Shrub densification heterogeneity in subarctic regions: the relative influence of historical and topographic variables

Expansion of shrub species is widely reported in northern regions, although its extent varies across the landscape. In subarctic Quebec (Canada), where dwarf birch (Betula glandulosa Michx) is the main species responsible for shrub expansion, little is known about the causes and consequences of this phenomenon. This study aims to identify the drivers of dwarf birch densification heterogeneity at the landscape level and to evaluate its influence on other shrub species. We used model selection with Akaike's information criterion to rank ecologically relevant models including topographic, historical and edaphic variables. The influence of dwarf birch densification was evaluated through regression analysis. We found that the best model explaining the heterogeneity in dwarf birch densification in western Nunavik includes factors related to both historical conditions (initial shrub cover, time elapsed since last wildfire) and topography (type of environment). Among these factors, only the initial shrub cover had a significant positive influence on the shrub densification. Increase in dwarf birch cover was found to negatively influence the cover of other shrub species. However, no relation was found between dwarf birch densification and other shrub species richness, suggesting that the densification did not yet lead to the exclusion of less competitive species.