COS 118-5 - Evapotranspiration and regulatory mechanisms in two semiarid Artemisia-dominated shrub steppes at opposite sides of the globe

Friday, August 7, 2009: 9:20 AM
Dona Ana, Albuquerque Convention Center
Burkhard Wilske1, Hyojung Kwon2, Long Wei3, Jing Xie4, Nan Lu5, Shiping Chen6, Guanghui Lin7, Wenbing Guan4, Elise Pendall8, Brent E. Ewers9 and Jiquan Chen10, (1)The University of Toledo, Toledo, OH, (2)Forest Ecosystems and Society, Oregon State University, Corvallis, OR, (3)Key State Laboratory of Vegetation and Environmental Change, The Chinese Academy of Sciences - Institute of Botany, Beijing, China, (4)College of Nature Conservation, Beijing Forestry University, Beijing, China, (5)Research Center for Eco-Enviromental Sciences, Chinese Academy of Science, Beijing, China, (6)Institute of Botany, the Chinese Academy of Sciences, Beijing, China, (7)Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, School of Life Sciences, Xiamen University, Xiamen, China, (8)Botany, University of Wyoming, Laramie, WY, (9)Botany, Program in Ecology, University of Wyoming, Laramie, WY, (10)Department of Environmental Sciences, University of Toledo, Toledo, OH 43606, Toledo, OH

Ecosystem Evapotranspiration (ET) is one of the main components in the water cycle affecting the local-to-regional water balance. Furthermore, ET links the long-term feedbacks between vegetation and climate including precipitation. Understanding the controls on ecosystem ET is crucial to comprehend present and potential future vegetation and water availability. This study investigated ET and the regulatory mechanisms in two semiarid shrub-dominated ecosystems in the Sierra Madre (SM, Wyoming, USA) and the Kubuqi Desert (KD, Inner Mongolia, China). The controlling mechanisms were analyzed based on ET and climate data from two eddy-covariance towers at SM and KD for the years 2004-2005 and 2006-2007, respectively. Climate variables were examined for their individual correlation with daily ET. Subsequently, the FAO Penman-Monteith model (FAO-PM syn. FAO-56) was stepwise modified to test the biophysical control of individual parameters in concert with other climate variables.

Located at opposite sites of the globe, the two ecosystems shared various similarities in vegetation and climate (e.g., mean temperature and amount of precipitation). At both sites, ET showed positive correlations with available energy (Rn - G), soil volumetric water content (VWC), and soil water potential (Ψs), but lacked positive correlation with vapor pressure deficit (VPD). Daily ET during growing seasons also correlated with air temperature (Ta) at KD, but not at SM.  The important modifications to the FAO-PM included the insertion of Ψs and the removal of VPD and wind speed (u). These modifications improved daily estimates of growing season ET as they reduced deviations between modeled and measured ET to 1-33%. Model tests corroborated that available energy and soil water potential were the main drivers of ET during growing seasons. VPD and u influenced ET mainly after precipitation events and at low Ψs (i.e., Ψs close to zero). The conditional sensitivity of ET to VPD during growing seasons may be regarded as an important difference between the control of ET in shrub-dominated and other ecosystems such as forests. This characteristic may be also considered in land management and conservation with respect to interferences that can potentially affect the water cycle and the water availability in areas that are naturally covered by shrub-dominated ecosystems.

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