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In Mediterranean climates, the season of water availability (winter) is out of phase with the season of light availability and atmospheric moisture demand (summer). We investigate the seasonality of evergreen tree transpiration in a Mediterranean climate, using observations from a small (4000 m2), forested, steep (32) hillslope, in the northern California Coast Range. We analyze 3 years of half-hourly measurements from 39 sap flow sensors in 26 trees, six depth profiles of soil moisture measured by TDR, and spatially distributed measurements of micrometeorology from five locations. The sap flow measurements show that two common evergreen tree species have different seasons of peak transpiration. Douglas-firs (Pseudotsuga menziesii) maintain significant transpiration through the winter rainy season and transpire maximally in the spring, followed by a sharp decline in transpiration in the summer dry season. Pacific madrones (Arbutus menziesii), and to a lesser extent other broadleaf evergreen species (Quercus wislizeni, Notholithocarpus densiflorus, Umbellularia californica), in contrast, transpire maximally in the summer dry season. The seasonal patterns are quantified using principal component analysis. Markov chain Monte Carlo estimation of response to environmental variables shows that the difference in transpiration seasonality arises from different sensitivities to atmospheric evaporative demand and root-zone moisture. The different sensitivities to atmospheric evaporative demand also create species differences in transpiration variability at synoptic time scales. Using the sap flow measurements and a regional forest inventory, a bottom-up regional transpiration estimate is constructed. The estimate suggests that sensitivity of Douglas-fir transpiration to water stress suppresses dry season evapotranspiration at the regional scale. Key Points Three year, half-hourly record of sap flow, micrometeorology, and soil moisture PCA shows offset in peak transpiration season between needleleaf and broadleaf species Douglas-fir water stress response could reduce dry season transpiration at large scale © 2014. American Geophysical Union. All Rights Reserved.

Abstract: To understand the effect of restoration thinning on the water balance of upland semi-arid ponderosa pine (Pinus ponderosa) forests of the southwestern US, we compared the components of forest water balance between an unthinned plot and a thinned plot using a paired water balance approach. Forest overstory transpiration (E O) was estimated from tree sapflow scaled to the plot level. Understory evapotranspiration (E U) was estimated from the difference between throughfall precipitation and changes in soil water content measured in trenched plots that excluded tree roots. The thinning treatment in 2001 reduced plot basal area by 82% and leaf area index by 45%. Difference in stand-level evapotranspiration (E) between the thinned and unthinned plots, and partitioning of E between E U and E O during the first post-treatment summer and spring, varied between drought and non-drought periods. The importance of E U in stand-level E was greater in thinned compared with unthinned plots and increased during extreme drought when E O was low due to stomatal closure. Our results highlight the importance of drought and climate as factors determining the impact of thinning on water balance in southwestern ponderosa pine forests. [Copyright &y& Elsevier]