Transpiration in Pacific madrone along a microclimatic gradient shows variable relationship with above ground climate

Abstract:

Plant transpiration is a major conduit for the transfer of water from the land to the atmosphere, and is determined by the interaction between plant physiology and local microclimate. Over a period of five months, we made continuous high frequency observations of microclimate and sap velocity in two populations of a single tree species, Pacific madrone (Arbutus menziesii), across a hillslope divide in Northern California containing a strong microclimatic gradient. The differences in sunlight between the two slopes lead not only to different temperatures and humidities, but also to differences in energy available for photosynthesis, and hence transpiration. As a result, the trees on the south-facing slope transpire 17% more water integrated over the dry Mediterranean summer, despite having lower near-surface soil moisture. Furthermore, we found that the relationship between water use and local environmental conditions differs between the two slopes, meaning that observed differences in water use are not due entirely to observed microclimate differences, but also due to population-level physiological differences, suggesting that the trees acclimate to their inhabited microclimate. While our present dataset and analytical tools do not positively identify any mechanism of possible acclimation, we speculate that differing proportions of sun-adapted and shade-adapted leaves, differences in stomatal regulation, and uncaptured cross-slope root zone moisture differences could explain some of the observed and modeled differences.