How do climate and topography influence the greening of the forest-tundra ecotone in northern Québec? A dendrochronological analysis of Betula glandulosa

1. NDVI analysis and repeated aerial photographs have revealed significant shrub expansion in many subarctic regions. While the recent increase in temperature is usually considered to be the main driver of this phenomenon at regional scales, very little is known about the local heterogeneity of shrub responses across the landscape. 2. In this study, we aim to identify the climatic factors controlling the growth of the largely distributed shrub species Betula glandulosa in three types of environments (terrace, hilltop and snowbed). We also aim to evaluate the relationship between B. glandulosa growth and the NDVI data for the Boniface River region, in north-western Québec, where the study took place. 3. In the field, we harvested 180 B. glandulosa individuals (20 per site, 3 sites per type of environment). We constructed specific growth-ring width chronologies and mean axial growth rate chronologies for each site and used them for dendroclimatic analysis (response functions). We also used linear regressions to evaluate the relative influence of dwarf birch growth on the NDVI trend. 4. We found a sharp increase in B. glandulosa radial growth in the 1990s followed by a sharp decreasing trend from 2002 on terraces and hilltops, while growth remained high in snowbeds. Betula glandulosa growth was positively correlated with summer temperatures on terraces and hilltops, whereas winter precipitation promoted growth on snowbeds. The NDVI trend was largely correlated to B. glandulosa growth on terraces and hilltops for the period between 1986 and 2002 (71—80% explained variance). 5. Synthesis. Our results suggest that topography plays a major role in B. glandulosa growth and therefore in shrub community dynamics. Because terraces and hilltops represent 70% of the land surface, the sharp B. glandulosa growth increase at these sites promoted an important overall expansion of the shrub community in the region. However, the decline in B. glandulosa growth observed after 2002 suggests that the expansion could be slowed down in the near future, therefore limiting shrub growth contribution to the regional NDVI signal.