The recent widespread expansion of deciduous shrubs across much of the Arctic has been largely attributed to climate warming. This study investigated decadal growth rates of dwarf birch (Betula glandulosa) across a low Arctic landscape in the continental interior of Canada. Detailed birch cover (100 m 2 replicate plots) and individual shrub stature measurement datasets for five representative habitat-types were compared between 2006 and 2016, and evaluated in relation to environmental characteristics. Furthermore, dendrochronologically-based annual growth rates were assessed in relation to the 20-year climate record. Birch height, lateral dimensions, and patch groundcover all increased 20-25% relative to 2006 values, but these increases were similar among the habitat-types. Together, the limited evidence of recent warming at this site, the absence of significant habitat-type growth rate differences, and the lack of correlation between annual climate and stem secondary growth strongly suggest that climate change was not the principal cause. Instead, we propose that release from caribou impacts following the recent severe herd decline may explain the net shrub growth. Individual shrub growth rates were correlated with soil nutrient availability, but the latter was highly variable, suggesting that growth rates are primarily determined by fine-scale rather than habitat-scale spatial heterogeneity in nutrient supply. Together, our results demonstrate that birch growth has been enhanced across a variety of habitat-types in the Daring Lake landscape over the decade since 2006, and suggest that the recent severe caribou herd declines may be at least as

Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1,153 individual shrubs and 22,363 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5-9°C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates’ potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions.

Climate change predominantly affects northern regions, and resultant vegetation change (particularly the expansion of arctic shrubs) has the potential to create large-scale, positive climate feedbacks, including the widespread release of CO2 from arctic soils. Understanding the intensity and distribution of arctic shrub expansion is therefore necessary to predict future climate trajectories. Few studies, however, have directly measured vegetation changes in the Canadian continental low Arctic, and similarly, there is a need to better understand the landscape-level factors that determine shrub growth responses to warming. Previous studies in Alaska indicate strong differences in shrub growth responses between habitat-types, attributed to higher nutrient and water supply in low-lying areas. Therefore, this study examines growth patterns of the dominant shrub (Dwarf Birch, Betula glandulosa) in a variety of habitat-types across a low arctic landscape. Significant increases in both shrub cover and stature over ten years were found, but surprisingly there were no differences in growth between habitat-types. Further analyses (pending) will measure inter-annual shrub growth to compare patterns/degrees of variability between habitat-types. Individual shrub growth rates over the past decade correlated to local soil nutrient concentrations, but no other variables, suggesting that local spatial variation in nutrient availability seems to be the primary factor determining shrub growth responses to climate change. Overall, our preliminary results stress the importance of local nutrient variability in controlling shrub responses to warming, and challenge previous studies indicating strong differences in shrub growth responses to warming among habitat-types.