Stem decomposition of temperate tree species is determined by stem traits and fungal community composition during early stem decay

Dead trees are vital structural elements in forests playing key roles in
the carbon and nutrient cycle. Stem traits and fungal community
composition are both important drivers of stem decay, and thereby affect
ecosystem functioning, but their relative importance for stem
decomposition over time remains unclear. To address this issue, we used a
common garden decomposition experiment in a Dutch larch forest hosting
fresh logs from 13 common temperate tree species. In total 25 fresh wood
and bark traits were measured as indicators of wood accessibility for
decomposers, nutritional quality, and chemical or physical defense
mechanisms. After one and four years of decay, we assessed the richness
and composition of wood-inhabiting fungi using amplicon sequencing and
determined the proportional wood density loss. Average proportional wood
density loss for the first year was 18.5%, with further decomposition
occurring at a rate of 4.3% yr-1 for the subsequent
three years across tree species. Proportional wood
density loss varied widely across tree species in the first year
(8.7-24.8% yr-1) and subsequent years (0-11.3% yr-1). The variation was
directly driven by initial wood traits during the first decay year, then
later directly driven by bark traits and fungal community
composition. Moreover, bark traits affected the
composition of wood-inhabiting fungi and thereby indirectly
affected decomposition rates. Specifically, traits
promoting resource acquisition of the living tree, such as wide conduits
that increase accessibility and high nutrient concentration, increased
initial wood decomposition rates. Fungal community composition, but not
fungal richness explained differences in wood decomposition after four
years of exposure in the field, where fungal communities
dominated by brown-rot and white-rot Basidiomycetes were linked to higher
wood decomposition rate. Synthesis. Understanding what drives
deadwood decomposition through time is important to understand the
dynamics of carbon stocks. Here, using a tailor-made experimental design
in a temperate forest setting, we have shown that stem trait variation is
key to understanding the roles of these drivers; Initially, wood traits
explained decomposition rates while subsequently, bark
traits and fungal decomposer composition drove
decomposition rates. These findings inform forest management with a view
to selecting tree species to promote carbon storage.