A Theoretical Model for Calculation of Xylem Sap Pressure from Climatological Data

A theoretical model is presented for the calculation of stem-xylem sap pressure from climatological data in an open-grown stand of Abies procera and A. amabilis located at 1206 m elevation in the Washington Cascades. Stem-xylem sap pressure is divided into two components: (1) a base level which is a function of soil moisture, and (2) a diurnal depression in sap pressure from this base level which is a function of current atmospheric evaporative demand. Predicted values of stem-xylem sap pressure were compared to observed values and no sig- nificant difference was observed. Other plant processes such as stomatal aperture were predicted from the model. Specific deficiencies in the model were observed and are discussed. Substantial apparent nighttime transpiration occurred within these two species under laboratory and field conditions, leading to a discrepancy between base level sap pressure and soil moisture. I NTRODUCTION The integrated abiotic and biotic factors affecting plants contribute to their water status (Fritts, 1966). Since growth frequently is limited by even mild foliar desiccation (Slatyer, 1967), the understanding of plant water status is essential to evaluating plant-environment inter- actions and the resulting ecological distribution of plants. Our effort to understand plant water status has led to the formulation of a model which predicts water status of forest trees from climatological data.