Early signals of climate change impacts on alpine plant diversity in Indian Himalaya

As the global climate scenario is leading towards warmer and dryer conditions, mountain ecosystems and their biota are among the most vulnerable because they are determined by low-temperature conditions. Owing to this, these ecosystems can be used as indicators for climate warming impacts. The present study uses a standardized multi-summit approach as outlined in the Global Observation Research Initiative in Alpine Environment (GLORIA) protocol to assess the temporal dynamics of vegetation in alpine to nival summits (ranging from 3773 m to 4266 m asl) in the western Himalaya. The summits were first established during 2014-15 and surveyed to document baseline vegetation data and then resurveyed after a period of five years. Between the baseline and resurvey datasets, we observed an overall significant increase in mean species richness (6.3%) and vegetation cover (13%). An apparent increase in species turnover (β_sim) was observed in resurvey, accompanied by a decrease in nestedness (β_sne), suggesting that temporal variation in β-diversity is being influenced mainly by species replacement, which in near future can lead to changes in the composition/assembly of plant communities. Accordingly, a shift in the vegetation composition at the summits was also evident in the Non-metric Multidimensional Scaling (NMDS) biplots. We provide evidence that this shift in composition favours a more warm-adapted plant community over the years, described here as thermophilization of mountain summits indicated by a significantly positive value of thermophilization indicator, D = 0.0345. Interestingly, temporal increase in plant diversity indices were pronouncedly higher in subnival and nival plots. Similar to diversity indices, lower elevation vegetation zones i.e., subalpine and alpine, exhibited a negative thermophilization indicator. Species-specific changes showed a marked increase in plot occupancy and cover of warmer-adapted species with distribution centres in lower elevations (alpine, treeline), which may lead to increased competition for cold adapted nival rare species in the near future. Thus, in view of the projected climate warming, the observed signals in the Himalayan Mountains suggest an initiation of community transformation in high-altitudes which may lead to local extinctions of alpine plant species.