Stem Response of Eit1JH ~ to Precipitation and Soil Moisture Dynamics: A Multi-Temporal Approach

Tarbox, Sharon

Plants in semi-arid regions utilize soil moisture resulting from both periodic precipitation events and seasonal recharge. How plants respond to multi-scale fluctuations in moisture availability is unknown. Stem growth is a temporally integrated response to intra-annual moisture variability, as are stem circumference changes- swelling during wet periods and shrinking during droughts. I tested: (a) whether total precipitation or soil moisture changes can be used to predict stem circumference changes of pinions (Pinus edulis, Engelm.), (b) if the relations between precipitation and soil moisture with change in stem circumference were significantly modified by the measurement time scale, and ( c) if stem water storage provided enough water to buffer pinions from reductions in soil moisture. I measured precipitation, soil moisture, and stem circumference over varying observation intervals ( one, ten, or sixty days) during the 1993 growing season drought to evaluate the relative roles of small precipitation events and prolonged seasonal drought. Total precipitation explained little of the variability in stem circumference change, while daily soil moisture explained most. Observation interval and soil moisture changes were highly correlated. Consequently, little variance in stem circumference change was explained by observation interval after removing the effect of soil moisture changes. Estimates of water use suggest that pinions do not store enough stem water to supply daily transpiration demands during seasonal droughts. The empirical relation between soil moisture and stem circumference change provides a basis for the prediction of growth of long-lived individuals within semi-arid ecosystems.