Hot or not? The effect of stemflow on infiltration and soil properties

Stemflow can form hotspots of precipitation in forests. The stemflow funneling ratio describes the degree of concentration compared to open land rainfall in reference to the tree basal area. But how strongly does stemflow actually concentrate at the point of precipitation input to the soil? This depends on the size of stemflow infiltration areas. Findings hereon vary widely, as they refer to different tree and rainfall characteristics. Furthermore, due to little representative data on stemflow and the separate evaluation of stemflow and throughfall, the importance of stemflow as a hotspot is still subject to controversy. Using an extensive and representative field data set, we want to investigate the effect of stemflow on soil infiltration and percolation. Measurements were conducted on a 1-ha mixed beech forest plot in central Germany with intermediate stemflow generation. In high-resolution statistical designs, stand precipitation (stemflow, n = 65, and throughfall, n = 350) were recorded during three summers, and soil undisturbed (n = 420) and disturbed (n = 100) samples were taken and analyzed for physical and chemical properties. We calculated the spatial distribution of infiltration from stand precipitation data, rain intensity and soil infiltration capacity. Soil properties near stems (< 1m) and farther away were compared to determine a stemflow impact. Results show that stemflow infiltration areas are very small. Stemflow funneling at infiltration exceeds the conventional funneling ratio. Therefore, infiltration depth (L m^-2) within stemflow infiltration areas is multiples of throughfall, even at dripping points. Soil properties close to trees are significantly different from the bulk soil, suggesting an accelerated soil formation process and a more developed soil structure. Stemflow-induced high soil water fluxes can be identified as an important driver for this pattern. Thus, the hotspot character of stemflow is confirmed by our findings. Stemflow-induced hotspots persist during infiltration and percolation. What is more, they have a direct and significant impact on the soil environment. Soil hydraulic properties facilitate quick water fluxes near stems. Such, trees might establish water flux bypasses from the canopy to the deeper subsurface.