PS 15-192 - Species differences in sap flux rates, transpiration amounts, and timing among dominant species of The Pacific Northwest

Abstract: Tree species can have significant influences on ecosystem processes through modification of ecohydrological processes. Previous research in the high-precipitation temperate rainforests of the Pacific Northwest have demonstrated that tree age, sapwood area, and species all can have significant influences on basin-level streamflow and hydrology. Similarly, we have recently demonstrated interspecific differences in radial interception profiles among four of the most common tree species in the Pacific Northwest (Alnus rubra, Acer macrophyllum, Pseudotsuga menziesii, and Thuja plicata). Here we use a two-year dataset of sapflux conducted on the same four tree species across two study sites in Washington state to examine differences among species in rates, amounts, and timing of transpiration at the whole-tree scale. Further, we use coincident measures of ambient temperature, humidity, soil moisture, and vapor pressure deficit (VPD) to examine trends in sensitivity of estimated leaf-specific transpiration to ambient conditions among species. Four central findings arose from these analyses: 1) Average daily rate of sap flux more than doubled between evergreen vs. deciduous species, and significant differences among all species were evident; 2) Similarly dramatic differences in total tree transpiration suggest the two deciduous species, and especially A. macrophyllum, transpire significantly more water in summer months than the two conifer species; 3) Distinct seasonality in continued sap flux in shoulder seasons among conifers is consistent with hypotheses about shoulder-season productivity and transpiration in temperate rainforest conifer species; 4) Patterns in leaf-specific conductance and transpiration both suggest that species differ significantly in sensitivity to vapor pressure deficit (VPD), and conifer species generally have much lower conductance and transpiration at high VPD than adjacent deciduous angiosperms. Overall, these data contribute to better understanding of patterns in transpiration among four of the most common species in the high-rainfall forests of The Pacific Northwest. These data have implications for tree interactions with local hydrology, stormwater, and seasonal drought, and they build on previous studies examining tree water-use at the whole-tree and basin scale.