Changes in regeneration and leaf traits of Rhododendron campanulatum along a treeline ecotone in central Nepal

Changes in population structure and plant functional traits across environmental gradient in the near natural treeline is a critical determinant of environmental changes in treelines. In this study we analyzed population characteristics and leaf traits of Rhododendron campanulatum across treeline ecotone in a trans-Himalayan region of Mustang district in central Nepal. Along 4 vertical transects, a total of 72 quadrats (5 m  5 m) - 34 above Betula utilis treeline and 38 below treeline - were sampled to analyze the population structure, regeneration, specific leaf area (SLA) and leaf-stem relationship of R. campanulatum. Our results showed that the regeneration of R. campanulatum was fairly good in both habitats, as evident from density-diameter curve. The species showed type Ⅲ survivorship curve in both habitats with the highest mortality during early stage of life. However, the seedling density and mortality was higher below treeline than above treeline. Canopy cover and basal cover were found to be the major factors affecting regeneration, as indicated by higher number of juveniles in relatively open areas above treeline. The total leaf area increased (p < 0.001) with stem cross-section area in both habitats. However, the SLA was higher below treeline (55 cm²/g) than above (47 cm²/g) (p < 0.001); it also increased with the canopy cover of trees (R² = 0.59, p < 0.001) below the treeline. In conclusion, high density of juvenile, low mortality of seedlings, and low SLA above treeline indicated that the species has successfully adapted in climatically stressful alpine habitat with potential to expand upward.