Forest structure and tree species diversity across a disturbance gradient in evergreen broadleaved secondary forests

Abstract

To provide scientific evidence for vegetative recovery and biodiversity protection of evergreen broadleaved secondary forests, changes in stand structure and tree species diversity across a disturbance gradient of evergreen broadleaved secondary forests in Jiangle Forest Farm, Fujian Province were studied. The representative locations with similar site condition were selected through the field surveys, then a total of 25(0.04-0.12 hm2) permanent sample plots were set using typical sampling methods for evergreen broadleaf secondary forests. Five disturbance gradients (Ⅰ-Ⅴ with Ⅴ having the most disturbance) were assigned based on the relative value of major disturbance indicators. Then using ANOVA and LSD multiple comparision to analyze changes in stand structure and species diversity along a disturbance gradient. Results showed that for the five disturbance gradients, individual tree diameter conformed to an inverse J-shape curve. The individual number of Ⅴ disturbance gradient was significantly lower than other disturbance gradients (P=0.022) indicating the lowest capacity of natural regeneration and productivity. The individual tree heights of Ⅰ-Ⅳ disturbance gradients conformed to a normal curve; whereas, the Ⅴ disturbance gradient conformed to a bimodal curve indicating the lower stability in the forests vertical structure. With an increasing disturbance gradient, uniform angle indexes(P=0.045) increased, but neighborhood comparison indexes(P=0.039) and mingling indexes(P=0.017) decreased. The alpha diversity measures declined(P < 0.05) along the disturbance gradient as dominance increased(P=0.000). In addition, the similarity in species composition between each pair of disturbed forests was generally low (Jaccard's similarity index < 47%). With an increase in disturbance gradient, average DBH, average tree height, and alpha diversity measures decreased(P < 0.05), but the dominance index increased(P=0.000). Overall, as the disturbance gradient increased in the evergreen broadleaf secondary forests, the diversity index and stability of the forest structure decreased, which was not conducive to vegetative recovery and species diversity.