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.