Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth patterns

The arctic-alpine biome is warming rapidly, resulting in a gradual replacement of low statured species by taller woody species in many tundra ecosystems. In northwest North America, the remotely sensed normalized difference vegetation index (NDVI), suggests an increase in productivity of the arctic and alpine tundra and a decrease in productivity of boreal forests. However, the responses of contrasting shrub species growing at the same sites to climate drivers remain largely unexplored.
 
 Here, we test growth, climate, and NDVI relationships of two contrasting species: the expanding tall deciduous shrub <i>Salix pulchra</i> and the circumarctic evergreen dwarf shrub <i>Cassiope tetragona</i> from an alpine tundra site in the Pika valley in the Kluane Region, southwest Yukon Territories, Canada.
 
 We found that annual growth variability of both species at this site is strongly driven by early summer temperatures, despite their contrasting traits and habitats. Shrub growth chronologies for both species were correlated with the regional climate signal and showed spatial correspondence with interannual variation in NDVI in surrounding alpine and Arctic regions. Our results suggest that early summer warming represents a common driver of vegetation change for contrasting shrub species growing in different habitats in the same alpine environments.