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Monitoring diseases within tree canopies is challenging due to their inaccessibility and the complexity of canopy ecosystems. Here, we explore the potential of stemflow sampling as a novel, ground-based method for detecting and monitoring canopy-associated pathogens. In a case study focused on Litylenchus crenatae ssp. mccannii (LCM), the nematode associated with Beech Leaf Disease (BLD), we collected stemflow samples from 18 Fagus grandifolia Ehrh. (American beech) trees across 12 storm events. eDNA assays detected LCM presence in 7 of those storms, with quantitative PCR-derived gene concentrations ranging from 80 to 158,000 copies mL−1. Higher detections and concentrations coincided with leaf senescence and bud formation periods, and they correlated conditionally with event rainfall amount and pre-storm changes in relative humidity. Although based on a single site and season, these findings demonstrate the potential for stemflow sampling to capture a pathogen’s eDNA (i.e., canopy distress signals) at ground level. This method could complement traditional monitoring, offering another affordable, non-invasive tool for pathogen detection. Additional validation, particularly regarding live versus dead organisms and across varied site conditions, will be essential to evaluate the breadth of value stemflow eDNA offers for canopy disease management and ecological research. 

Precipitation channelled down tree stems (stemflow) or into drip points of ‘throughfall’ beneath trees results in spatially concentrated inputs of water and chemicals to the ground. Currently, these flows are poorly characterised due to uncertainties about which branches redirect rainfall to stemflow or throughfall drip points.We introduce a graph theoretic algorithm that ‘prunes’ quantitative structural models of trees (derived from terrestrial LiDAR) to identify branches contributing to stemflow and those contributing to throughfall drip points. To demonstrate the method's utility, we analysed two trees with similar canopy sizes but contrasting canopy architecture and rainfall partitioning behaviours.For both trees, the branch ‘watershed’ area contributing to stemflow (under conditions assumed to represent moderate precipitation intensity) was found to be only half of the total ground area covered by the canopy. The study also revealed significant variations between trees in the number and median contribution areas of modelled throughfall drip points (69 vs. 94 drip points tree⁻¹, with contributing projected areas of 28.6 vs. 7.8 m²tree⁻¹, respectively). Branch diameter, surface area, volumes and woody area index of components contributing to stemflow and throughfall drip points may play a role in the trees' differing rainfall partitioning behaviours.Our pruning algorithm, enabled by the proliferation of LiDAR observations of canopy structure, promises to enhance studies of canopy hydrology. It offers a novel approach to refine our understanding of how trees interact with rainfall, thereby broadening the utility of existing LiDAR data in environmental research. 

Plant canopies divert a portion of precipitation to the base of their stems through “stemflow”, a phenomenon that influences the canopy water balance, soil microbial ecology, and intrasystem nutrient cycling. However, a comprehensive integration of stemflow into theoretical and numerical models in natural science remains limited. This perspective examines three unresolved, fundamental questions hindering this integration, spanning the canopy to the soil. First, the precise source area within the canopy that generates stemflow is undefined. Thus, we asked, “whence stemflow?” Current common assumptions equate it to the entire tree canopy, a potentially misleading simplification that could affect our interpretation of stemflow variability. Second, we asked what are the various conditions contributing to stemflow generation—beyond rain, to dew and intercepted ice melt—and could the exclusion of these volumes consequently obscure an understanding of the broader implications of stemflow? Third, we explored ”whither stemflow?” This question extends beyond how much stemflow infiltrates where, into what uptakes it and from where. Addressing these questions is constrained by current observational and analytical methods. Nevertheless, by confronting these challenges, the stemflow research community stands to make significant strides in comprehending this unique hydrological component and situating it within the broader context of natural science.