Spatial and temporal variation of water use efficiency in a blue oak (Quercus douglasii) savanna: A combined analysis of stable isotopes and eddy covariance measurements

Blue oak savannas are exposed to broad variation in water availability and to severe droughts during the summer months. Previous findings from instantaneous gas exchange measurements showed that very large oaks maintain constant water-use efficiency (WUE; carbon fixed per water lost) even in the face of very dry summers. Here, we expanded upon these results and investigated the influence of tree size using a more integrated index of WUE, the stable carbon isotope composition (δ¹³C) of leaf soluble carbohydrates. Trees that varied in diameter (at breast height, ranging from 0.2 to 0.7m) were sampled during the summer drought (June-September) in two subsequent years. Preliminary results showed large differences among trees in δ¹³C of leaf soluble carbohydrates (from -24‰ to -28‰), but each tree's δ¹³C remained relatively constant over time. This indicated that intrinsic WUE (ratio of carbon assimilation to stomatal conductance to water loss) was also constant, despite decreasing soil water content and leaf water potentials. Differences among trees could not be explained by either tree size or differential use of soil water sources (estimated by δ¹⁸O of soil and xylem water). Leaf-level, isotope based WUE estimates on individual trees were compared with ecosystem-level WUE estimates derived from continuous eddy covariance measurements of water and carbon exchange. These comparisons indicate that these savanna trees cope with severe drought conditions through coordinated down-regulation of carbon and water fluxes. Our findings improve our understanding of the manner by which trees in extremely dry environments can respond to drought.