Radial growth of subarctic tree and shrub species: relationships with climate and association with the greening of the forest-tundra ecotone of subarctic Quebec, Canada

Climate change has resulted in a widespread increase in primary productivity in northern regions. This "greening" can alter terrestrial ecosystems dynamics and trigger positive climate feedbacks but often exhibits spatial heterogeneity. Few studies have focused on the differential responses of plant functional groups to warming as a potential driver of heterogeneity in greening rates. Our objective was to determine whether climate-growth relationships are species-specific, and whether they can explain the heterogeneity of the subarctic greening rates. We compared climate-growth relationships and associations to normalized difference vegetation index (NDVI) of the dominant tree (Picea mariana) and shrub (Betula glandulosa) species at two locations in the forest-tundra ecotone in Nunavik (Quebec, Canada). Correlation coefficients of the climate-growth relationship varied between species and locations, being higher for P. mariana and at the northernmost location. The lower association between NDVI and P. mariana radial growth at the northernmost location appeared to be compensated by a stronger association between NDVI and B. glandulosa radial growth. Our results show that response to climate varies spatially and between species, both of which could potentially be used to explain the heterogeneity of subarctic greening rates. This improved understanding of the species-specific response to climate change will help predict forthcoming changes in primary productivity