Impact of forest cover and land use change on tree species diversity and carbon stocks in Shivapuri Nagarjun National Park, Nepal

The relationship between tree species and carbon stock is significantly influenced by changes in forest cover and quality. Forests, as renewable resources, are facing rapid decline, leading to land degradation and biodiversity loss. In this study, field observation, mapping, and sampling were conducted across 63 blocks with dimensions of 10 × 10 m for trees and 5 × 5 m for sapling. Secondary materials and Landsat images, sources from the United States Geological Survey (USGS) website, provided valuable spatial data. Data analysis employed ArcGIS and Microsoft Excel, incorporating calculations for the Normalized Difference Vegetation Index (NDVI), Land use and land cover (LULC), carbon stock, and biodiversity. The study revealed an increase in forest cover from 2001 (703.73 hectares) to 2022 (955.8 hectares), with an intermediate cover of 830.88 hectares in 2013. The highest NDVI value was recorded in 2001 at 0.65, which then decreased to 0.41 in 2013 and rose slightly to 0.47 in 2022. Likewise, high-vegetation land (HVL) cover decreased from 76.3% in 2001 to 11.8% in 2013 but increased again to 41.7% in 2022. This drastic decline followed by significant recovery in HVL reflects the complex interplay of human activity, policy, and natural factors. The recovery to 41.7% by 2022 suggests a positive trajectory, highlighting the effectiveness of recent conservation and restoration efforts. The analysis revealed distinct trends in species prevalence, diversity, and richness. Alnus nepalensis was notably the most prevalent species (18.2%), while Quercus semicarpifolia was the least (2.55%). Diversity patterns varied significantly, with block 16 showing the highest Shannon–Wiener index (4.8), reflecting balanced species composition, while blocks 57 and 58 had no diversity (0.0). Conversely, these same blocks showed the highest Simpson index (1.0), indicating complete dominance by a single species. Exceptional cases include blocks 11, 16, and 23, which exhibited the highest species richness (7.0), contrasting sharply with the lowest richness in blocks 57 and 58. These trends highlight considerable spatial variability in forest structure, with blocks 57 and 58 standing out for their low diversity and richness but high dominance. The total aboveground biomass (AGB) and carbon stock across all 63 blocks were 255.83 tons/ha and 120.24 tons/ha, respectively, with mean and standard error values for AGB and carbon stock at 4.06 ± 0.45 and 1.91 ± 0.21, respectively. The study highlights the long-term ecological value of Shivapuri-Nagarjun National Park, especially due to its proximity to the capital city, where it provides essential natural resources such as clean water and air to the urban population. The park also serves as a vital habitat for a wide variety of wildlife. Government efforts to protect these species and maintain a healthy ecosystem are crucial. Further research focusing on GIS and remote sensing could provide valuable insights into the park’s conservation efforts on both spatial and temporal scales.