How does the stand structure of treeline-forming species shape the treeline ecotone in different regions of the Nepal Himalayas?

Abstract

Stand structure dynamics are considered as major happenings in any forest as a response to environmental changes. However, this important topic is underrepresented in the treeline studies in the Nepal Himalayas. We aimed to investigate site- as well as species-specific changes in morphometric features (basal diameter, crown cover, density, and height) along the elevational gradient across treeline ecotones in response to recent environmental changes. The stand structure characteristics of Abies spectabilis, Pinus wallichiana, and Betula utilis across the treeline ecotone of three study sites in Eastern (Barun), Central (Manang), and Western (Dhorpatan) Nepal were analyzed to elucidate structural heterogeneities. Altogether, eight transects (20 m × (60–250 m)) across the treeline ecotone were established. Trees of all life forms, trees (> 2 m), saplings (0.5–2 m), and seedlings (< 0.5 m), within each transect were enumerated and sampled for the morphometric features and age. Site-specific and species-specific stand structure dynamics were found. The rate of basal area increment was higher in Barun, but the Manang treeline, despite profound regeneration in recent years, had a low annual basal area increment. Moreover, the altitudinal distribution of age and morphometry were not consistent among those ecotones. Furthermore, intra-specific competition was not significant. The site-specific stand structure dynamics explain why treelines do not respond uniformly to increasing temperature. It invokes, in further studies, the incorporation of the tree’s morphometric adaptation traits, phenotypic plasticity, and interactions between species genotype and the environment.