Rainfall onto trees entrains dissolved organic matter (tree‐DOM). Tree‐DOM is then exported down stems in stemflow and through leaves, branches, and gaps as throughfall. We synthesize tree‐DOM literature, presenting trends in and controls of tree‐DOM concentrations, fluxes, and chemistry. Tree‐DOM concentrations are higher in stemflow (7–482 mg‐C L⁻¹) than throughfall (1–61 mg‐C L⁻¹) with both being enriched in DOM compared to rainwater. Per unit area of landscape, trees yield similar amounts of DOM as the streams draining those landscapes, indicating tree‐DOM may provide a quantitatively significant carbon subsidy to stream and soil ecosystems. Tree‐DOM chemistry is indicative of mixed sources likely dominated by organic molecules produced by the trees and supplemented by inputs from epiphytes and atmospheric deposition. Increased research is required to understand the sources of tree‐DOM, the controls on tree‐DOM concentrations and fluxes, and the significance of tree‐DOM to aquatic C cycling.

Stemflow, a precipitation and solute supply to soils near tree stems, can play a wide array of roles in ecosystem functioning. However, stemflow’s ecohydrological functions have been primarily studied in forests with voluminous stemflow because resource subsidy is currently considered stemflow’s only impact on near-stem soils. This common assumption ignores controls that stemflow generation may exert via resource limitation (when stemflow < open rainfall and near-stem throughfall is negligible). We reviewed selected literature across numerous forests to evaluate the predominance of stemflow as a potential resource limitation to near-stem soils and characterized the concentrated, but meager, solute flux from low stemflow generators. Global observations of stemflow were highly skewed (skewness = 4.6) and leptokurtic (kurtosis = 28.8), where 69% of observations were ≤2% of rainfall. Stemflow ≤ 2% of rainfall is 10–100 times more chemically enriched than open rainfall, yet low volumes result in negligible solute fluxes (under 1 g m^-2 y^-1). Reduced stemflow may be the global and regional norm, creating persistently dry near-stem soils that receive infrequent, salty, and paltry precipitation flux if throughfall is also low. Ignoring stemflow because it results in scarcity likely limits our understanding of ecosystem functioning as resource limitations alter the fate of soil nutrients, energy flows, and spatial patterning of biogeochemical processes.

Trees concentrate rainfall to near-stem soils via stemflow. When canopy structures are organized appropriately, stemflow can even induce preferential flow through soils, transporting nutrients to biogeochemically active areas. Bark structure significantly affects stemflow, yet bark-stemflow studies are primarily qualitative. We used a LaserBark to compute bark microrelief (MR), ridge-to-furrow amplitude (R) and slope (S) metrics per American Society of Mechanical Engineering standards (ASME-B46.1–2009) for two morphologically contrasting species (Fagus sylvatica L. (European beech), Quercus robur L. (pendunculate oak)) under storm conditions with strong bark water storage capacity (BWSC) influence in central Germany. Smaller R and S for F. sylvatica significantly lowered BWSC, which strongly and inversely correlated to maximum funnelling ratios and permitted stemflow generation at lower rain magnitudes. Larger R and S values in Q. robur reduced funnelling, diminishing stemflow drainage for larger storms. Quercus robur funnelling and stemflow was more reliant on intermediate rain intensities and intermittency to maintain bark channel-dependent drainage pathways. Shelter provided by Q. robur’s ridged bark also appears to protect entrained water, lengthening mean intrastorm dry periods necessary to affect stemflow. Storm conditions where BWSC plays a major role in stemflow accounted for much of 2013’s rainfall at the nearest meteorological station (Wulferstedt).