Precipitation Partitioning—Hydrologic Highways Between Microbial Communities of the Plant Microbiome?

Abstract

There are multiple distinct habitats for microbiota inhabiting the plant microbiome (phyllosphere, endosphere, litter, rhizosphere) and habitats that act as additional sources (and sinks) of microbes and nutrients for the plant microbiome (atmosphere, pedosphere, bedrock, and fauna). These habitats harbor distinctive microbial communities that differ in structure, composition, function and spatiotemporal dynamics. Each habitat also differs in the mechanisms that provide "gateways" of exchange of microbes (and microbial products) between two communities, or in their access to "highways" that connect multiple communities. Of the environmental processes driving microbial community exchanges, precipitation events seem to represent the only one highway that can connect all the above-mentioned habitats-the "hydrologic highway." When precipitation contacts plants, it is partitioned into interception (water stored on, and evaporated from, plant surfaces), throughfall (water that drips from canopy surfaces and through gaps), and stemflow (water that is drained down the stem). This chapter describes the ways that precipitation partitioning in vegetated ecosystems (into interception, throughfall and stemflow) may connect microbial communities from the top (atmospheric boundary layer) to the bottom (bedrock face) of the critical zone via these hydrologic highways.

Hypothesis and Theory: Fungal Spores in Stemflow and Potential Bark Sources

“…This suggests that stemflow conidia fluxes may represent "hot spots and moments" of fungal dispersal to litter, soil, and potentially root areas. Stemflow infiltration areas range from 10 −1 -10 1 m 2 tree −1 (Van Stan and Allen, 2020) and stemflow pulses have been found to influence litter layers (Tanaka et al, 1991;Iida et al, 2005;Rashid and Askari, 2014) and drain throughout habitats of the plant microbiome, including the rhizosphere and pedosphere (Van Stan et al, 2020b).…”

“…Within tree canopies, branchflows can enable exchanges between the tree's external and internal microbial communities (Aung et al, 2018), or spread pathogens through resident animal communities (e.g., D' Amico and Elkinton, 1995). Branchflows, and their suspended and dissolved constituents, that contribute to dendrotelmata can affect mosquito control and plant health (Carpenter, 1982;Van Stan et al, 2020b). Before stemflow reaches the ground, it may serve as a drinking water source for canopy-dwelling animals, e.g., koalas (Mella et al, 2020).…”

Bark-Water Interactions Across Ecosystem States and Fluxes

“…Bark is both spatially expansive and temporally persistent and, therefore, its interactions with the hydrologic cycle may be as well. Regarding the spatial extent, if we estimate bark's global surface area from the same type of land surface model input data used to estimate global leaf surface area (i.e., Vorholt, 2012), then the bark surface is nearly as large as the Asian continent, ∼41 million km 2 (Van Stan et al, 2020). The surface area of this "bark continent" is likely an underestimate as it is based on stem area index (SAI) of standing plants (Figures 1a,b), which does not include the added surface area due to bark surface structural complexity or due to bark on fallen woody debris -see Fang et al (2019), and references therein, regarding SAI estimation methods.…”

Conceptual analysis: What signals might plant canopies send via stemflow?

“…Regardless of the technique, the view from climbers and remote sensing both reveal that, like watersheds, the tree canopy environment is spatially and temporally heterogenous (e.g., Merrick et al, 2021), resulting in a diversity of canopy microclimates (Ehbrecht et al, 2019) and microhabitats (Larrieu et al, 2018). During precipitation and condensation events, the heterogeneity of forest canopy structure can form complex water runoff patterns that may drain across/through a diversity of canopy microhabitats (Van Stan et al, 2020b). Like a watershed's soils, branches and bark pore spaces can generate a complex network of heterogenous pores that may act as an accumulator, transporter, substrate, and reactor for draining waters and their suspended and dissolved materials (Ponette-González, 2021) (Figure 1).…”

“…Within these arboreal soils, on the leaf and bark surfaces, and throughout the various microhabitats, is a wide range of microbes (Koskella, 2020;Looby et al, 2020), flora ( Van Stan and Pypker, 2015;Mendieta-Leiva et al, 2020) and fauna (Nadkarni, 1994) (Figure 1). These canopy communities exist at a complex interface that modulates interactions between the atmosphere and the internal physiology of the tree ( Van Stan et al, 2020b). Given these properties and processes, the forest canopy is a highly complex and difficult-to-access system that poses substantial challenges to interested researchers.…”