Variability and temporal stability of throughfall along a hillslope

Abstract

Tree species composition, tree size, and microclimate can vary along hillslopes in forest ecosystems. This will affect the amount of throughfall (TF) that reaches the ground. However, few studies have focused on the effects of topography on the spatio-temporal variability of throughfall. We, therefore, measured throughfall on a steep roughly 110 m long hillslope covered by European beech trees (Fagus sylvatica) in the Re della Pietra experimental catchment, Tuscany Apennines, Central Italy. Although the trees all have a similar age, the basal area, Plant Area Index (PAI), and canopy cover are higher on the lower part of the hillslope than the upper part of the hillslope. We installed 49 throughfall collectors in two 144-m2 square grids: one on the lower hillslope and one on the upper hillslope. In addition, we installed 29 collectors on a transect connecting the lower and upper grid. Throughfall was measured from all collectors at an approximately monthly interval and compared with gross precipitation (GP) measured in a nearby open area. The overall TF/GP ratio for the 61 manual measurements during the July 2020 to September 2023 study period was 70 ± 3 %. It was higher for the growing season (71 ± 4 %) characterised by higher rainfall intensities than the dormant season (69 ± 3 %). TF/GP was lower for the lower grid than the upper grid (61 ± 5 vs 73 ± 6 % of GP, respectively). The differences between the measurements at the lower and upper grid were significant for almost all measurement days and reflect the differences in tree size and stand density. Temporal stability analysis indicated a high persistence of the spatial TF pattern for both grids, especially in the dormant season. The higher temporal stability for in the upper grid seems to be related to the lower variability in tree size. Although these results need to be confirmed by larger-scale studies at different hillslopes, they suggest that topographic differences in stand characteristics should be taken into account when determining the water balance for forested catchments.