roots and tap-roots of P. velutina. Reverse flow (away from the stem)...
In semiarid ecosystems, physiography (landscape setting) may interact with woody-plant and soil microbe communities to constrain seasonal exchanges of material and energy at the ecosystem scale. In an upland and riparian shrubland, we examined the seasonally dynamic linkage between ecosystem CO2 exchange, woody-plant water status and photosynthesis, and soil respiration responses to summer rainfall. At each site, we compared tower-based measurements of net ecosystem CO2 exchange (NEE) with ecophysiological measurements among velvet mesquite (Prosopis velutina Woot.) in three size classes and soil respiration in sub-canopy and inter-canopy micro-sites. Monsoonal rainfall influenced a greater shift in the magnitude...
? The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall (?pulses?) on carbon and water exchange in water-limited ecosystems are poorly quantified. ? In the present study, we estimated net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased (?wet?) or decreased (?dry?) monsoon-season (July?September) rainfall during 2002 and...
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