Abstract :
[en] The interaction between topography and climate has a crucial role in shaping forest composition and structure. The understating of how the ecohydrological processes across the landscape affect tree performance becomes especially important with the expected reduction in water availability and increase in water demand, which could enhance the thermal and hydrologic gradient along the slope. Incorporating soil moisture variation and groundwater gradient across the landscape has been found to improve the capacity to predict forest vulnerability and water fluxes in complex terrains. However, most of the information that can be retrieved by remote sensing technique cannot capture small scale-processes. Therefore, hillslope-catchment scale studies can shed light on ecosystem responses to spatially and temporally variable growing conditions.
In the present work, I investigated how hillslope position affects tree physiological response to environmental controls (i.e. soil moisture, vapor pressure deficit, groundwater proximity to the surface) and tree water use in two hillslope transects (Chapter 1 and 3). Sap velocity measurements and isotopic measurements have been applied along two hillslope transects, characterized by contrasting slopes angle, climate, and species composition. We found that the different hydrological processes occurring at the two sites lead to contrasting physiological responses and water uptake strategies. In the Weierbach catchment, the lack of shallow downslope water redistribution through interflow leads to no substantial differences in vadose zone water supply between hillslope positions and ultimately no spatial differences in the tree’s physiological response to environmental drivers. Furthermore, beech and oak trees displayed different stomatal control resulting from their water uptake strategies and physiology. In the Lecciona catchment, the greater soil moisture content at the footslope, promoted by the steep slope, led to more suitable growing conditions and a longer growing season in the piedmont zone. These results emphasize the strong interconnection between vegetation, climate, and hydrological processes in complex terrains, and the need to consider them as a whole to better understand future ecosystem responses to changing climate.
Additionally, the present work sheds new light on the complex interaction between sapwood and heartwood. In Chapter 2, I provide experimental evidence about water isotopic exchange between the two compartments in four tree species (Fagus sylvatica, Quercus petraea, Pseudotsuga menziesii, and Picea abies) characterized by different xylem anatomy, and timing of physiological activity. While the two functional parts display a consistent difference in isotopic composition in conifers, they are characterized by more similar values in broadleaved species in broadleaved species, suggesting a higher degree of water exchange. These results highlight the value of accounting for radial isotopic variation, which might potentially lead to uncertainties concerning the origin of the extracted water for water uptake studies.
