²ÊÃñ±¦µä

Szutu, Daphne J ¹ ; Papuga, Shirley A ²

¹ ²ÊÃñ±¦µä, INSTAAR
² University of Arizona

Dryland ecosystems account for nearly 40% of terrestrial biomes, and understanding transpiration dynamics in these environments is critical to understanding how climate change will impact global water and carbon budgets. Semiarid shrublands and other dryland ecosystems are highly responsive to precipitation pulses that, depending on their size, differentially influence the distribution of moisture in the soil profile. Recent field studies have shown that transpiration dynamics and plant productivity are largely a function of deep soil moisture available after large precipitation events, regardless of where the majority of plant roots occur. We suggest that adopting a hydrologically defined two-layer conceptual framework of the soil profile is more appropriate for understanding plant water use in dryland ecosystems than a framework that is based on rooting depth. We make the assumption that shallow and deep soil layers have different isotopic signatures and use this framework to show how transpiration dynamics vary with the availability of deep soil moisture in dryland ecosystems. We present continuous eddy covariance, sap flow transpiration and soil moisture data with discrete isotopic samples of precipitation, soil, and stems taken over 18 months at a creosotebush-dominated shrubland ecosystem in southern Arizona. We found that transpiration is associated with the availability of deep soil moisture, and when transpiration rates were highest, both deep moisture and stem water were more isotopically similar to winter precipitation than summer precipitation, suggesting that winter precipitation can play an important role in supporting these ecosystems. Our study suggests that integrating sap flow and stable isotope techniques with soil moisture measurements offers a better understanding of how plant water use strategies shift with changes in source water and its availability than either technique could offer on its own. We contend that semiarid shrubs depend on deep moisture for growth and functioning and are therefore vulnerable to shifts in precipitation. Ultimately these findings should help to improve the representation of drylands within regional and global models of land surface atmosphere exchange and their linkages to the hydrologic cycle.