Complex drivers of population dynamics across treeline: Expected and unexpected responses in Himalayan systems

Background/Question/Methods:     Alpine treelines are responding to current climate change worldwide. However, the global drivers interact with local drivers to elicit a response that is both site and species specific. To understand tree line dynamics and its potential drivers, we studied two most dominant tree species, Abies spectabilis (AS) and Rhododendron campanulatum (RC), on the north facing slope of two mountains in central Nepal. We set up 13 elevational transects, each crossing tree line; a transect had upto eight plots of 10m×10m size, spaced 50m apart in elevation. We recorded height and state (dead or alive) of all individuals, and measured a range of potential covariates at each plot: slope, aspect, canopy of adults, proxies for anthropogenic and animal disturbance. We determined spatial pattern of regeneration potential, mortality and abundance for various size/age classes, and we identified the most important drivers of such patterns. A random subset of individuals within plots were chosen for tree coring, to determine age and growth patterns using tree-ring analyses.

Results/Conclusions:     Plant age strongly and linearly relates to height above treeline (r = 0.71 and 0.81 for two mountains). Young plants (<2m tall) of RC have higher density above treeline than below treeline. Mature plants (>2m tall) of RC, on the contrary, show insignificant trend towards higher density below treeline than above. Mortality of RC was always lower above treeline than below, independent of sizeclass. Away from treeline, density of RC decreases with elevational distance both upslope and downslope for all size classes. AS saplings have extremely low density above treeline than below, with mature plants being virtually absent above treeline. These stark differences between two species – RC luxuriantly growing above treeline, upto ~300m above, and AS stuck at treeline, growing upto ~25m above treeline – however, are contrasted by the fact that elevation is the only significant predictor of the decrease in density of both species above treeline. The saplings are progressively younger and shorter with distance above treeline. Age structure analysis shows that the populations above treeline of both species are regenerating faster than below. These results from analysis of natural (in-situ) mortality and density across elevations, their discrepancies between young and mature plants, important predictors of the plant responses, and regeneration potential on two sides of treeline are consistent with upward shift of the tree line of RC  as a result of recent amelioration of temperature.