Throughfall and stemflow are major hydrologic highways for particulate traffic through tree canopies

Abstract

For one‐third of Earth’s land surface, precipitation passes through tree canopies (as throughfall or stemflow) before entering watersheds. Over a century of research has described fluxes of water and solutes along these “hydrologic highways”, yet little is known about their “traffic” – that is, the organisms and nonliving particulates frequently discarded from water samples after filtration in the lab. A comprehensive understanding of the composition of sub‐canopy precipitation is necessary to estimate the total nutrient and pollutant inputs to watersheds for redistribution downstream, as well as to systematically investigate precipitation effects on organismal exchanges along the atmosphere–plant–soil continuum. Here, we review current concepts and research showing that the hydrologic highways from tree canopies to soil carry ecologically relevant quantities of biologic (viruses, microbes, microfauna, and meiofauna) and abiotic particulates. Their fate may have important consequences for the biogeochemistry and biodiversity of terrestrial systems.

Under the canopy: disentangling the role of stemflow in shaping spatial patterns of soil microbial community structure underneath trees

“…3). Hypothetically, microbial taxa that decrease in abundance with proximity to the stem may represent bacteria that are either outcompeted by stemflow-favoured bacteria or killed by stemflow, perhaps by hydrolysis or salinity-related lysisboth of which are plausible given chemically concentrated and/or voluminous stemflow (Van Stan and Gordon, 2018). Additionally, if stemflow promotes the growth of other microbes (i.e.…”

“…Throughfall is the portion of rainwater that drips from the canopy or passes through gaps, while that which drains from outlying crown areas to the stem base is stemflow. Globally, most trees' mean annual stemflow represents <2% of rainfall (Van Stan and Gordon, 2018); however, even a small amount of rainfall (e.g. $0.1 cm) over a single tree's canopy (e.g.…”

“…$0.1-10 m 2 ) in a single storm (Van Stan and Allen, 2020). Because stemflow drains down substantial bark surface area, it can also become significantly chemically enriched from materials on and within the bark (Van Stan and Gordon, 2018). Thus, for over a century [since Riegler, 1881] the stemflow process has been reported to be able to magnify small fractions of rainwater across a tree canopy by 10-100 times at the surface surrounding tree stems (Herwitz, 1986;Friesen and Van Stan, 2019).…”

Accumulator, Transporter, Substrate, and Reactor: Multidimensional Perspectives and Approaches to the Study of Bark

“…Bark accumulates particulates from the atmosphere and plant canopies (Van Stan et al, 2021b), including organic matter (e.g., pollen and microbes), crustal matter (i.e., dust), and pollutants (e.g., heavy metals). It has been estimated that ∼80% of a tree's bark surface deposit is organic matter, with the remaining 20% comprising similar amounts of crustal and anthropogenic particulates (Catinon et al, 2009).…”

Routing stemflow water through the soil: a dual labelling approach with artificial tracers

“…Recent research has unveiled biotic and abiotic factors that regulate stemflow production before it is funnelled belowground (Levia 35 and Germer, 2015;Cayuela et al, 2018;Zhang et al, 2021). Although stemflow is not a major proportion of the overall catchment water balance, it can saturate near-stem soil and may enhance overland, preferential or subsurface flow, contribute to soil water replenishment and groundwater recharge, produce soil erosion (Levia and Germer, 2015), transport chemical, particulate and biological materials (Van Stan et al, 2021), and influence root water uptake (Hildebrandt, 2020). Due to its importance in soil hydrology, biogeochemistry and ecohydrology, stemflow has become an active topic of research in recent 40 years (see Levia et al, 2011;Van Stan et al, 2020).…”