Comparative Plant Water Relations and Soil Water Depletion Patterns of Three Seral Shrub Species on Forest Sites in Southwestern Oregon

Abstract

Abstract We compared stomatal conductance, transpiration, plant water potential, and soil moisture depletion patterns for three shrub species common on early seral forest sites in southwestern Oregon following logging or fire. Our goal was to determine which of these species were more likely to be the strongest competitors with regenerating conifers. The three species, Arctostaphylos patula Greene, Ceanothus sanguineus Pursh., and Holodiscus discolor (Pursh.) Maxim., were selected to represent a range in leaf morphology and expected water use patterns. Diurnal patterns of leaf conductance, plant water potential, and environmental parameters were measured throughout the growing season, along with seasonal patterns in soil moisture. As with any data obtained under ambient field conditions, environmental parameters and exact timing of measurements varied among shrubs. To better evaluate the response of individual species to common environmental parameters, we constructed models of leaf conductance based on field data and used these models to estimate responses of conductance and transpiration to averaged environmental conditions. This allowed us to better compare species responses to seasonal and diurnal trends in environmental variables. C. sanguineus typically had the highest transpiration rates per unit leaf area, and H. discolor the lowest; however, due to much higher leaf area indices of H. discolor, the two species depleted soil moisture at about the same rate. C. sanguineus and A. patula both had high predawn water potentials throughout the season, even when soil water potential at 1 m depth was less than -1.2 Mpa, suggesting that these species, but not H. discolor, had roots in deeper soil layers. We predict that the two deciduous species, C. sanguineus and H. discolor, will be stronger competitors for soil moisture than A. patula, at least in the top meter of soil. In contrast, A. patula and C. sanguineus are capable of depleting moisture from deeper in the soil and may therefore strongly compete even with deep-rooted conifers late in the season. Our use of empirical models allowed us to compare species responses to common environmental conditions, which facilitated ecological interpretation of species differences in water use patterns. By early August, soil water potentials under all species were low enough to significantly inhibit conifer transpiration, photosynthesis, and growth. For. Sci. 43(3):336-347.