Site- and species-specific treeline responses to climatic variability in eastern Nepal Himalaya

Alpine treelines act as bio-indicators and bio-monitors of environmental change impacts in high elevation forests. This dendro-ecological study carried out in treeline ecotones in the Sagarmatha (Mt. Everest) National Park (SNP), eastern Nepal Himalaya, aimed to assess treeline dynamics and to understand the response of treeline forming Abies spectabilis (D. Don, Mirb) and Betula utilis (D. Don) to environmental change. At three treeline sites we placed two to four belt transects (size: 20 m wide, variable length) which bisected the treeline as well as the tree species limit. The results revealed spatio-temporally heterogeneous regeneration with a higher regeneration of A. spectabilis compared to B. utilis. Warm temperatures during summer (JJA) growing seasons combined with sufficient moisture favored the growth of A. spectabilis while moisture stress during spring seasons (MAM) mainly limited the growth of B. utilis. The regeneration of A. spectabilis was favored by high temperatures throughout the year with sufficient moisture. The climatic response of the regeneration of B. utilis was spatiotemporally different and variable. Results predict a changing community structure in the treeline in response to future environmental change. During the past 200 years, A. spectabilis shifted upward by about 0.93 m/yr and B. utilis by 0.42 m/yr, with stabilization during the second half of the 20th century at the majority of the sites. The recent stability in treeline position of both species at most sites indicated that in addition to favorable climate, species-specific competitive abilities during the recruitment phase, recruitment suppression in the Krummholz and dwarf scrub belts, and grazing determine regeneration success and treeline position in the region.