Arboreal Epiphytes in the Soil-Atmosphere Interface: How Often Are the Biggest “Buckets” in the Canopy Empty?

Abstract

Arboreal epiphytes (plants residing in forest canopies) are present across all major climate zones and play important roles in forest biogeochemistry. The substantial water storage capacity per unit area of the epiphyte "bucket" is a key attribute underlying their capability to influence forest hydrological processes and their related mass and energy flows. It is commonly assumed that the epiphyte bucket remains saturated, or near-saturated, most of the time; thus, epiphytes (particularly vascular epiphytes) can store little precipitation, limiting their impact on the forest canopy water budget. We present evidence that contradicts this common assumption from (i) an examination of past research; (ii) new datasets on vascular epiphyte and epi-soil water relations at a tropical montane cloud forest (Monteverde, Costa Rica); and (iii) a global evaluation of non-vascular epiphyte saturation state using a process-based vegetation model, LiBry. All analyses found that the external and internal water storage capacity of epiphyte communities is highly dynamic and frequently available to intercept precipitation. Globally, non-vascular epiphytes spend <20% of their time near saturation and regionally, including the humid tropics, model results found that non-vascular epiphytes spend~1/3 of their time in the dry state (0-10% of water storage capacity). Even data from Costa Rican cloud forest sites found the epiphyte community was saturated only 1/3 of the time and that internal leaf water storage was temporally dynamic enough to aid in precipitation interception. Analysis of the epi-soils associated with epiphytes further revealed the extent to which the epiphyte bucket emptied-as even the canopy soils were often <50% saturated (29-53% of all days observed). Results clearly show that the epiphyte bucket is more dynamic than currently assumed, meriting further research on epiphyte roles in precipitation interception, redistribution to the surface and chemical composition of "net" precipitation waters reaching the surface.

Leaf trait co‐variation and trade‐offs in gallery forest C3 and CAM epiphytes

“…Epiphytes and their associated decomposing litter and substrate play an important role in nutrient cycling of tropical forests, eventually transferring intercepted aerial nutrient supplies to the forest floor (Gotsch et al., 2016; Nadkarni & Matelson, 1992). They also modify host tree topography providing habitats and food for many organisms (Gotsch et al., 2016), influencing forest hydrological processes (Hargis et al 2019) and tree crown microclimate (Stuntz et al., 2002).…”

Stand-Level Variation Drives Canopy Water Storage by Non-vascular Epiphytes Across a Temperate-Boreal Ecotone

“…As with leaves and bark (Eller et al, 2013;Holder, 2013;Klamerus-Iwan and Błońska, 2018), the surface hydrophobicity of lichens and bryophytes can vary not just across species, but also with hydration status, with dry thalli often more hydrophobic than those "primed" by high humidity or wetting (Lakatos et al, 2006). Epiphytes may also still be retaining water from previous precipitation or fog (Hargis et al, 2019). As such, the humidity conditions preceding a rain event may impact the amount of water retained by epiphytes.…”

Vascular epiphytes of the Serra do Relógio: the surprising richness of the seasonal forests in the mountains of Minas Gerais, Southeastern Brazil

“…They play an important role in ecosystems, providing food, water, and shelter, and are even compared to small ecosystems (Rommel & Baights 1999;Zanin & Tusset 2007;Seidl et al 2020). Epiphytes also contribute to maintaining nutrients and local air moisture (Benzing 1998;Hargis et al 2019), and this characteristic can be highly relevant in the dry period in forests such as seasonal semi-deciduous forests (SSF) (Mestre et al 2001). About 9% of all vascular plants in the world are epiphytes (Zotz 2013), while this proportion is approximately 15% in the Atlantic Forest (Freitas et al 2016), and this number can be up to 50% of the total in smaller scales (Kelly et al 1994).…”