Determining transpiration rates from beech and spruce trees with measurements of sapflow, leaf water potential and stomatal conductance

Beech and spruce trees are dominant species in prealpine forests. Thus, plant-specific physiological traits of beech and spruce are key to determine evapotranspiration fluxes from these forests. During dry periods trees adapt to the decreased soil water availability, however these adaptation strategies are not yet well determined by observation data. Adaptations to water limitations are different between species and if not accounted for, may lead to an overestimation of evapotranspiration fluxes. These unknowns add additional uncertainty to the simulation of transpiration patterns with ecohydrological models under water limited conditions. Here we present a comparison of field methods to measure (directly and indirectly) the transpiration process for the purpose of supporting mechanistic ecohydrological modelling. At our mixed beech and spruce forest field site at Waldlabor Zurich we equipped multiple trees with sapflow sensors (hourly measurements) and frequently measured stomatal conductance and leaf water potential (weekly to twice a week) during the 2021 growing season. Along with these plant-physiological measurements, we recorded timeseries of meteorological variables and soil water content and matric potential in different depths (10, 20, 40 and 80cm).

Sapflow measurements suggest that transpiration rates are tightly linked to the magnitude of solar radiation and vapor pressure deficit. Summer transpiration rates were higher in beech trees compared to spruce trees. Most of the early summer of the 2021 growing season was relatively wet, but the months August and September had considerably lower precipitation than the long-term average. This period with low precipitation during August and September led to decreasing soil water content and matric potential, which caused leaf water potentials to decrease accordingly. On the contrary, stomatal conductance remained relatively constant for beech and even increased for spruce, suggesting that under the encountered conditions, stomatal control is not depending directly on leaf water potential. Sapflow rates gradually decreased as the growing season proceeded, but it remains unclear to what degree this decrease was due to phenology, meteorological conditions and/or limited water availability. We compared our measurements to the simulations of an existing mechanistic ecohydrological model (Tethys-Chloris) to test the performance on the observed diurnal dynamics. The comparisons between observed and simulated transpiration rates showed that uncertainties are larger when water availability is limited in the dry periods of August and September.

Our work provides insight into the processes at the soil-plant-atmosphere continuum by the combination of highly resolved measurements and an established mechanistic ecohydrological model. Results highlight how well different measurements of transpiration proxies agree with each other, how suitable they are to assess the actual transpiration rates, and which conditions have larger simulation uncertainties in ecohydrological models and thus need to be better constrained by field observations.