The outer bark of tree branches and stems (i.e., phellem or rhytidome) constitutes a critical interface between the atmosphere, water, and vegetation that has important implications for the cycling of materials in woody plant-dominated ecosystems. Bark is a passive receptor surface to which materials deposit during precipitation (wet deposition) and via dry deposition. Some fraction of these materials can sorb to or be absorbed by bark surfaces, resulting in retention. Materials also leach from bark surfaces, moving through the bark into external solution that drains to the surface during storms. Growing on and within bark surfaces, epiphytic plants (e.g., mosses, ferns, and bromeliads) intercept, retain, and leach substances (Mendieta-Leiva et al., 2020), while bark-dwelling microorganisms and fauna produce, transform, and decompose materials (Aguirre-von-Wobeser, 2020). Exchanges between epiphytic communities and their substrates create additional pathways for material cycling within and below canopies. Thus, bark surfaces directly and indirectly influence the physical, chemical, and biological characteristics of water flowing through woody canopies (Ponette-González et al., 2020).

Water that drips from leaves, twigs, and branches (throughfall) and that flows down tree stems (stemflow) washes canopy surfaces, integrating deposition, retention, and leaching processes and the outcomes of bark-epiphyte interactions (Decina et al., 2020). As such, understanding the complete network of surfaces—including the non-leafy components—that links the top of the canopy to the soil is critical for a more complete and comprehensive view of how woody plants alter biogeochemical inputs to soils and the potential consequences for ecosystem functions, such as carbon and nutrient cycling (Van Stan et al., 2021a).

Bark exhibits a diverse array of physical and chemical properties that affect the chemistry and composition of waters draining over its surface (Oka et al., 2021). Importantly, bark can comprise a significant proportion of the total plant or ecosystem surface area available for passive interception and active exchange (i.e., uptake and leaching) of materials. Early estimates from temperate deciduous forest indicate that branch and stem bark surface areas combined range from 1.5 to 2.8 m² per m², while leaf surface area ranges from 3 to 6 m² per m² (Whittaker and Woodwell, 1967). Recent estimates from temperate evergreen coniferous forest dominated by redwood (Sequoia sempervirens) show exceptionally high and nearly equivalent bark and leaf surface areas (Sillett et al., 2019). In other words, as much as 30–50% of the total plant or ecosystem surface area exposed to the atmosphere (and precipitation) is bark. While the relative importance of the bark interface varies spatially due to species- and community-specific differences in outer surface areas, the bark interface varies temporally as well. The ratio of bark to leaf surfaces increases with tree age (Whittaker and Woodwell, 1967), during periods of leaf abscission, and after disturbances (e.g., hurricanes and insect outbreaks) that result in partial or complete canopy defoliation. Bark surfaces are also more temporally persistent than leaves (Van Stan et al., 2021a), meaning that they accumulate and exchange materials continuously over multiple seasons, years, and often over the entirety of a plant’s life.

Bark surfaces are rough, porous, hygroscopic (absorb and retain water), and sorptive (Supplementary Figure 1), characteristics that influence deposition, leaching, and retention and interactions with epiphytes. The sorptive properties of bark and its effectiveness at removing metal ions from aqueous solution has resulted in growing interest in using bark in water and wastewater treatment (Şen et al., 2015). The hygroscopicity of bark is of relevance as it represents a potentially significant component of total bark water storage. In temperate forests, water adsorbed from the atmosphere during dry periods can constitute 10–30% of maximum bark water storage capacity, with values exceeding 60% at humid forest sites (Ilek et al., 2016, 2021). These findings suggest that bark surfaces with lower hygroscopicity will retain more water during storms, increasing water residence time and opportunities for canopy exchange on bark. Surface roughness is another bark property affecting both bark water storage and dry deposition. As is the case with leaves and whole canopies (Rindy et al., 2019), increased roughness enhances particulate capture (Oka et al., 2021). Deposited particulates can wash off bark surfaces or accumulate within porous bark “traps” (Magyar et al., 2021; Supplementary Figure 1). Additionally, some tree species, such as paper birch (Betula papyrifera) and copperwood (Bursera simaruba), undergo periodic exfoliation. Bark shedding releases materials retained on and in bark tissues and leads to renewal of the bark surface. Finally, bark surfaces are diverse in chemical and elemental composition. Species differ in resource allocation to chemical defenses against insects, pests, and pathogens (e.g., Franceschi et al., 2005) and may translocate elements such as manganese to the bark to avoid toxic concentrations in leaves and other tissues (Hauck and Paul, 2005). Taken together, the structural heterogeneity and complex composition of bark give rise to unique associations with flora and fauna that in turn participate in material cycling and alteration of water quality.

The role and potential significance of the outer bark in atmosphere-water-vegetation interactions is often examined qualitatively or overlooked in field and modeling studies (Butler et al., 2020; Pace and Grote, 2020). In many fields, leaves still rule. As a result, the processes of deposition, retention, leaching, and washoff are relatively well described for leaves, but not so for bark. This precludes our ability to fully understand how woody plants influence the quality of water transported within (branch bark) and below (stem bark) tree canopies. In this mini-review, I briefly explore diverse perspectives and approaches to the study of bark and what they reveal about the myriad ways bark surfaces influence the chemistry and composition of sub-canopy precipitation.

