Soil water hydraulic redistribution in a subtropical monsoon evergreen forest.

Hydraulic redistribution (HR), which is the passive movement of water through plant roots from wet to dry soil due to the water gradient, is important for plant physiology and ecohydrological processes. However, our poor knowledge on HR in the humid monsoon climate zone hampers the understanding of the interactions between vegetation and soil water during frequent droughts in evergreen forests. Thus, 5 years (2011-2015) of data, including meteorological parameters and soil moisture content at depths of 10, 30, 50, and 100 cm in soil profiles, were compared at one evergreen broad-leaved forest and at one clear-cutting forest site in south China. Analyses of soil moisture dynamics show that HR was frequently triggered within the depth of 30 cm at the evergreen broad-leaved forest, while (if any) was less visible at the clear-cutting forest site. The daily averaged magnitude of redistributed soil water reached the maximum of 0.81 mm/d. The HR mainly occurred during the monsoon dry season (i.e., from October to March of the following year), possibly indicating a different cause, i.e., asynchronous variations in rainfall and plant water use shape the seasonal patterns of soil water HR, compared to other humid zones. During the study period when HR occurred, the average daily HR in the soil profiles replenished approximately 34-50% of the water consumption in the 0-30 cm soil layer. The simulation results of a distributed hydrology-soil-vegetation model incorporating a HR scheme indicate that evapotranspiration enhanced during drought periods when HR occurred. In the future climate change context, comprehensive investigations on the water fluxes in the atmosphere-vegetation-soil continuum are needed to fully understand the effects of HR on the physiological responses of plants and on the water cycle.