Composition, age-structure and dendroecology of high altitude treeline forest in the western Himalaya, India

Abstract

In the natural treeline ecotones, population structure and tree recruitment dynamics are strong indicators of vegetation responses to global climate change. We dendroecologically investigated age-structure, regeneration and tree-growth climate relationship of Abies spectabilis (D. Don) Mirb. forests along an elevational gradient at high altitude treeline ecotone (3020–3450 m asl) in the Kedarnath wildlife sanctuary, western Himalaya, India. The stand age structure of A. spectabilis was determined by ring counts of tree individuals (n = 95) core and bud scar count in seedlings and saplings. Years taken to reach coring height (CBH-1.37 m) were determined by fitting height-age regression model. Range shift and radial growth-climate relationships were studied to capture the advancement rate and climatic influence on tree growth. The result shows poor regeneration of A. spectabilis and low seedlings and sapling recruitment at the treeline ecotone (∼3340 m), in contrast higher recruitment of Rhododendron campanulatum in meadows zone, forming krummholz line at ∼ 3550 m. The stand age structure shows that older tree individuals were recruited at lower elevations while younger ones were at higher elevations. The treeline migrated upward at ∼10.01 m/decade. A 263-year old ring-width chronology of A. spectabilis shows precipitation of the current year's April month was conducive for radial growth, while the previous year's September and current year's April, May temperature negatively influenced radial growth. The presence of a few A. spectabilis seedlings and saplings within the ecotone limit could be attributed to the combined response of climate change and anthropogenic factors. Despite rapid warming, A. spectabilis treeline in the Tungnath, western Himalaya, has little advanced upslope in contrast with central Himalaya. Positive correlation with winter month’s (DJF) temperature indicated its sensitivity to warming scenario in western Himalaya. It appears that the beneficial effect of warming on tree growth is offset by water stress caused by increased evapotranspiration.