Variation in Montane Forest Transpiration Dormancy and Seasonality Along an Elevation Gradient.

Abstract:

Forest transpiration is a primary component of water- and energy-balance studies as a mechanism for mass and energy transfer. During winter, however, there is a period of transpiration quiescence, due to lower air temperatures and physiological inactivity. While the importance of transpiration is well understood, the cues that initiate transpiration in the winter-spring-summer transition, and their variability across gradients of elevation and climate are not. The goal of this study was to characterize seasonal variability in daily transpiration activity (DTA) and timing of winter transpiration dormancy release among eight study sites across an elevation gradient. We defined DTA as the daily coefficient of determination (i.e. R²) of linear regressions between 30-minute measurements of vapor pressure deficit (VPD) and tree sapflow. Transpiration dormancy date (EOD) was defined as the day that DTA became consistently > 0.3. Across sites, DTA was low (i.e. < 0.15) in winter, higher in the spring reaching a peak in early summer, indicating VPD and transpiration decouple in the winter but are coupled transiently in the spring. Lower elevation sites had more days with high DTA (i.e. > 0.3) than higher sites (82% and 49% respectively). Among sites, DTA was significantly correlated with air temperature and soil temperature (p < 0.05 for both). Site EODs were also significantly correlated with site elevation (p < 0.05); with the lower elevation sites experiencing EODs > 40 days earlier than higher elevation sites in 2013 and 2014 but only by 25 days earlier in 2015 due to cooler temperatures in May and above-average precipitation. Correlations between EOD and meteorological and edaphic conditions were inconsistent among study sites, requiring further study. However, in fall/winter of water year 2015 there were two distinct warm periods (20-Nov to 9-Dec and 5-Jan to 10-Feb) that temporarily initiated transpiration activity – the magnitude of which was correlated with site elevation. This study highlights the need for understanding of seasonal variability in forest transpiration that, if left unaccounted for, may skew representations of forest transpiration. Moreover, understanding the cues that end dormancy provides insight into how upper montane forests will react to shifts in future climate and snow melt dynamics.