Tree-to-tree interactions slow down Himalayan treeline shifts as inferred from tree spatial patterns

Aim: The spatial patterns of tree populations reflect multiple ecological processes. However, little is known whether these patterns mediate responses to climate in marginal tree populations as those forming alpine treelines. Harsh conditions at these ecotones imply the existence of positive interactions which should lead to tree clustering. In fact, densification in response to climate warming is more widely reported than upward shifts in most treelines. This suggests that more intense tree-to-tree interactions could buffer the treeline responses to climate warming, resulting in low treeline shift rates. Location: Central Himalayas. Methods: We examined influence of tree-to-tree interactions on the responsiveness of treelines to climate warming by analyzing a network of 17 treeline sites located across the central Himalayas, and encompassing a wide longitudinal gradient characterized by increasing precipitation eastwards. We quantified the changes in density and the spatial patterns of three 50-year age classes of the two main tree species found at treeline (Betula utilis and Abies spectabilis), and related them to reconstructed shifts in treeline elevation. Results: Young trees showed clustering near the treeline, while older trees tended to show random spatial distribution. Clustering decreased as climate conditions ameliorated, i.e. in the wetter eastern sites. A negative association between upward treeline shift rate and clustering indicates that tree aggregation weakens treeline responsiveness to climate warming. Thus, warming-induced drought stress tends to lower treeline shift rates by enhancing clustering. Main conclusions: Our results highlight the complexity and contingency of site-dependent treeline responses to climate. Hence, to advance our understanding on treeline processes, we should consider both direct and indirect influences of relevant biotic (tree-to-tree interactions) and abiotic (climate) drivers of treeline dynamics.