Bark interactions with the atmosphere, water, and vegetation have been the subject of research in the fields of environmental science, ecohydrology, epiphyte ecology, and microbiology for over five decades (e.g., Staxäng, 1969; Johnsen and Søchting, 1976; Farmer et al., 1991). A review of selected peer-reviewed literature indicates that within these fields bark is conceptualized as: (1) accumulator and biomonitor of atmospheric pollution; (2) transport surface; (3) substrate for epiphytic communities; and (4) reactor.

A Web of Science search for articles published between 1945 and present conducted with the terms “bark” and “biomonitoring” (n = 148), “bark” and “stemflow” (n = 122), and “bark” and “epiphytes” (n = 212) suggests that bark is most often considered in the context of plant ecology. Less than 10% of the articles identified in the “bark” and “epiphytes” group focused on microorganisms and animals. While the number of publications in all research areas has grown steadily since 1976 (Supplementary Figure 2), there are important differences in where, at what scale, and how bark is studied (Figure 1).

Bark biomonitoring often takes place within urban and industrial areas, where atmospheric pollution is a health and environmental concern. Bark is considered advantageous in biomonitoring research given its widespread distribution, accessibility, and capacity to intercept and retain pollutants. Indeed, bark is widely used to map and measure past and present impacts of airborne pollution downwind of point sources (e.g., incinerator and smelter; Cocozza et al., 2016) and near city and heavily trafficked roads (e.g., Catinon et al., 2012). The objective often is to monitor pollution changes over long-time frames or on local scales (e.g., Guéguen et al., 2012)—that is, within kilometers of major emissions sources. Most studies that utilize tree bark as a passive biomonitor determine trace element concentrations in bark tissue, with heavy metals such as lead, copper, cadmium, mercury, and uranium of particular interest due to their toxicity to human populations (e.g., Fujiwara et al., 2011; Chiarantini et al., 2016). For example, Flett et al. (2021) determined uranium concentrations in tree bark on tribal lands in the western United States and found that these were highest along an abandoned mine access road and near a mill where uranium was processed; concentrations decreased with increasing distance from pollution hotspots. The effectiveness of bark relative to other biomonitors such as leaves, lichens, and mosses has also been examined. Comparisons of multiple biomonitors consistently show that pollutant concentrations decrease in the order lichen/moss > bark > leaf surface > leaf wax (e.g., Cucu-Man and Steinnes, 2013).

Insights on how bark alters stemflow material inputs to soils generally derive from measurements conducted in non-urban forests. In this context, stemflow water is generally collected and analyzed for dissolved inorganic nutrients, organic carbon, organic nitrogen, and pH. While bark characteristics influence solute chemistry and fluxes, beyond the role of bark in altering stemflow water volumes, it is unclear how (Levia and Germer, 2015). In ecohydrological research, rarely are concentrations in bark leachate compared to those in stemflow (but see Tucker et al., 2020).

Ecologists and microbiologists frequently investigate bark-epiphyte interactions in non-urban forested environments. For example, in the northeastern United States, United Kingdom, and Sweden, pollution gradient studies underscore the effects of increasing pollutant deposition and acidification on bark substrates and epiphytic lichens and bryophytes along their length (e.g., Schmull et al., 2002; Mitchell et al., 2005; Fritz et al., 2009). In contrast to studies focused on epiphytic vegetation, we are only beginning to understand the role of bark and resident epiphytes as habitat for microorganisms and faunal communities (e.g., Aguirre-von-Wobeser, 2020). To understand the various factors influencing epiphytic microbial, plant, and animal communities, it is not uncommon for researchers to collect samples from diverse environmental matrices, including bark, precipitation (rainfall, throughfall, and stemflow), epiphytic tissue, and in some cases soils, for analysis of nutrients and pollutants (e.g., Farmer et al., 1991). Despite differences in scale, approach, and method, knowledge gained from all research areas elucidates how bark can influence biogeochemical cycling in woody ecosystems now and in the future.

Bark is a passive accumulator, an active transport surface, a substrate, and a reactor. Although bark has been shown to accumulate considerable amounts of particulates, the contribution of bark to total nutrient and pollutant loading has not been quantified. Further, we know little about exchanges of both water, nutrients, and pollutants across tree branch and stem surfaces. In the future, changes in atmospheric composition, precipitation, and disturbance regimes will alter what is deposited to bark surfaces, as well as woody plant species composition and species’ expression of bark. The latter may occur via changes in the age structure of stands or intraspecific variation in bark structure and chemistry. In sum, the interaction of bark and stormy conditions may represent a critical influence on the accumulation, exchange and transport of elements between atmosphere, water, and vegetation.

AGP-G conceived the mini-review, reviewed the literature, created the figures, and wrote the manuscript.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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