Double-funneling of trees: Stemflow and root-induced preferential flow

Trees partition rainfall into throughfall and stemflow, resulting in a spatial distribution of nutrient and water fluxes reaching the soil centred on the trunks of trees. Stemflow fluxes of water and nutrients are then funneled preferentially belowground along tree roots and other preferential flow paths, bypassing much of the bulk soil. This double funneling leads to increased soil chemical, biological, and hydrological heterogeneity, which has been shown to persist for decades. In this paper, we review nutrient fluxes of stemflow water for a variety of tree species and climates. The amount of precipitation partitioned by trees to stemflow ranges over more than three orders of magnitude, accounting for 0.07-22% of incident rainfall in a range of precipitation regimes (600-7100 mm.y(-1)). Sternflow fluxes of NO3- and K were found to be larger for species with greater stemflow partitioning, regardless of climate type. While stemflow volumes may increase in relation to increasing precipitation, stemflow nutrient concentrations tend to become more dilute. On an annual basis, however, it appears that plant canopy morphology is strongly related to stemflow fluxes for plant-mobile nutrients such as K (r(2) = 0.64) and NO3- (r(2) = 0.61). Root-induced preferential flow provides an additional feedback mechanism in nutrient cycling by which stemflow-derived nutrient fluxes are delivered to the rhizosphere.