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

<p>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<sup>-2</sup>) 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.</p>

Temporally dependent effects of rainfall characteristics on inter- and intra-event branch-scale stemflow variability in two xerophytic shrubs

Stemflow is important for recharging root-zone soil moisture in arid regions. Previous studies have generally focused on stemflow volume, efficiency and influential factors but have failed to depict stemflow processes and quantify their relations with rainfall characteristics within events, particularly for xerophytic shrubs. Here, we measured the stemflow volume, intensity, funneling ratio and time lags to rain at two dominant shrub species (Caragana korshinskii and Salix psammophila) and rainfall characteristics during 54 events at the semiarid Liudaogou catchment of the Loess Plateau, China, during the 2014–2015 rainy seasons. The funneling ratio was calculated as the ratio between stemflow and rainfall intensities at the inter- and intra-event scales. Our results indicated that the stemflow of C. korshinskii and S. psammophila, on average, started at 66.2 and 54.8 min, maximized 109.4 and 120.5 min after rain began, and ended 20.0 and 13.5 min after rain ceased. The two shrubs had shorter stemflow duration (3.8 and 3.4 h) and significantly larger stemflow intensities (517.5 and 367.3 mm h−1) than those of rain (4.7 h and 4.5 mm h−1). As branch size increased, both species shared the decreasing funneling ratios (97.7–163.7 and 44.2–212.0) and stemflow intensities (333.8–716.2 and 197.2–738.7 mm h−1). Tested by the multiple correspondence analysis and stepwise regression, rainfall amount and duration controlled stemflow volume and duration, respectively, at the event scale by linear relations (p < 0.01). Rainfall intensity and raindrop momentum controlled stemflow intensity and time lags to rain for both species within the event by linear or power relationships (p < 0.01). Rainfall intensity was the key factor affecting stemflow process of C. korshinskii, whereas raindrop momentum had the greatest influence on stemflow process of S. psammophila. Therefore, rainfall characteristics had temporally dependent influences on corresponding stemflow variables, and the influence also depended on specific species.

Connecting ecohydrology and hydropedology in desert shrubs: stemflow as a source of preferential flow in soils

Ecohydrology and hydropedology are two emerging fields that are interconnected. In this study, we demonstrate stemflow hydrology and preferential water flow along roots in two desert shrubs (H. scoparium and S. psammophila) in the south fringe of Mu Us sandy land in North China. Stemflow generation and subsequent movement within soil-root system were investigated during the growing seasons from 2006 to 2008. The results indicated that the amount of stemflow in H. scoparium averaged 3.4% of incident gross rainfall with a range of 2.3–7.0%, while in S. psammophila stemflow averaged 6.3% with a range of 0.2–14.2%. Stemflow was produced from rainfall events with total amount more than 1 mm for both shrubs. The average funneling ratio (the ratio of rainfall amount delivered to the base of the tree to the rainfall that would have reached the ground should the tree were not present) was 77.8 and 48.7 for H. scoparium and S. psammophila, respectively, indicating that branches and stems were fully contributing to stemflow generation and thereby provided sources of water for possible preferential flow into deeper soil layer. Analysis of Rhodamine-B dye distribution under the shrubs showed that root channels were preferential pathways for the movement of most stemflow water into the soil. Distribution of soil water content under the shrubs with and without stemflow ascertained that stemflow was conducive to concentrate and store water in deeper layers in the soil profiles, which may create favorable soil water conditions for plant growth under arid conditions. Accordingly, a clear linkage between aboveground ecohydrology and belowground hydropedology in the desert shrubs is worth noticing, whereby an increase in stemflow would result in an increase in soil hydrologic heterogeneity.