Moisture‐mediated responsiveness of treeline shifts to global warming in the Himalayas

Abstract

Among forest ecosystems, the alpine treeline ecotone can be considered to be a simplified model to study global ecology and climate change. Alpine treelines are expected to shift upwards in response to global warming given that tree recruitment and growth are assumed to be mainly limited by low temperatures. However, little is known whether precipitation and temperature interact to drive long-term Himalayan treeline dynamics. Tree growth is affected by spring rainfall in the central Himalayan treelines, being good locations for testing if, in addition to temperature, precipitation mediates treeline dynamics. To test this hypothesis, we reconstructed spatiotemporal variations in treeline dynamics in 20 plots located at six alpine treeline sites, dominated by two tree species (birch, fir), and situated along an east-west precipitation gradient in the central Himalayas. Our reconstructions evidenced that treelines shifted upward in response to recent climate warming, but their shift rates were primarily mediated by spring precipitation. The rate of upward shift was higher in the wettest eastern Himalayas, suggesting that its ascent rate was facilitated by spring precipitation. The drying tendency in association with the recent warming trends observed in the central Himalayas, however, will likely hinder an upslope advancement of alpine treelines and promote downward treeline shifts if moisture availability crosses a critical minimum threshold. Our study highlights the complexity of plant responses to climate and the need to consider multiple climate factors when analyzing treeline dynamics.

Long‐term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

“…(moisture demand) of tree growth . Tree growth at upper treeline in the dry regions of the Himalaya was also found to be moisture limited in other studies (Liang et al, 2014;Sigdel et al, 2018). Similarly, positive (winter and summer) temperature sensitivity at higher (mainly 3,900 and 3,600 m) and lowest (2,800 m) elevation belts is clearly visible in the wet region, and was supported by the earlier findings of Shrestha et al (2017) in the humid northfacing slopes of the Himalaya.…”

“…High year‐to‐year fluctuations in temperature and precipitation (moisture availability) at lower elevation belts are likely related to stronger temperature sensitivity in the dry region and thus enhanced warming‐induced drought stress (moisture demand) of tree growth (Liang et al, ). Tree growth at upper treeline in the dry regions of the Himalaya was also found to be moisture limited in other studies (Liang et al, ; Sigdel et al, ). Similarly, positive (winter and summer) temperature sensitivity at higher (mainly 3,900 and 3,600 m) and lowest (2,800 m) elevation belts is clearly visible in the wet region, and was supported by the earlier findings of Shrestha et al () in the humid north‐facing slopes of the Himalaya.…”

Past the climate optimum: Recruitment is declining at the world's highest juniper shrublines on the Tibetan Plateau

“…, Sigdel et al. ). Additionally, a total of 277 juniper shrubs with different basal stem diameters located just outside the shrubline plots were selected to represent different age classes.…”

“…Juniper shrubs form well-defined annual growth rings that have proven to be reliable for dendrochronological dating (Liang et al 2012). Thus, we used standard dendroecological techniques to estimate the ages of all the measured shrubs, as has often been done in treeline and shrubline studies (Camarero and Guti errez 2004, Liang et al 2011, Garc ıa-Cervig on et al 2017, Sigdel et al 2018. Additionally, a total of 277 juniper shrubs with different basal stem diameters located just outside the shrubline plots were selected to represent different age classes.…”

Remotely-Sensed Identification of a Transition for the Two Ecosystem States Along the Elevation Gradient: A Case Study of Xinjiang Tianshan Bogda World Heritage Site

“…Some studies have taken the limitations of the alpine tree line formation as the entry point and used the experimental methods of physiology and ecology to conduct studies on temperature as the main controlling factors [2,[8][9][10][11][12]. Precipitation, propagation limitation, carbon limitation, environmental stress, and external disturbances have been analyzed as the causes of formation for the alpine tree line [3,9,[12][13][14][15][16][17]. The direct effects of climate change with stored terrestrial carbon include high temperatures, drought, and windstorms.…”