Studies on the fate and transport of dissolved organic matter (DOM) along the rainfall‐to‐discharge flow pathway typically begin in streams or soils, neglecting the initial enrichment of rainfall with DOM during contact with plant canopies. However, rain water can gather significant amounts of tree‐derived DOM (tree‐DOM) when it drains from the canopy, as throughfall, and down the stem, as stemflow. We examined the temporal variability of event‐scale tree‐DOM concentrations, yield, and optical (light absorbance and fluorescence) characteristics from an epiphyte‐laden Quercus virginiana‐Juniperus virginiana forest on Skidaway Island, Savannah, Georgia (USA). All tree‐DOM fluxes were highly enriched in dissolved organic carbon (DOC) compared to rainfall, and epiphytes further increased concentrations. Stemflow DOC concentrations were greater than throughfall across study species, yet larger throughfall water yields produced greater DOC yields versus stemflow. Tree‐DOM optical characteristics indicate it is aromatic‐rich with fluorescent DOM dominated by humic‐like fluorescence, containing 10–20% protein‐like (tryptophan‐like) fluorescence. Storm size was the only storm condition that strongly correlated with tree‐DOM concentration and flux; however, throughfall and stemflow optical characteristics varied little across a wide range of storm conditions (from low magnitude events to intense tropical storms). Annual tree‐DOM yields from the study forest (0.8–46 g C m−2 yr−1) were similar to other yields from discrete down‐gradient fluxes (litter leachates, soil leachates, and stream discharge) along the rainfall‐to‐discharge flow path.

Transport pathways of microbes between ecosystem spheres (atmosphere, phyllosphere, and pedosphere) represent major fluxes in nutrient cycles and have the potential to affect microbially mediated biogeochemical processes. Novel data on bacterial fluxes from the phyllosphere to the pedosphere during rainfall via throughfall (rain dripping from/through the canopy) and stemflow (rain funneled down tree stems) are reported. Bacterial concentrations were quantified using flow cytometry and validated with quantitative polymerase chain reaction assays in rainfall samples from an oak‐cedar forest in coastal Georgia (southeastern U.S.). Bacteria concentrations (cells mL−1) and storm‐normalized fluxes (cells m−2 h−1, cells m−2 mm−1) were greater for cedar versus oak. Total bacterial flux was 1.5 × 1016 cells ha−1 yr−1. These previously unexamined bacterial fluxes are interpreted in the context of major elemental pools and fluxes in forests and could represent inoculum‐level sources of bacteria (if alive), and organic matter and inorganic solute inputs (if lysed) to soils.

Dissolved organic matter (DOM) drives carbon (C) cycling in soils. Current DOM work has paid little attention to interactions between rain and plant canopies (including their epiphytes), where rainfall is enriched with tree-derived DOM (tree-DOM) prior to reaching the soil. Tree-DOM during storms reaches soils as throughfall (drip through canopy gaps and from canopy surfaces) and stemflow (rainwater drained down the trunk). This study (1) assessed the susceptibility of tree-DOM to the consumption by microbes (biolability); (2) evaluated interstorm variability in the proportion and decay kinetics of biolabile tree-DOM (tree-BDOM), and (3) determined whether the presence of arboreal epiphytes affected tree-BDOM. Tree-BDOM from Juniperus virginiana L. was determined by subjecting throughfall and stemflow samples from five storms to 14-day microbial incubations. Tree-DOM was highly biolabile, decreasing in concentration by 36-73% within 1-4 days. Tree-BDOM yield was 3-63 mg-C m −2 mm −1 rainfall, which could represent 33-47% of annual net ecosystem exchange in Georgia (USA) forests. Amount and decay kinetics of tree-BDOM were not significantly different between throughfall versus stemflow, or epiphyte-covered versus bare canopy. However, epiphyte presence reduced water yields which reduced tree-BDOM yields. Interstorm proportions, rates and yields of tree-BDOM were highly variable, but throughfall and stemflow consistently contained high tree-BDOM proportions (>30%) compared to previously-published litter and soil leachate data (10-30%). The high biolability of tree-DOM indicates that tree-BDOM likely provides C subsidies to microbial communities at the forest floor, in soils and the rhizosphere.

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