Forest degradation caused by dwarf bamboo overabundance reduces soil C, N and P stocks in giant panda habitat

Abstract

Although forest degradation caused by dwarf bamboo overabundance has a significant effect on the survival of wild giant pandas, the degradation process and its impact on soil C, N and P stocks remain unclear. By investigating the vegetation structure and physicochemical analysis of litter and soil, we explored the changes in soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), total phosphorus (TP) and available phosphorus (AP) stocks and the driving forces at three degradation levels triggered by dwarf bamboo overabundance in an important giant panda habitat—the subalpine coniferous forest. Dwarf bamboo overabundance following forest logging prevented tree regeneration, resulting in simplified a vegetation structure; even after 23 years of natural recovery, the clear-cut forestlands remained in the shrub stage, and the tree density and basal area were only 1.6% and 38%, respectively, of those in primary subalpine coniferous forest. With increasing degradation intensity, the litter stock and litter quality showed a decreasing trend, as did the SOC, N and P stocks in the litter layer and topsoil. The vegetation structure and litter quality together explained more than 90% of the variance in the C, N and P stocks (SOC: 97.3%, TNsoil: 93.5%, ANsoil: 96.2%, TPsoil: 97.2% and APsoil: 97.0%), and litter quality had greater explanatory power than the vegetation structure. The SEM analysis results showed that the increase in dwarf bamboo density caused by the reduction in tree basal area affected the litter C/N and C/P ratios, and ultimately reduced the SOC, Nsoil and Psoil stocks. This study highlights that dwarf bamboo overabundance not only causes forest degradation but also reduces the carbon sequestration function of giant panda habitat, leading to the loss of soil N and P. We suggest that reducing bamboo abundance can promote the recovery of the soil carbon, nitrogen and phosphorus stocks in degraded giant panda habitats.