Controls on transpiration in a semiarid riparian cottonwood forest

Cottonwood(Populus spp.)forestsareconspicuousandfunctionallyimportantelementsofriparianvegetationthroughoutmuchof the western U.S. Understanding how transpiration by this vegetation type responds to environmental forcing is important for determining the water balance dynamics of riparian ecosystems threatened by groundwater depletion. Transpiration was measured in semiarid riparian cottonwood (Populus fremontii) stands along a perennial and an intermittent reach of the San Pedro River in southeastern Arizona. Sap flow was measured using thermal dissipation probesandscaled to the standlevel to investigate standwater useinrelationtocanopystructure,depthtogroundwaterandclimateforcing.Thecottonwoodstandlocatedattheperennialstreamsite hadhigherleaf-to-sapwoodarearatio(0.31 � 0.04 m 2 cm � 2 ),leafareaindex(2.75)andshallowergroundwaterdepth(1.1‐1.8m)than the standattheintermittentstreamsite (0.21 � 0.04 m 2 cm � 2 , 1.75and3.1‐3.9 m,respectively).Moreover,totalannualtranspiration was higher at the perennial stream site (966 mm) than at the intermittent stream site (484 mm). The significant positive and linear correlation between transpiration and vapor pressure deficit indicated high hydraulic conductance along the root‐shoot pathway of cottonwoodtreesatthe perennialstreamsite.Duringthepeakdryperiodpriortothe summerrainyseason,the treesattheintermittent streamsiteexhibitedgreaterwaterstressastranspirationdidnotincreasebeyonditsmid-morningpeakwithincreasingvaporpressure deficit, which was likely due to leaf stomatal closure. However, this stress was alleviated after significant monsoonal rains and runoff events had recharged soil moisture and raised groundwater levels. Riparian cottonwood forests are exposed to extremefluctuations in water availability and transpiration demand throughout the growing season, and their access to shallow groundwater sources determines their structural and physiological responses to drought. Spatial and temporal variation in depth to groundwater induces drought stress in cottonwood threatening their productivity and existence along the river systems throughout much of western U.S. # 2006 Elsevier B.V. All rights reserved.

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