Dispersal and nutrient limitations of decomposition above the forest floor: Evidence from experimental manipulations of epiphytes and macronutrients

Abstract

Decomposition is a major component of global carbon cycling. However, approximately 50% of wood necromass and a small proportion of leaf litter do not contact the forest floor, and the factors that regulate the decomposition above the forest floor are largely untested. We hypothesized that separation from soil resources causes slower decomposition rates above the forest floor. Specifically, we tested whether slower decomposition results from decreased nutrient availability (the nutrient limitation hypothesis) and/or microbial dispersal limitation (the dispersal limitation hypothesis) in the absence of soil resources. We tested these hypotheses by combining experimental manipulations of epiphytes and macronutrient fertilization with elemental analyses and community metabarcoding (fungi and prokaryotes). Specifically, we compared wood stick and cellulose decomposition among three treatments: an unaltered trunk section, an epiphyte mat, and a ‘removal treatment’ where an epiphyte mat was removed to test the effect of soil resources. We also performed a factorial fertilization experiment to test the effects of nitrogen (N) and phosphorus (P) on the decomposition of suspended cellulose. Decomposition rates were fastest on the epiphyte mats, intermediate in the removal treatment and slowest in the controls. Phosphorus addition increased decomposition rates in the fertilization experiment, and greater P concentrations, along with some micronutrients, were associated with increased rates of decomposition on the epiphyte mats and in the removal treatments. Locally dispersed fungi dominated the wood stick communities, indicating that fungal dispersal is limited in the canopy, and fungal saprotrophs were associated with increased rates of decomposition on the epiphytes. These experiments show that slowed decomposition above the forest floor is caused, in part, by separation from soil resources. Moreover, our findings provide support for both the nutrient limitation and dispersal limitation hypotheses and indicate that mechanisms regulating canopy‐level decomposition differ from those documented on the forest floor. This demonstrates the need for a holistic approach to decomposition that considers the vertical position of necromass as it decomposes. Further experimentation is necessary to quantify interactions between community assembly processes, nutrient availability, substrate traits, and microclimate. A free Plain Language Summary can be found within the Supporting Information of this article.

Decay classes of coarse woody debris in a lowland Dipterocarp forest: implications for volume, density, and carbon estimates

“…This result is in line with earlier reported findings that in rainforests termite activity in deadwood depends on its vertical location (Law et al, 2019). Logs and branches on the ground are attacked and consumed by termites more quickly than suspended ones (Collins, 1981) as soil maintains a high moisture content and provides easy access for soil micro-and meso-fauna to colonize decomposing wood (Gora & Lucas, 2019). 2005).…”

“…This result is in line with earlier reported findings that in rainforests termite activity in deadwood depends on its vertical location (Law et al, 2019). Logs and branches on the ground are attacked and consumed by termites more quickly than suspended ones (Collins, 1981) as soil maintains a high moisture content and provides easy access for soil micro‐ and meso‐fauna to colonize decomposing wood (Gora & Lucas, 2019). Law et al, (2019) examined the relative contribution of macro‐invertebrates (predominantly termites) and microbes to initial decomposition of suspended and downed CWD in a lowland tropical rainforest in Malaysia.…”

Microsites and early litter decomposition patterns in the soil and forest canopy at regional scale

“…3a, b). Epiphytes and litter-trapping plants affect temperature in the canopy, increase humidity (Ortega-Solís et al, 2017;Scheffers et al, 2014;Stuntz et al, 2002), and mitigate dispersal limitations of decomposers while increasing their density and diversity (Vance and Nadkarni 1990;Paoletti et al 1991;Gora and Lucas 2019). Unlike branches, bromeliads represent humid habitats (Zotz et al 2020) in which the density of fungi, bacteria, and invertebrate decomposers can be higher than in terrestrial soil (Paoletti et al 1991;Pittl et al 2010).…”

Functional traits of fossil plants

“…It represents the economic trade‐off between investment in structural tissues, stem maintenance, and access to light (Falster & Westoby, 2003). Taller plants are linked to increased biomass, acting as carbon sinks (Moles et al ., 2009), and woody debris of taller plants such as trees slows decomposition (Gora et al ., 2019 and references therein). Taller canopies provide niche partitioning by opening up understory and aerial habitats.…”