Contrasting treeline dynamics of pine and birch in response to climate warming in the Karakoram

[Key message] Pine benefited from a warming climate with increasing recruitment and upslope shifts of alpine treeline but birch treelines were mostly static highlighting the influence of other confounding, local factors.
[Abstract] Alpine treeline is considered as a sensitive indicator to climate change. Given that low temperature is a primary constraint of treeline dynamics, they are expected to shift upslope in response to climate warming. The Karakoram, under westerly-dominated climate, has experienced significant warming since the mid-nineteenth century. However, in comparison to monsoon-dominated wetter areas, little is known how alpine treelines have responded to rising temperatures in westerly-dominated dry areas, such as the Karakoram. To test if climate warming triggered upslope shift of alpine treelines in the Karakoram, we compared the recent dynamics of blue pine (Pinus wallichiana) and Himalayan birch (Betula utilis) treeline populations. We expected a higher responsiveness of pine recruitment to temperature as compared with birch due to the higher drought-tolerance of pine species. Tree mapping and dendrochronological methods were used to determine stand age structure, and to reconstruct spatiotemporal treeline dynamics during the past 150 years. The decadal recruitment rate of pine was positively correlated with summer and winter temperatures, whereas the birch recruitment was negatively correlated. Accordingly, pine treelines showed significant upslope shifts in two out of three plots, whilst two birch treelines remained stable in the two studied plots. Such different treeline shift rates between blue pine and birch species agree with their divergent recruitment responses to climate warming. Thus, projected warming in the Karakoram will differently impact treelines depending on species-specific responses with pine showing a more dynamic and rapid upward treeline advancement and birch showing less rapid responses. However, we should consider other non-climatic factors such as anthropogenic disturbances for better understanding treeline dynamics in the Karakoram.