1 Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, 100101 Beijing, China

2 Tree Ring Laboratory, Lamont-Doherty Earth Observatory of Columbia University, Palisades,12 NY 10964, USA

3 Centre of Wood Sciences, University of Hamburg, Leuschnerstrasse 91, 21031 Hamburg, 14 Germany

4 Central Department of Hydrology and Meteorology, Tribhuvan University, Kathmandu, Nepal

5 University of Chinese Academy of Sciences, Beijing 100049, China

 

*corresponding author:

Email: liangey@itpcas.ac.cn

 

Abstract：Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf timberline species that

28 survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper timberlines between 3,900 and 4,150 m a.s.l. is primarily limited by moisture availability rather than by low temperature. To verify this assumption, a total of 292 increment cores from 211 birch trees at nine timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross-dating and a common climatic signal in the tree-ring width variations. From March-May, all nine tree-ring width site chronologies showed a strong positive response to total precipitation and a less strong negative response to temperature. During the instrumental meteorological record (from 1960 to the present), years with a high percentage of locally missing rings coincided with dry and warm pre-monsoon seasons. Moreover, periods of below-average growth are in phase with well-known drought events all over monsoon Asia, showing additional evidence that Himalayan birch growth at the upper timberlines is persistently limited by moisture availability. Our study describes the rare case of a drought-induced alpine timberline that is comprised of a broadleaf tree species.

 

Key-words: Betula utilis, Nepal, central Himalayas, alpine timberline, tree-ring width, missing ring, dendroecology, climate sensitivity, precipitation, warming, drought, pre-monsoon season