Seasonal and Diurnal Patterns of Whole-Tree Plant Water Relations in Three Pacific Northwest Conifer Species: Thuja plicata, Pseudotsuga menziesii, and Tsuga heterophylla

Abstract

I examined the diurnal and seasonal patterns of sap flow and stem increment in a conifer stand in western Washington. The study site, at the Charles L. Pack Experimental Forest, is on a south-facing slope and at approximately 460-m elevation. Two <italic>Pseudotsgua menziesii</italic>, two <italic>Tsuga heterophylla</italic>, and four <italic>Thuja plicata</italic>, of various diameters and heights, were outfitted with sap flow modules and dendrometer bands. I also estimated water use efficiency for the study trees and approximated total stand transpiration by scaling up normalized daily sap flow of the study trees. <italic>T. plicata</italic> demonstrated a large lag in peak daily sap flow in the summer compared with the other two species, while <italic>P. menziesii</italic> and <italic>T. heterophylla</italic> appeared to down-regulate their transpiration via stomatal closure during the hottest, driest parts of the day. Normalized sap flow rates were highest in <italic>T. heterophylla</italic> in spring (0.24 kg per cm DBH), early summer (~0.5 kg per cm DBH), late summer (0.71 kg per cm DBH), and in the fall (0.44 kg per cm DBH). Flow rates were lowest in <italic>P. menziesii</italic> in spring (.02 - .04 kg per cm DBH) and early summer (0.15 kg per cm DBH in the suppressed tree), in the shortest <italic>T. plicata</italic> in late summer (0.13 kg per cm DBH), in the shortest and the codominant <italic>T. plicata</italic> in fall (0.04 and 0.03 kg per cm DBH, respectively), and in the codominant <italic>T. plicata</italic> and smaller <italic>T. heterophylla</italic> in winter. In contrast to the greater average sap flow per unit circumference noted in <italic>T. heterophylla</italic>, the maximum daily value noted during my study was observed in the largest <italic>T. plicata</italic> and was 176.32 kg (the value for the largest <italic>T. heterophylla</italic> on the same day was 50.72 kg). A strong drought response was evident in <italic>T. plicata</italic> in late summer and early fall as stomatal conductance and sap flow fell, while <italic>T. heterophylla</italic> and <italic>P. menziesii</italic> did not show such a response. Overall stem growth during the study period was lowest in P. menziesii</italic>. I used mean daily sap flow rates of each tree to estimate total stand transpiration, which reached 1.23 mm in late summer and fell to 0.18 mm in winter. Overall, the study provided insight into ecological and physiological differences and similarities among these three species.