Hotspots of soil water movement induced by vegetation canopies

Abstract

Vegetation and soil form a highly interactive system, within which water is one of the most important factors. By the redistribution of precipitation and its separation into interception, throughfall and stemflow, vegetation canopies introduce a strong small-scale heterogeneity to downwards-directed water fluxes in forests. This could importantly affect subsequent hydrological and biogeochemical processes. In my study, I addressed the formation of patterns and hotspots of below-canopy precipitation and their imprint on soil water conditions. In a comprehensive experimental approach, I used a high-resolution statistical design to capture overall patterns, and hotspot locations trees to identify extreme impacts of canopy-induced water flow on soil water and properties. In Chapter 1, I show that soil properties, instead of net precipitation patterns, most prominently shaped spatial patterns of soil water content. Soil properties, yet, showed to be spatially organized due to the position of trees, forming areas of enhanced soil drainage around the trunks. In Chapter2, the effects of tree, neighborhood and stand properties on stemflow were identified using linear mixed effects models. Stand density and species diversity increased stemflow due to high woody surface area. The temporal stability of stemflow variation indicates that vegetational impacts are highly relevant. Chapter 3 assesses the spatial distribution of infiltration from stemflow and throughfall and the impact of hotspots on soil properties. Stemflow infiltration areas proved to be extremely small and infiltration depth high. These hotspots formed distinct soil microsites at the base of trees by accelerating soil formation. Thus, vegetation induces water flow hotspots from the canopy to below the rooting zone. This is likely to influence hydrological responses and the separation of rainfall to plant available water in contrast to deep percolation and groundwater recharge.