Population structure and dynamics of Arctic willow (Salix arctica) in the High Arctic

AimAlterations to the abundance and distribution of plants, especially shrubs, have been predicted by warming experiments and confirmed by remote-sensing analyses and land-based observations in the Low Arctic. In the High Arctic, analyses of satellite images suggest a recent increase in the normalized difference vegetation index (NDVI) and warming experiments have shown a positive correlation between temperature and shrub cover, but little is known about the responses of natural systems. In this context, we investigated populations of Arctic willow (Salix arctica Pall.) to assess the potential contribution of this structuring species to High Arctic shrubification. LocationSeven polar semidesert sites, including two glacier forelands, in High Arctic Canada and Greenland. MethodsWe evaluated the size distribution and establishment age of Arctic willow populations through field measurements and dendrochronological analysis. We also assessed the cover of Arctic willow at each site and compared current data with historical records. ResultsAge determination of over 200 large individuals established by seed suggests that colonization events were associated with the melting of permanent snow and glacial ice. Size distributions showed that recent recruitment was more frequent on glacier forelands, even though seedling establishment was common in other sites. Comparing our measurements with previous studies at the same sites revealed that significant increases in cover over the past few decades occurred only on glacier forelands. Main conclusionsOur results suggest that the population structure and dynamics of Arctic willow in the High Arctic are influenced by site-specific factors. Rapid glacier retreat is increasing the extent of the glacier forelands, which are currently experiencing relatively rapid plant colonization. Some sheltered sites with greater snow accumulation will probably experience increased shrub colonization under current and predicted climate change. It appears, however, that shrub recruitment and growth are still strongly limited by water availability, and the reduction in permanent snow cover may exacerbate this stress.