Å·ÃÀ¿Ú±¬ÊÓƵ

Barnard, David M ¹ ; Barnard, Holly R ² ; Molotch, Noah P ³

¹ University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder: INSTAAR
² University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder: INSTAAR
³ University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder: INSTAAR

Forest transpiration is a primary component of water- and energy-balance studies as a mechanism for mass and energy transfer. During the winter months, however, there is a period of transpiration quiescence, due to physiological inactivity. While the importance of transpiration is understood, the cues that initiate transpiration in the winter-spring-summer transition are not. Hence, a better understanding of the processes that determine transpiration activity and how they vary across gradients of elevation and climate are essential for predicting how forests will react to future climate change. The goal of this study was to characterize seasonal variability in daily transpiration activity (DTA) the date that winter transpiration dormancy ends among eight study sites covering a 900 m elevation range in the Front Range of Å·ÃÀ¿Ú±¬ÊÓƵ. 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 stem sapflow. We defined the end of transpiration dormancy date (EOD) as the day that DTA became consistently > 0.3. There was a distinct trend at all sites over the three study years of low DTA (i.e. < 0.15) in winter, higher DTA in the spring and peaked DTA in the early summer, indicating that VPD and transpiration are primarily decoupled in early winter but can have transient coupling in late winter and spring. Lower elevation and south facing sites had more days during the study period (82%) with DTA > 0.3 than the higher elevation north facing sites (< 50% of days). Among all sites, DTA was significantly correlated with soil temperature. Site EODs were also significantly correlated with site elevation (p < 0.05); with the lower elevation sites experiencing EODs up to 40 days earlier than higher elevation. 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 these two events was greatest in the lowest elevation sites and the south-facing sites at mid-elevation and the magnitude of change in DTA at higher elevation sites and north-facing sites was less. This study underscores the importance of understanding seasonal forest transpiration dynamics and how they respond to meteorological and edaphic signals. Such variation should be accounted for in representations of forest transpiration as standard evapotranspiration calculations may not properly account for seasonal variability in physiological control of transpiration, potentially resulting in unrealistic estimates. Moreover, understanding the cues that end dormancy will provide useful information for how montane forests will react in the future as the climate changes resulting in warmer spring time temperatures and earlier snow melt.