Climate as a driver of shrub expansion and tundra greening

The April 2014 IPCC Impacts report assigned a high confidence in the detection of an increase in shrubs in Arctic ecosystems and attributes shrub expansion to climate warming. However currently, the relative importance of climate versus other biological or environmental factors as drivers of tundra shrub growth and expansion remains poorly understood. Over the next century tundra ecosystems are predicted to warm by 2 – 10C. A significant unknown is whether shrubs will rapidly respond to warming conditions, or whether environmental factors such as water availability, nutrient limitation, herbivory, pathogen outbreaks, or fire will begin to limit the rate of future vegetation change. To address this knowledge gap, we propose to: 1) quantify the importance of climate as a driver of tundra shrub growth and expansion at sites across the tundra biome, 2) test the correspondence between plot-scale field observations with remotely-sensed data of tundra greening, 3) analyse the correlations between time series of shrub growth, tundra greening and climate and 4) assess the agreement between patterns of tundra greening and vegetation change at the landscape-scale. Our key objectives are:
1) To scale tundra vegetation change-climate interactions from plot to biome: We will integrate locally-collected data from sites around the Arctic and remotely-sensed data to test a) whether growing season temperatures are the primary driver of tundra vegetation change and b) whether remotely-sensed greening data capture a signal that correlates with spatial and temporal variation in local-scale shrub expansion. 2) To link patterns of tundra greening and vegetation change at the landscape scale: To scale from plot to biome at our focal research site Herschel Island, we will collect high spatial resolution (< 10 cm pixel) tundra greening data using an unmanned aircraft system (UAS) to compare with vegetation monitoring data, plant trait measurements, disturbance rates and terrain characteristics. 3) To improve predictions of tundra vegetation change: We will integrate our research findings to make data-driven predictions of shrub increase across the tundra biome. We will conduct a data synthesis workshop in 2017 to integrate our research objectives, evaluate the future research priorities and synthesize our findings. This proposed research is both timely and urgent, as the required biome-scale datasets have only recently become available in long-enough duration, and the technologies, methodological approaches and statistical techniques are only now refined sufficiently to test mechanisms of shrub expansion across spatial and temporal scales. Our findings will provide fundamental insights into the drivers of shrub growth and tundra greening and will enable improved predictions of future tundra vegetation change. The proposed research will result in the generation of a publically available dataset that will provide key estimates of tundra vegetation change to the broad scientific community. We will disseminate results through timely publications in internationally recognized journals and presentations at major conferences, knowledge exchange, ecological assessments, public engagement, and media. Lastly, the project will provide comprehensive training, mentorship and networking opportunities for early career researchers.