Stemflow Infiltration Hotspots Create Soil Microsites Near Tree Stems in an Unmanaged Mixed Beech Forest

Abstract

In stemflow, rainfall is collected and channeled to a concentrated soil water input. It can constitute up to 30% of incident precipitation in some ecosystems. However, the size of the zone influenced by stemflow is unclear, and statistically representative measurement of stemflow (on and in between sites) is scarce. Therefore, whether stemflow creates hotspots of infiltration and potential impacts on forest soils remain subject to controversy. In this study, we investigated the areal dimension of infiltrating stemflow fluxes as well as effects on near-stem soils. We measured throughfall, stemflow and soil properties in high-resolution statistical designs on a mixed forest plot in Germany receiving moderate stemflow. From this data, we modeled the spatial distribution of net precipitation infiltration depth on the plot. Furthermore, we examined soil chemical and physical properties around tree stems to test for and assess a stemflow impact. Results show that stemflow infiltration areas are much smaller than typically assumed and constitute strong infiltration hotspots compared to throughfall. This is also mirrored in soil properties, which are significantly altered near stems. Here, accelerated soil formation and enhanced translocation processes indicate increased soil water fluxes due to high inputs. Additionally, altered soil hydraulic properties enable quicker soil water fluxes near stems. Our findings attest that even comparatively low stemflow fractions (of gross precipitation) can generate strong hotspots of water and matter inputs, which are impactful to subsequent hydrological and biogeochemical processes and properties. Trees shape their direct soil environment, thereby establishing pathways of preferential water flow connecting the canopy and the deeper subsurface.

Stem distance as an explanatory variable for the spatial distribution and chemical conditions of stand precipitation and soil solution under beech (Fagus sylvatica L.) trees

“…In the solid soil phase, our investigations found no evidence of this effect with regard to soil pH and base saturation. This confirms the results by (Metzger et al, 2021) from a mixed European beech forest, who also found no pH differences and attributed this to low air pollution. Our study site is also located in an area with relatively low atmospheric deposition (Wellbrock et al, 2005), as indicated by similar pH values in SF, TF and PR (Supplement 1).…”

Under the canopy: disentangling the role of stemflow in shaping spatial patterns of soil microbial community structure underneath trees

“…Recently, in German temperate forests, Metzger et al . ( 2021 ) found near‐stem microsites of unique physiochemistry, including increased soil organic C content and macropore proportion, and Jochheim et al . ( 2022 ) found higher macronutrient ion and heavy metal concentrations in soil solutions within the stemflow infiltration area.…”

“…The significant influence of stemflow on soil moisture, physiochemistry and the composition of soil solutions has been reported for trees generating similar amounts of stemflow as observed at our site. Recently, in German temperate forests, Metzger et al (2021) found near-stem microsites of unique physiochemistry, including increased soil organic C content and macropore proportion, and Jochheim et al (2022) found higher macronutrient ion and heavy metal concentrations in soil solutions within the stemflow infiltration area. At a forest site on the Canary Islands, Aboal et al (2015) studied a range of tree species with differing stemflow generation, finding multiple soil chemical variables, pH, conductivity, and available phosphorus, correlated with the corresponding tree's stemflow inputs.…”

Stemflow infiltration areas into forest soils around American beech (Fagus grandifolia Ehrh.) trees

“…Considering that the topsoils near our studied trees have a mean clay content greater than 15%, there is a high probability of having macropore structures and consequently preferential flow in the forest soils of our study. Metzger et al (2021) found that soil microsites near tree stems had lower field capacity and higher hydraulic conductivities, which indicated increased macroporosity near tree stems. In addition, tree root systems, both dead and alive, allow the funnelling of stemflow water as preferential flow, bypassing the soil matrix (e.g., Johnson & Lehmann, 2006; Liang et al, 2011; Martinez‐Meza & Whitford, 1996; Schwärzel et al, 2012; Spencer & van Meerveld, 2016).…”