Species-Specific Interactions Between Canopy Cover, Soil Water Dynamics, and Root Water Uptake in Temperate Forest Ecosystems

In forested environments, precipitation is intercepted and redistributed within the tree canopy, thereby generating spatially heterogeneous water fluxes to the forest floor. Water from the forest soil is taken up by roots and subsequently released back into the atmosphere through the process of transpiration. However, the effect of either of these processes on spatial variation of soil water content and water fluxes remains to be elucidated.

In this study, we examined the temporal variability in soil water content, measured along a canopy cover gradient of three distinct tree species over two years. Our analysis focused on two phases of soil drying following precipitation events during the growing season. Specifically, we evaluated the immediate effect of precipitation on soil wetting and the subsequent root water uptake during longer dry periods. To identify the factors significantly influencing the soil water response to precipitation, we employed a linear mixed effects modeling approach.

The results indicate that spatial patterns of throughfall had a weak yet significant influence on the soil water response. The effect of position along the canopy cover gradient depended on event gross precipitation, which suggests that the canopy cover gradient only reflected the spatial patterns of water input when gross precipitation was low. Soil wetting was less pronounced under Fagus sylvatica than under Picea abies and Pinus sylvestris. The soil water response to precipitation was found to be influenced by the spatial patterns of the pre-event soil moisture, with soil profiles that were locally wetter responding more strongly to precipitation,  as has previously been observed for other sites and vegetation covers. This effect was more pronounced in overall dry soils and higher event gross precipitation, hence indicating preferential flow. Furthermore, root water uptake was found to be considerably higher under F. sylvatica and P. sylvestris than under P. abies. The root water uptake depth profiles of F. sylvatica and P. sylvestris exhibited substantial uptake from soil layers as deep as one meter below the soil surface, whereas root water uptake of P. abies was more confined to the topsoil, despite occasional observations of deeper root water uptake.

The findings of this work emphasize the influence of the tree canopy on belowground water fluxes and illustrate pronounced species-specific differences in soil wetting and root water uptake.