Variation in Water Stress of Ponderosa Pine

Seasonal and diurnal variations in water stress of the one year old needles in ponderosa pine were followed using the Schardakow method of estimating diffusion pressure deficit and the relative turgidity method of estimating moisture deficit. Soil moisture, vapor pressure deficit of the atmosphere and changes in trunk radius were measured to ascertain their relationships to needle water stress. Diffusion pressure deficit increased with decreasing solid moisture until growth was initiated. At that time diffusion pressure deficit values were markedly reduced and this is attributed to changes in solute content resulting from high carbohydrate metabolism and translocation. Diurnal variations in diffusion pressure deficit followed closely the diurnal variations in vapor pressure deficit, but the measured changes in relative turgidity did not follow closely the changes in diffusion pressure deficit. Measurements of changes in truck radius appear useable for estimating needle tissue water status except during periods of terminal and cambial activity.