Comment on esurf-2023-16

Abstract

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The degradation of ground vegetation cover caused by large grazing herbivores frequently results in enhanced erosion rates in forest ecosystems. Splash erosion can be caused by drop impacts with high throughfall kinetic energy (TKE) from the canopy of the trees. Notably bigger canopy drips from structurally-mediated woody surface points appear to induce even higher TKE and generate concentrated impact locations causing severe focus points of soil erosion. However, TKE at these locations has rarely been reported. This study investigated the intensity of TKE at a concentrated impact location and compared it to general TKE locations under the canopy and freefall kinetic energy (FKE) outside the forest. We measured precipitation, TKE and FKE using splash cups at seven locations under Japanese beech trees and five locations outside the forest in the leafless and leafed seasons in a deciduous broadleaved forest of Japan, respectively. TKE at the concentrated impact location was 15.2 and 49.7 times higher than that at general locations under beech and FKE, respectively. This study confirmed that canopy drip from woody surfaces can be a hotspot of soil erosion in temperate forest ecosystems. Throughfall precipitation at the concentrated impact location was 11.4 and 8.1 times higher than that at general locations and freefall, respectively. TKE per 1 mm precipitation (unit TKE) at the concentrated impact location (39.2 ± 23.7 J m -2 mm -1 ) was much higher than that at general locations (22.0 ± 12.7 J m -2 mm -1 ) and unit FKE (4.5 ± 3.5 J m -2 mm -1 ). Unit TKE in the leafless season was significantly lower than in the leafed season because of fewer redistribution of canopy drips induced only by woody tissue. Nevertheless, unit TKE at the concentrated impact location in the leafless season (36.4 J m -2 mm -1 ) was still higher than at general locations in the leafed season. These results show that potentially high rates of sediment detachment can be induced by not only throughfall precipitation, but also larger throughfall drop size distributions at concentrated impact locations, even in the leafless season.