Effects of forest gaps on the humification process of deadwood in the subalpine forest

Knowledge regarding forest gap effects on deadwood humification and humus accumulation remains unknown, although the decomposition and humification of deadwood, a structural component of forest ecosystems, play a crucial role in the global carbon cycle and soil organic matter accumulation. We conducted an in situ decomposition experiment of Abies faxoniana (Minjiang fir) to assess how gap positions (closed canopy, gap edge, and gap center) influence the humification and accumulation of humic substances (HS), humic acid (HA), and fulvic acid (FA) in bark, sapwood, and heartwood across five decay classes (I–V) during the warming (Feb–Aug) and cooling (Aug–Feb) seasons. Results showed that forest gaps primarily facilitated bark humification but inhibited heartwood humification. The interactive effects of gap position and decay class on HS, HA, and FA accumulation were weak across all wood tissues. Furthermore, under the closed canopy, the HS and FA amounts increased during the warming season and decreased during the cooling season for all wood tissues, whereas the opposite seasonal trend occurred in sapwood and heartwood in the gap center. Notably, bark in the gap center showed marked increases in HS, HA, and FA during the warming season. These findings suggest that forest gaps enhance the contribution of bark humification but reduce that of heartwood to soil organic carbon accumulation, and that seasonal warming further amplifies this forest gap effect, which has significant implications for predicting deadwood carbon cycling under climate change in subalpine forests.