Editorial: Bark-Water Interactions (Volume II)

John T Van Stan^1*Salli F Dymond²Anna Klamerus-Iwan³Julieta A Rosell⁴

• ¹Cleveland State University, Cleveland, United States
• ²Northern Arizona University College of the Environment Forestry and Natural Sciences, Flagstaff, United States
• ³Uniwersytet Rolniczy im Hugona Kollataja w Krakowie, Kraków, Poland
• ⁴Universidad Nacional Autonoma de Mexico Laboratorio Nacional de Ciencias de la Sostenibilidad, Mexico City, Mexico

Natural science has overlooked the "bark side" of the water cycle. Spanning millions of square kilometers, this complex surface is a major component of forest hydrology (Van Stan et al., 2021), but models of water, vegetation, and landscapes often treat it as an afterthought or give it a borrowed identity. For instance, bark's ability to intercept rainwater is often (enough) reduced to that of a leaf or "a soggy piece of blotting paper" (Van Stan and Simmons, 2025). Vegetation models currently lack bark-specific vapor conductance pathways, utilizing "stem desiccation through residual water loss from leaves, not bark" (Wolfe, 2020). Bark–xylem water exchange may also influence isotopic signals used to trace plant water sources—an unmodeled pathway with profound implications for ecohydrological inference (Nehemy et al., 2021; Treydte et al., 2021). Even bark hydrologic traits are shaped by legacy assumptions, e.g., hydrophobicity is typically tested with deionized water (see discussions in Tonello et al., 2021; Ossola and Farmer, 2024), as if precipitation were purified by the canopy rather than enriched by it. The result is trait values that assume clean droplets fall on clean bark (Noren et al., 2023). And when trees become woody debris, entering streams, models often render it as static blockage, ignoring bark's role in shaping sediment texture, routing organic matter, and altering channel dynamics. This second volume in the Bark–Water Interactions Community Series presents new work that reveals bark's overlooked potential to transform forest–water modeling routines, from vapor exchange (Ávila-Lovera and Winter, 2024) and isotope tracing (Vega Grau et al., 2025) to sediment dynamics (Słowik-Opoka et al., 2024) and surface tension physics (Noren et al., 2023). Noren et al. (2023) delivered the first field observations showing that rainwater exiting tree canopies can have lower liquid–vapor surface tension (γlv) than the rain that entered. Their work is connected to bark water storage capacity, a key variable in hydrologic and plant physiology models. As rainwater becomes enriched with canopy-derived solutes (e.g., dissolved organic carbon concentrations up to ~100 mg L-1) (Stubbins et al., 2020), its γlv may drop by 12–18 mN m-1 via Gibbs isotherm. This chemical shift has direct physical consequences—based on Young's equation, it can increase a droplet's contact angle on bark, potentially increasing water storage by 10–20% before drainage occurs. This creates a testable hypothesis (γlv as a driving variable in hydrologic models' canopy storage states) that suggests changes in rainwater chemistry can influence drainage dynamics, surface evaporation, and microbial habitats. Ávila-Lovera and Winter (2024) investigate bark conductance to water vapor (gbark) across Neotropical tree species and its implications for drought stress, finding it varies widely among species. Notably, even "low" gbark values (~5–20 mmol m-2 s-1) can represent a persistent drain on a tree's stored water. In practical terms, during drought this seemingly minor flux could equal the entire amount of water a stem recharges overnight. Compounded over several rainless days, this steady desiccation can theoretically lower midday stem water potential by an additional 0.1–0.2 MPa, pushing a tree closer to its hydraulic failure threshold. By showing that gbark varies predictably with relative bark thickness and temperature, the authors provide a clear path to incorporate this process into drought mortality models that may improve forecasts of when and why trees succumb to water deficit. Vega Grau et al. (2025) show that water held in bark in tropical forest species can differ from outer‑xylem water by ±4–10‰ δ2H during a morning transpiration surge, before equilibrating by midday. Alternatively, these isotope values may converge during other times of day, with similarity between xylem and bark interpreted as radial exchange between the two pools. Transient offsets in isotope ratios between xylem and bark suggest times at which there is and is not exchange, providing insights into a model of within-tree flows that counters classical perspectives. From the perspective of using plant-water isotope data to infer depth of water uptake, artefacts from co-extracting bark and xylem waters (e.g., +8‰) could lead to misattribution of uptake depths if there are small differences between shallow and deep soil water δ2H. Thus, by accounting for bark's influence, we may get a clearer picture of what is happening below ground. Even after death, bark and its associated wood continue to shape the water cycle. In a Carpathian mountain stream, Słowik-Opoka et al. (2024) found that accumulations of woody debris acted like natural sieves, trapping finer gravel and sand. Where debris piled up, mean grain size was about 8 mm in diameter; downstream, after the debris barrier, it averaged closer to 2 mm. These debris patches held modest amounts of organic matter (around 5–9 % by weight) depending on forest age. Thus, hypothetically during high-flow events, partial dam failures could release some of the stored sediment and organic material, creating brief pulses of turbidity and carbon export. Although commonly treated as fixed roughness, woody debris dynamically filters and then flushes sediments and nutrients. Together, these studies invite ecohydrologists to explore the "bark side" of the water cycle (from treetops to streambeds) and to recognize bark as hydrologically active infrastructure embedded in every forest stem. Bark emerges as a multifaceted regulator, depending on setting and conditions: it could release rainwater with altered surface physics, 'bleed' vapor during drought, skew the isotopic fingerprints used to infer plant water sources, and (via woody debris) moderate the routing of sediment and carbon in forest streams. Each process examined in this volume carries open questions: How dynamically does canopy chemistry modify interception storage? To what extent does bark transpiration tip trees from survival to mortality during drought? When should we deconvolve bark-water mixing from root uptake in isotope models? And, how often do woody debris jams switch from buffers to sudden sources of turbidity and nutrients? Addressing these uncertainties will require dedicated measurement, process‑based representation, and integration into models. As global changes intensify drought stress and alter precipitation patterns, bringing the "bark side" of the water cycle into mainstream hydrological science will close a long-standing, likely massive and thus critical gap in our understanding of how forests store, release, and transform water across scales.