Impact of climate change on growth of Abies spectabilis at alpine treeline ecotone in Nepal Himalaya

Himalaya of Nepal is experiencing rapid, and elevation-dependent warming along with increasing climatic extremes. However, the long-term growth dynamics and climate sensitivity of high elevation tree taxa from their upper distribution range remain inadequately understood. The present dendro-ecological study evaluates the long-term growth trends and climatic responses of Himalayan silver fir (Abies spectabilis D. Don) from marginal treeline habitats in Lauribina, Branakharka, and Paralkharka area within Langtang National Park (LNP), central Nepal Himalaya. Increment core samples of Abies spectabilis were obtained from the selected treeline sites of the LNP using tree increment borer. Tree-ring width site-chronologies spanning 97 to 219 years were developed, along with a 223-year regional composite chronology spanning 1789 to 2011 CE. The mean annual radial growth of Abies spectabilis at the treeline ecotone was 2.22 mm ± 0.79 SD. Tree growth-climate relationship indicate that temperature is primarily the limiting factor regulating the growth of the Abies spectabilis at the humid treeline ecotone, with A. spectabilis showing significant negative correlation to spring temperatures, and positive relationship with summer mean and maximum temperatures. Secondarily, moisture stress restricts the growth of the fir during the spring season showing positive relationship with precipitation during spring season and months. The long-term growth derived from basal area increment chronologies suggests that these fir forests are in an active growing phase, with growth enhancement observed across most sites in recent decades, likely driven by climate change. These findings imply that A. spectabilis at high elevation, humid region of Nepal may benefit from ongoing warming, potentially leading to increased growth and upward treeline advancement in the future.