COS 201-6 - Sapwood water storage and transpiration in two tree species with contrasting water management strategies

Abstract: Understanding hydraulic properties of trees is vital to knowing how they function in relation to environmental stress and climate change. Wood water storage and release, also known as capacitance, is an often-neglected functional trait of trees, which varies among species and within individual trees, such as with sapwood depth. The objective of this study was to compare wood water storage in deep (12 cm) and shallow sapwood (4 or 6 cm) in two species, Douglas fir (Pseudotsuga menziesii) and Rocky Mountain juniper, in a snow-dominated forest and shrubland ecosystem. These species were selected for their differing water use strategies (isohydric vs. anisohydric). To compare the water storage of these species, we installed soil moisture sensors into the trunks of four individual trees of each species at depths of four or six and twelve centimeters. We also installed sap flow sensors to monitor whole-plant transpiration. Comparisons of water storage between early summer (June) and late summer (August) indicate that both species store a greater amount of water in deep sapwood than in shallow sapwood. Additionally, sap flow velocity of Pseudotsuga decreased by late summer, whereas sap flow velocity did not change in Juniperus. Preliminary results indicate that Pseudotsuga (isohydric) maintains a constant midday leaf water potential by reducing stomatal conductance to limit transpiration during drought, whereas Juniperus (anisohydric) maintains constant transpiration, despite the presence of a drought. Ongoing measurements are focused on characterizing differences in stored water between shallow and deep sapwood and quantifying how much of stored water is moved into the transpiration stream. While the functional significance of stored water is poorly understood, the implications could improve land management and land surface models.