Effects of forest gap and coarse woody debris on biomass and diversity of epixylous moss in an alpine forest

Both epixylous moss plant and coarse woody debris(CWD) are two essential components in most primary forest ecosystems, and their interaction exerts critical roles in carbon and nutrient cycles, soil and water conservation, and maintain biodiversity. Meanwhile, epixylous moss growing on the woody substrate is very sensitive to substrate quality change and microclimate as affected by canopy and CWD properties. However, the effects of gap position and CWD on epixylous moss plant communities in the forest ecosystem remains unknown. The study aimed to understand the effects of gaps and CWD type and decay class on epixylous moss communities. The biomass, Shannon, Simpson, and Pielou diversity indices for epixylous moss communities on fallen logs, large dead branches, snags, and stumps in the gap center, gap edge, and closed canopy were investigated in an alpine Minjiang fir(Abies faxoniana Rehd.) forest ecosystem in the upper reaches of the Yangtze River. The results showed that the biomass stock of epixylous moss on CWD totaled 141.14 kg/hm~2 in the studied alpine forest, and varied greatly with CWD type, decay class and gap position. The largest and smallest biomass stocks of epixylous moss were respectively observed on fallen logs(78.80 kg/hm~2) and snags(3.11 kg/hm~2). Meanwhile, higher stocks of epixylous moss were found on CWD with decay classes of III and IV, and lower stock on CWD with decay class I. In addition, the highest epixylous moss biomass per unit area on fallen logs, large dead branches, and snags were observed in the gap edge. The biomass accumulation of epixylous moss on CWD varied greatly with gap position and CWD type. The diversity of epixylous moss communities was also affected by forest gap position and CWD type. First, the Simpson index of epixylous moss communities grown on fallen logs, large dead branches, and stumps decreased from the gap center to closed canopy. Second, the highest and lowest Shannon and Pielou diversity indices for epixylous moss on fallen logs were observed in the closed canopy and gap edge, respectively. Third, the Shannon, Simpson, and Pielou diversity indices of epixylous moss communities on large dead branches in the gap center were higher than those on fallen logs. In addition, the diversity indices of epixylous moss on snags and stumps varied with gap position, and Dicranum and Neckera dominated the epixylic moss communities. The "edge effect" well explained the higher expixylous moss biomass in the gap edge