The climate sensitivity of shrub growth: A synthesis of 20 arctic and alpine sites

Background/Question/Methods

Recent evidence indicates widespread expansion of canopy-forming shrubs in tundra ecosystems. Satellite data show a ‘greening’ of Arctic tundra which has been partially attributed to increasing shrub cover. This increase in woody shrubs has occurred concurrently with increasing temperatures but the mechanisms, magnitude of change in cover and feedbacks promoting expansion over time have yet to be quantified. Increases in woody shrubs in tundra ecosystems will alter ecosystem functioning by changing albedo, soil temperatures, nutrient cycling, carbon storage and biodiversity and could create positive feedbacks to further shrub expansion and Arctic warming. To better understand shrub expansion, we compiled annual growth data collected from 23 shrub species sampled at 22 sites around the circumpolar Arctic and in alpine locations and compared these data to climate variables including seasonal temperatures and precipitation.  We analyzed variability in the growth sensitivity to climate.  We hypothesized that the relative influence of summer and winter temperatures and precipitation will differ between sites, with more temperature sensitive growth being found at sites with less extreme growing conditions, and growing season length and snow cover being more important controls on growth in High Arctic and alpine environments. 

Results/Conclusions

Initial results indicate that growing season temperatures were positively correlated with growth, with early growing season temperatures explaining almost half of the interannual variation in growth for some species at some sites. Winter temperatures, snow and rain were also important factors explaining growth for some sites and species. Some tundra ecosystems have experienced greater warming than others in recent decades.  Only six of 14 geographic regions included in the analysis showed a significant positive growing season warming trend between the years 1980 and 2009.  We predict that the sites showing the most temperature sensitive growth and experiencing the greatest growing season warming will most likely show the greatest future change in shrub cover, given that other growth regulating factors (such as herbivory) remain at present-day levels. Better quantifications of the relationship between the growth of shrub species and climate will inform the development of models for estimating future shrub expansion. These improved projections of vegetation change will contribute to better quantifications of both the resulting ecological impacts of shrub expansion in tundra ecosystems, and the strength of climate-ecosystem feedbacks.

This data synthesis is an activity of the Shrub Hub Research Network (http://shrubhub.biology.ualberta.ca/).