Characteristics and intrinsic influencing factors of log humification depend on wood traits in a subalpine forest

Deadwood is an indispensable component of forest ecosystems, with important consequences for global carbon cycling. Although research into carbon release from decaying logs and its response to wood traits has increased, little attention has been paid to how wood traits affect the potential of carbon sequestration in decaying logs through humification. To evaluate the wood trait effects, we measured the concentrations (mg g-1) and content (mg cm-3) of humus, including humic substrates, humic acid, and fulvic acid, and the degree of humification (%) separately in bark, sapwood, and heartwood in a four-year log decomposition experiment in a subalpine Minjiang fir forest on the eastern Qinghai-Tibet Plateau, China. We found the highest humus concentrations and humi-fication degree in bark and the lowest in sapwood. The humus mass had a net loss in bark and did not change in sapwood over time. As decomposition proceeded, the humus mass in heartwood firstly accumulated and then decreased. In addition, wood traits strongly affected the humification of sapwood and heartwood compared to bark. Lignin and its interactions with cellulose and N concentrations were associated with sapwood humification. Nutrient conditions like N and P concentrations and the C/N ratio were related to heartwood humification. These findings suggest that wood traits can determine the potential of carbon sequestration of decaying logs through humification and that the large proportion of sapwood in decaying logs has a positive effect on long-term and stable carbon sequestration in poor soils in subalpine forests.