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To accurately predict ecosystem responses induced by climate warming at local-to-global scales, models are in need of more precise knowledge of response during periods of environmental stress such as drought. In this paper, we studied environmental control of canopy-level water use efficiency (WUE) during drought at an eddy flux site in an oak-hickory forest in central Missouri, USA. Two consecutive severe droughts in the summers of 2006 and 2007 afforded coverage of a broad range of environmental conditions. We stratified data to obtain subranges that minimized cross-correlations among putative WUE-controlling factors. Our results showed that WUE was subject to control by atmospheric saturation deficit (ASD), soil water potential (SWP) and the ratio of diffuse to total photosynthetically active radiation ( If/ It). Generally, WUE was found to scale with 1/(ASD)0.5, consistent with predictions from stomatal optimization theory. In contrast, SWP and If/ It were related to WUE in a linear fashion. ASD was better correlated with WUE than either of the other two factors. It was also observed that the relationship between WUE and any single controlling factor was subject to influence of the other two. One such example was an opposite response of WUE to SWP between low and high ASD values, suggesting a breakdown of stomatal optimality under severe environmental stresses and a shift from optimal stomatal regulation to nonstomatal regulation at leaf scale. We have demonstrated that different data handling (stratified vs. nonstratified) or selection (hourly vs. daily) could lead to different conclusions on the relationship between WUE and its controls. For this reason, we recommend modelers to be cautious when applying WUE-response formulas at environmental conditions or at time scales different from those at which they are derived. [ABSTRACT FROM AUTHOR]

Abstract: The seasonal and interannual variability in surface energy exchange and evapotranspiration (E) of two temperate semi-arid grasslands in southeastern Arizona, USA, were investigated using multi-year (2004–2007) eddy covariance measurements. The study sites, a post-fire site (AG) and an unburned site (KG), received 43–87% of the annual precipitation (P) during the North American Monsoon season (July–September), with the lowest values in the drought years of 2004 and 2005. Irrespective of the differences in temperature, surface albedo, vegetation cover and soil characteristics both sites responded similarly to changes in environmental conditions. The seasonal and interannual variations in the partitioning of net radiation to turbulent fluxes were mainly controlled by P through the changes in soil water content (θ) and vegetation growth. Drastic changes in albedo, vegetation growth and energy fluxes occurred following the onset of the monsoon season in July. During the dry or cool periods of autumn, winter and spring, the sensible heat flux was the largest component of the energy balance, whereas latent heat flux dominated during the warm and wet periods of summer. The dry-foliage Priestley–Taylor coefficient (α) declined when θ in the 0–15cm soil layer dropped below 0.08m3 m−3 at AG, and 0.09m3 m−3 at KG, respectively. The July–September average of dry-foliage surface conductance, α and E, reached their lowest values in 2004 at AG and in 2005 at KG. During July–September, monthly E was linearly correlated to the monthly mean θ and the broadband normalized difference vegetation index (NDVI), whereas during May–June the relationship between NDVI and E was not significant. Annual E varied from 264 to 322mm at AG and from 196 to 284mm at KG with the lowest value during the severe drought year at the site. July–September E had positive correlation with total P, the mean NDVI and the number of growing season days during that period. Annual P explained more than 80% of the variance in annual E. [Copyright &y& Elsevier]