Ecological Dynamics and Regeneration Expansion of Treeline Ecotones in Response to Climate Change in Northern Bhutan Himalayas

Abstract

The alpine treeline ecotones are an early indicator of vegetation’s response to changes in climate, and the advancement of diffuse treeline ecotones has been associated with mean annual warming temperatures. However, the knowledge of how tree demographic size, age and population distribution, and regeneration decrease with increasing elevation and mean annual temperature remain fragmentary in Bhutan. There was no explanation of how treelines migrate in response to the climate. Therefore, the objectives of this study were to investigate tree demographic size and age and population distribution, as well as the regeneration expansion of treeline ecotones of Abies densa trees in response to climate change. Demographic data from thirty transect bands from treeline ecotones and reconstructed mean annual temperatures from tree-rings were used. Regression analysis was used to establish a relationship between elevation/temperature and demographic tree size and age, as well as to determine recruitment frequency distributions and whether these could be driven by climate change. The tree demography indicated that the treeline ecotone in our sampling site is temperature limited. Hence, cooler temperatures at higher elevations should drive decreases in basal diameter, age and recruitment frequencies. From the dendroecological analysis, the diffuse treeline ecotones appear to be climbing on average 1.00 m per year in Northern Bhutan. We also found that the recruitment frequency has increased over recent years (1850–2017), as temperatures continue to rise. The thermal treeline ecotones will be likely to serve as a line of bioclimatic reference against which other zones of bioclimate can be defined. With documented responses of treeline ecotones toward mean annual temperatures, the expectation is that additional warming will continue to influence regeneration expansion in the future. This dynamic response of treeline ecotones towards the climate acts as an indicator of climate change. Information about climbing treelines and altered ecotones should be a vital part of the material for decision makers to consider, to assess impacts and threats to Himalayan alpine biota.

Increasing growth sensitivity of Pinus wallichiana to summer season maximum temperature – evidence from central Bhutan Himalaya

“…This summer, temperature sensitivity is also consistently observed in the radial growth of P. wallichiana from different parts of Bhutan (Wangchuk et al, 2018;. Similar temperature sensitivity has also been noted for P. spinulosa from central Bhutan (Krusic et al, 2015) and A. densa from North Bhutan (Khandu, Polthanee, & Ayutthaya, 2022a;. A warm summer provides optimal conditions for photosynthesis and xylem production, particularly in temperature-limited environments.…”

“…Multi-centennial temperature reconstructions have also been developed from the region using tree-ring width chronologies of Picea spinulosa (Krusic et al, 2015) and Abies densa (Khandu, Polthanee, & Ayutthaya, 2022b). Advancing tree lines have recently been reported in the northern extreme of Bhutan based on observations of regeneration and expansion rates of Abies densa (Khandu, Polthanee, & Ayutthaya, 2022a). The density fluctuations and scars observed in tree rings of multiple species were used to reconstruct flood history and estimate the streamflow of the Dhur River in north central Bhutan (Speer et al, 2019).…”