Forest gaps accelerate the degradation of cellulose and lignin in decaying logs in a subalpine forest

Knowledge of the patterns in the degradation of cellulose and lignin in decaying logs under different climatic conditions is necessary to understand the carbon cycling in forest ecosystems since cellulose and lignin are the dominant carbon fractions in decaying logs. However, the effects of the environmental heterogeneity by variation in canopy cover on the degradation of cellulose and lignin in decaying logs remain unknown. To evaluate the effect of forest gaps, we measured the concentration and mass loss of cellulose and lignin in experimentally exposed decaying logs in gaps, at the edge of gaps, and under the closed canopy during a four-year in situ log incubation experiment in a subalpine Minjiang fir (Abies faxoniana) forest on the eastern Qinghai-Tibet Plateau, China. We found the lowest cellulose concentration in gaps for all wood tissues, while we observed the opposite for lignin except for bark. Based on the linear relationship between cellulose and lignin concentrations, the rate of lignin enrichment in bark and heartwood was slowest in gaps and fastest under closed canopy, while in sapwood gaps had almost no influence. The mass loss of cellulose was highest in gaps for all wood tissues in any decay classes. The gap effect on lignin mass loss in decaying logs showed high variability with wood tissues and decay classes. Our results show that forest gaps accelerate the degradation of cellulose and lignin during the decomposition of logs by increasing cellulose mass loss and slowing lignin enrichment. The observed gap effects were mediated by wood tissues but not by decay classes.