Assessing the Impacts of Livestock-Induced Disturbance on the Vegetation Composition and Forest Regeneration of Sikkim's Mid-Elevation Forests

Abstract

1. Livestock rearing earlier contributed sizeably to Sikkim’s economy, thus putting immense pressure on natural forests, which were converted into artificial pastures. These forests have not been subjected to evolutionary grazing pressure and differ in their sensitivity to the effects of livestock grazing, as compared to the historically-grazed tropical grasslands and rangelands. 2. This study aims to assess the impacts of livestock-induced disturbance on the vegetation and forest regeneration of the Sikkim Himalayan mid-elevation forests. 3. We sampled vegetation in and around pastures under different levels of livestock grazing pressure - from currently grazed to abandoned; and from heavily grazed to lightly grazed. 4. We found highest species richness and diversity values in abandoned pastures, as compared to areas which are currently grazed as well as areas which have not had a historical pasture presence. We found highest species richness and diversity values in abandoned pastures, as compared to areas which are currently grazed as well as areas which have not had a historical pasture presence. 5. We found significant difference between plant species composition among various livestock grazing levels. Species composition differed due to (1) grazing-tolerant herbaceous species, and (2) encroaching shrub or bamboo at the pasture boundaries in the absence of livestock as a control. 6. Seedling densities were overall lower in actively grazed pastures than in abandoned pastures. Seedling densities for buffer forested areas of the different livestock grazing levels were comparable, apart from the lightly grazed treatment, which had very low seedling densities. Sapling densities was higher in and around lightly grazed pastures, than the other livestock grazing levels; this contradicts the trend seen in seedling densities. 7. Seedlings and saplings in lightly grazed areas were from the same species pool; we propose that the decrease in the former and the increase in the latter’s densities is due to successful recruitment of the seedlings to the sapling stage. 8. Livestock grazing level, slope, shrub volume and position of plot (either within the pasture area or in the buffer forested area) explained seedling density. Seedling densities were highest in abandoned pastures, low in areas with high slope values, low in areas with high shrub volume and higher in buffer forested areas, compared to pastures. 9. Livestock grazing levels, slope value, shrub volume and position of sapling (wither in buffer forested areas or pasture areas) influenced sapling densities 10. Areas with and without active livestock grazing had similar proportions of unpalatable – palatable species; suggesting that direct herbivory may not be an important regulation by which density of regenerating is affected. Instead, other indirect livestock-related impacts, such as trampling or dung deposition, could lead to changes in regeneration metrics. 11. Successful regeneration occurs across the different livestock grazing levels, however the composition of regenerating species differs. Restoration of disturbed forests by artificial regeneration may not be required, as it may interfere with natural successional dynamics already in motion.