Himalaya to Hengduan: dynamics of alpine treelines under climate change

Abstract

Alpine treelines serve as vital indicators of the impacts of climate change on tree growth and forest distribution. They offer valuable insights into how shifting temperature and precipitation patterns affect ecosystems in treeline ecotones. Analyzing the age structure of tree stands at treelines provides a glimpse into how different generations of trees have responded to changing environmental conditions and aids in predicting future changes. Moreover, studying the spatiotemporal distribution of tree species at treelines helps us gain a comprehensive understanding of how forests adapt to climate variations. Tree rings at treelines can elucidate the climatic factors that limit tree growth and establishment patterns. Mountain environments, characterized by low temperatures at higher elevations, create constraints on tree growth. However, the intricate interplay between temperature and water availability, driven by precipitation gradients, means that predicting treeline shifts based solely on temperature changes is overly simplistic and may not fully reflect the complex reality. To assess the potential for such interactions, we contrasted the dendroecological performance of different tree species (Abies spectabilis, Betula utilis, Abies georgei and Larix potaninii) in the trans-Himalayan zone, Nepal and Hengduan Mountains, China. We reconstructed the stand age structure by using dendrochronology. Statistical determination of climate-growth responses demonstrated that treeline is moisture sensitive in Himalaya, and temperature as well as moisture sensitive in Hengduan region. There was abundant seedling recruitment with consistent range shift of A. spectabilis and B. utilis treelines in Nepal, and lower seedling recruitment with lower shifting rates of treelines of A. georgei and L. potaninii in Hengduan Mountains. We identify both moisture and temperature as critical environmental factors in determining tree radial growth and treeline response to climate. However, modifying factors such as microhabitat conditions and biotic interactions are also highly important to improve accuracy of treeline dynamics.