Variation of Treeline Mountain Birch Establishment Under Herbivory Pressure

Abstract

Alpine and arctic treelines have been viewed as sensitive indicators of global climate change. While many treelines have advanced under warmer climate regimes in recent decades, the response has not been uniform. Some of this variability may be attributable to the impacts of herbivores. This study investigates the interacting effects of herbivory, climate, and understory vegetation on mountain birch establishment at treeline in the Scandes Mountains of northern Sweden. An extensive dendrochronological database was created to determine periods of establishment, which were then regressed against reindeer (Rangifer tarandus, L.) population data and historical climate data. Vegetation classifications were also created and analyzed to determine if establishment patterns vary by understory vegetation type. I have tested the hypothesis that tree establishment varies within the treeline ecotone and that high reindeer stocking levels negatively impact establishment.

Weakly positive responses to herbivory were observed in patterns of tree establishment at treeline. This indicates that reindeer may modestly promote treeline advance at low densities, contradicting some previous research, but many of the results were not statistically significant. The climate variables found to have significant relationships with establishment were inconsistent across herding districts and aggregation levels. No connections between vegetation assemblages and establishment or between vegetation assemblages and reindeer use were observed.