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.

We took advantage of the LOGLIFE project (Cornelissen et al. 2012), in which freshly cut stems of 13 temperate tree species were left to decompose in a common garden experiment. Ten tree species were incubated in February 2012, and three tree species were added in February 2015. Firstly, five individual trees were cut for each tree species and distributed to five blocks (i.e. forest plots) to decay. From trunk parts without major side branches five logs were cut per individual tree and treated as replicates, each with 1 m length and a diameter of 25 ± 3 cm, thus assuring a similar exposed stem cross-sectional area and substrate volume accessible to decomposers. These logs obtained from the same tree shared similar physical-chemical traits. One of the five logs was randomly harvested after one and four years of decay, respectively, and used for wood density measurement and molecular sampling. Corresponding data are archived here.