Tree density, distribution and regeneration status in relation to soil quality at different alpine treeline regions of North-west Himalaya

The Himalayan alpine treeline possesses a unique identity and plays a vital role in the ecosystem. This study explores the relationship between soil quality and the distribution, diversity, and regeneration patterns of tree species in the alpine treeline regions of Uttarakhand Himalaya. The research focuses on five different treeline sites in Uttarakhand: Dayara Bugyal, Tungnath, Valley of Flowers, Ali-Bedni Bugyal, and Khaliya Top. Tree diversity and regeneration sampling in the treeline region were conducted by laying out 0.01 hectares quadrats, which were selected using the belt transect method along the treeline and soil samples were collected from each quadrate at 0-15 and 15-30 cm soil depths. The Rhododendron campanulatum, Quercus semecarpifolia, Abies spectabilis and Betula utilis are predominant in the treeline region of Uttarakhand Himalaya. Analysis of tree regeneration indicates generally poor regeneration for most species, with specific site variations. The additive Soil Quality Index (SQI) ranged from 2.30 to 2.84, 2.35 to 2.84, and 2.32 to 2.84 at soil depths of 0–15 cm, 15–30 cm, and 0–30 cm, respectively. Similarly, the weighted SQI showed a comparable trend, with Ali-Bedni Bugyal recording the highest values (0.95–0.96 across all depths). The reported SQI values exhibited a positive correlation with soil physicochemical properties and a negative correlation with vegetation density at the seedling, sapling, and tree stages. The site-specific variations in tree species distribution, diversity, and soil quality reflect distinct ecological dynamics and species interactions, while the poor regeneration status of most tree species highlights the need for targeted conservation strategies.