Treeline Ecotonal Dynamics: A Case Study from Pinus wallichiana Forest in Ngawal, Manang, North-Central Nepal

Abstract

Global warming is now a scientifically established fact and this is also known to influence treeline structure. This study tried to have a glance and inkling through size, age structure and regeneration of Himalayan Blue Pine (Pinus wallichiana) on the treeline ecotone of Ngawal, Manang in south facing slope of the trans-Himalayan dry valley. Three vertical transects (20 m width) were laid across the treeline ecotone from forest line to treeline. Age of individual trees within the transects was determined based on the number of annual rings counted from increment cores whereas age of juveniles saplings and seedlings was estimated by counting branch whorls and scars left on stem. Almost reverse J-shaped size and age structure showed that the majority of the pine population was recruited from 1970s. The current timberline position is ca. 3930 m a.s.l. whereas the treeline position in the area is ca. 4020 m a.s.l. that with age-height regression model is estimated to be attained by early 1990s. Despite the absence of recent treeline advance, the progressive densification in the treeline ecotone as well as seedlings and saplings establishment above the treeline may indicate the future potential for the advance. Even under the current warming scenario of Nepal Himalaya, the ecotonal responses could not be predicted by climate warming alone but marked release in human pressure might have synchronous effect and for the disentangling pursuit, future studies incorporating both climate and anthropogenic aspects are proposed.