Species‐specific browsing constraints on tree recruitment may curb conifer treeline expansion in the Arctic

Treelines at the forest–tundra ecotone are often shifting to higher elevation and latitudes in response to global warming, yet not all treeline positions match the climatic maximum range. This mismatch often reflects the interplay between various factors, including historical land use legacies, propagule dispersal capacity and herbivore impacts on seedling recruitment. Much of Scandinavia's current treeline is formed by deciduous mountain birch Betula pubescens forest, with pines Pinus sylvestris and spruce Picea abies most often forming a secondary conifer treeline at lower elevations. Climate change could alter this composition, enabling conifer expansion to higher elevation and latitudes as already reported for some locations. We investigated how herbivory by cervids shapes the secondary conifer treeline structure using long‐term exclosure experiments in Abisko, Sweden and a network of plots along elevational gradients at ten Scandinavian mountain locations. We asked: is there a browsing trap for the conifer treeline? Is this trap tree species‐specific? Does it vary across space? Browsing trap is defined as a demographic bottleneck in which repeated herbivore damage prevents recruitment from escaping vulnerable height classes at a population level. The experiment revealed strong suppression of pine establishment and growth under herbivory, while field patterns were consistent with a browsing trap. The height and the strength of the browsing trap is tree‐species specific: lower in height and stronger for pine but taller in height and weaker for birch. Our results indicate that the intensity of the browsing trap on tree recruitment varies with elevation, cervid abundance and distance to forest edge. Species‐specific browsing traps and herbivory interactions may therefore contribute to the mismatch between climate warming and observed rates of treeline advance.