Estimates of Water Loss and Its Relation to Environmental Parameters in Douglas‐Fir Saplings

Water use by plantation Douglas—fir (Pseudotsuga menziesii [Mirb.] Franco) cannot always be predicted from environmental parameters. Under conditions of low soil—water content, no significant correlations were found between solar radiation or air temperature, or both, and sap flow. However, highly significant correlations were observed after soil—moisture recharge had occurred. Zones of xylem sap pressure and relative stomatal apertures were found within the crowns of 6—m high trees. Vertical zones of xylem sap pressure seemed to be controlled by differential stomatal closure which, in some cases, acted independently of crown microenvironment. However, differences in xylem sap pressures between the north and south sides of the trees could readily be explained by differences in microenvironment.