COMPARATIVE PLANT WATER RELATIONS AND SOIL-WATER DEPLETION PATTERNS OF 3 SERAL SHRUB SPECIES ON FOREST SITES IN SOUTHWESTERN OREGON

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 o r fire. Our goal was to determine which of these species were move lik ely to be the strongest competitors with regenerating conifers. The th ree 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 pat terns of leaf conductance, plant water potential, and environmental pa rameters were measured throughout the growing season, along with seaso nal patterns in soil moisture. As with any data obtained under ambient field conditions, environmental parameters and exact timing of measur ements varied among shrubs. To better evaluate the response of individ ual species to common environmental parameters, we constructed models of leaf conductance based on field data and used these models to estim ate responses of conductance and transpiration to averaged environment al 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 sa me rate. C. sanguineus and A. patula both had high predawn water poten tials throughout the season, even when soil water potential at 1 m dep th was less than -1.2 Mpa, suggesting that these species, but not H. d iscolor, had roots in deeper soil layers. We predict that the two deci duous species, C. sanguineus and H. discolor, will be stronger competi tors for soil moisture than A. patula, at least in the top meter of so il. In contrast, A. patula and C. sanguineus are capable of depleting moisture from deeper in the soil and may therefore strongly compete ev en 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 di fferences in water use patterns. By early August, soil water potential s under all species were low enough to significantly inhibit conifer t ranspiration, photosynthesis, and growth.