A hillslope scale comparison of tree species influence on soil moisture dynamics and runoff processes during intense rainfall

This study investigates how different tree species influence soil hydrological properties that are relevant for the rainfall-runoff response of a given soil type. We hypothesize that for the same soil type, tree species that differ in rooting system, water consumption and associated soil fauna and soil flora lead to different soil moisture dynamics and lateral flow processes during rainfall and hence to different runoff responses. To test this hypothesis, we compare soil moisture patterns and interflow at different soil depths in a Norway spruce (Picea abies (L) Karst) forest and in a European beech (Fagus sylvatica L) forest during sprinkling experiments on two 6 x 10 m hillslope segments with the same soil type. Spruce with a shallow rooting system and sinkers that remain very shallow on poorly aerated soils and beech with a heart shaped, often deeper rooting system are two of the most important tree species in Central Europe. At each hillslope, volumetric soil water contents were measured in 6 min intervals with 48 TDR waveguides during and after sprinkling with intensities of 100 mm/h and 60 mm/h (for 1 h). The waveguides were installed in 12 soil pits, whereby a single soil pit consisted of four 20 cm buriable waveguides installed in 10 cm, 30 cm, 50 cm and 70 cm soil depth. Surface and shallow interflow at 10 cm soil depth and interflow at soil depths of 30 cm and 60 cm was automatically recorded. Despite the high rainfall intensities, no surface flow was observed in any of the experiments and only small amounts of shallow interflow were measured. Soil moisture patterns of lateral cross sections during and after the sprinkling reveal how tree species can alter runoff dynamics: under spruce, coinciding with rooting patterns, a water table develops in approximately 30 cm soil depth while the soil water content in 50 and 70 cm depth remains low. At the beech site, where coarse roots are found in deeper soil horizons, more water is directed towards deeper, already wetter soil horizons, from where the water table raises into the topsoil with high lateral conductivity. Because the higher water content on top of the stagnic layer allows segments of macropores like old root channels to connect earlier under beech, the beech hillslope exhibits a faster runoff response than the spruce hillslope. A lower water table and a higher macro-porosity makes saturation excess overland flow unlikely under beech. With the shallow water table and a lower available soil volume for preferential flow, a site planted with spruce is prone to saturation excess overland flow under natural rainfall conditions with inflow from the top. The results suggest that different tree species can lead to different rainfall-runoff responses at the same soil type. Though the study site showed minimal variation in soil properties, we cannot exclude that some of the differences in runoff processes we observed are caused by factors other than tree species, because only one large hillslope segment in each forest stand was sprinkled. (C) 2011 Elsevier B.V. All rights reserved.