The influence of topography on beech water use: evidence from a comparative study

The ongoing climate change is significantly impacting forest ecosystems, intensifying environmental extremes such as droughts and heatwaves. The increased atmospheric evaporative demand, coupled with reduced soil water availability, poses a risk of water deficit to trees, threatening their health status. These extremes can amplify the thermal and hydrologic gradients along hillslopes, driving spatially variable thermal and hydraulic stress for trees. Until now, site-specific case studies have offered only a limited understanding of how hydrological processes at the hillslope scale influence tree water use. Comparative studies hold the potential to offer a more generalized understating of how trees growing in complex terrains respond to spatially varying growing conditions.

To address this, we set up a comparative study on two forested hillslopes located in the Weierbach catchment (Luxembourg) and the Lecciona catchment in Tuscany (Italy), respectively. The investigated sites differ in steepness, climate, geology, and soil characteristics, but both are dominated by beech (Fagus sylvatica L.) trees. We combined sap velocity, isotope measures, and wood moisture content with environmental monitoring (soil moisture, groundwater level, and hydro-meteorological variables) at different locations to capture beech trees’ water response to heterogeneous environmental conditions. This combination of measurements allowed us to link the transpiration response of trees to water availability along the two investigated hillslopes over the 2019 and 2020 growing seasons in the Weierbach catchment and over the 2021 growing season in the Lecciona catchment.

We observed that surface topography and hillslope structure result in differing sap velocities in response to environmental controls (i.e., vapor pressure deficit and relative extractable water), but not consistently across our study sites. In the Weierbach catchment, we noted a uniform physiological response to environmental controls among trees, even during the drier conditions of 2020, compared to 2019. We attribute this consistency to the homogeneous growing conditions across the slope. On the contrary, in the Lecciona catchment, trees located upslope displayed a more conservative hydraulic behaviour compared to the footslope location. This finding suggests a stronger edaphic and environmental gradient across the hillslope topography. Here, water redistribution through shallow subsurface flow results in more favourable growing conditions and longer growing seasons for trees at the footslope location, compared to the rather uniform growing season observed in the Weierbach catchment. These contrasting results between the two investigated hillslopes suggest that in landscapes where the hydraulic and climatic gradients are stronger, the physiological response among locations will be more spatially variable.