Abstract ID: 3.12723 | Not reviewed | Requested as: Talk | TBA | TBA

Evapotranspiration (ET) from forested ecosystems is a major component of the Alpine water cycle, influencing soil moisture, groundwater recharge, and streamflow. ET also modulates local and regional climate through latent heat exchange, affecting temperature and humidity in forest ecosystems. However, long-term measurements of ET are rare, thus the effects of a warming climate on forest ET fluxes remain poorly understood. Here we present a comprehensive analysis of long-term evapotranspiration data from eddy covariance measurements from two different forest sites in Switzerland: a Alpine evergreen coniferous forest (CH-Dav) and a montane mixed deciduous forest (CH-Lae), where H2O fluxes have been measured using eddy covariance (EC) since 1997 and 2005, respectively. Using this long-term data collection we i) analyzed changes in annual and seasonal ET fluxes and ii) assessed the major drivers of ET fluxes across the two forest sites. The subalpine Davos site received slightly higher precipitation and had a lower temperature compared to the Laegeren site, thus soil moisture and vapor pressure deficit were less important drivers of ET at Davos. Instead, ET limitations in Davos were mainly related to lower net radiation. At the montane Laegern site, compound dry events combining soil water deficits and higher VPD forced the trees to downregulate transpiration during the growing season, decreasing forest ecosystem ET fluxes. Analyses of the especially dry years (i.e., 2003 – only Davos, 2015, 2018 and 2022) indicate that the spruce dominated Davos site showed higher ET compared to average years, due to the favorable growing conditions at this typically energy-limited subalpine site. However, we found that even at the Davos site extended dry periods (i.e., as observed in the year 2018) may lead to lower water availability in the soil and thus force spruce trees to down-regulate transpiration. Overall, our results indicate that i) forest ecosystems in drier lower-elevated places are already experiencing frequent periods of ET reduction due to water limitations in the soil and atmospheric demand exceeding trees’ limitations while ii) higher elevation forest ecosystems might become more vulnerable when the durations of dry conditions and high temperatures are extending over longer time periods as projected in a future climate.