Anthropogenic disturbances shift diameter distribution of woody plant species in Shorea robustaGaertn. (Sal) mixed forests of Nepal

Assessing diameter distribution is an approach of observing regeneration strategies and impacts of anthropogenic disturbances on forest structural attributes. In this study, we evaluated the differences in diameter distributions in highly disturbed, moderately disturbed, and undisturbed forest sites of Buffer Zone Community Forests (BZCFs), viz. Janajagaran BZCF, Musharni Mai BZCF, and Radha Krishna BZCF of Central Terai, Nepal. Weibull and gamma distributions were fitted to explain changes in frequency distribution of diameter data collected. We observed a significant decrease in species richness (overall: p < 0.001) and density of smallest diameter class (0–10 cm diameter at breast height) stems (overall: p < 0.001) with increasing disturbances. Similarly, increase in median diameters (p < 0.01) and parameters of Weibull’s and gamma distributions along gradients from undisturbed to highly disturbed forest sites (except between moderately disturbed and undisturbed sites of Musharni Mai BZCF) were observed. Bowley coefficient of skewness was lower in all highly disturbed sites of BZCFs showing reduction in right skewness of the distribution curves with disturbances. These results indicate anthropogenic disturbances shift diameter structure of forests stands, negatively affecting abundance of small diameter–sized woody species and shifting Shorea robustamixed forests towards even-aged stands.