Warming-Induced Shrub Expansion and Lichen Decline in the Western Canadian Arctic
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Strong evidence for a pan-Arctic expansion of upright shrubs comes from analysis of satellite imagery, historical photographs, vegetation plots, and growth rings. However, there are still uncertainties related to local-scale patterns of shrub growth, resulting interactions among vegetation functional groups, and the relative roles of disturbance and climate as drivers of observed change. Here, we present evidence that widespread and rapid shrub expansion and lichen declines over a 15,000 km2 area of the western Canadian Arctic have been driven by regional increases in temperature. Using 30 m resolution Landsat satellite imagery and high resolution repeat color-infrared aerial photographs, we show that 85% of the land surface has a positive 1985–2011 trend (P < 0.05) in NDVI, making this one of the most intensely greening regions in the Arctic. Strong positive trends (>0.03 NDVI/decade) occurred consistently across all landscape positions and most vegetation types. Comparison of 208, 1:2,000 scale vertical air photo pairs from 1980 and 2013 clearly shows that this greening was driven by increased canopy cover of erect dwarf and tall shrubs, with declines in terricolous lichen cover. Disturbances caused by wildfires, exploratory gas wells, and drained lakes all produced strong, yet localized increases in NDVI due to shrub growth. Our analysis also shows that a 4°C winter temperature increase over the past 30 years, leading to warmer soils and enhanced nutrient mineralization provides the best explanation for observed vegetation change. These observations thus provide early corroboration for modeling studies predicting large-scale vegetation shifts in low-Arctic ecosystems from climate change.
Keywordsclimate change landsat lichen remote sensing satellite air photos shrubs
We thank Ken Baldwin from the Canadian Forest Service for help in locating and shipping the 1980 CIR film canisters, Alice Deschamps and Marilee Pregitzer from CCMEO for assisting with film scanning and photo processing, and Alex Brooker from University of Ottawa for processing the Landsat image scenes. Wenjun Chen and Darren Pouliot from CCMEO, and Jan Adamczewski from Government of NWT offered helpful comments to improve the paper. The Polar Continental Shelf Program of Natural Resources Canada provided helicopter time from Great Slave Helicopters to acquire the 2013 photos and conduct ground surveys. Funding for this work was provided by the NWT Cumulative Impacts Monitoring Program under the project “A Multi-scale Assessment of Cumulative Impacts in the Northern Mackenzie Basin” led by Claire Marchildon of Aboriginal Affairs and Northern Development Canada, and a Natural Sciences and Engineering Research Council Discovery Grant to Trevor Lantz. For field assistance and logistical support we thank: Richard Binder, Jeff Moore, Emanuel Adam, Douglas Panaktalok, Yu Zhang, the Aurora Research Institute, and the Inuvialuit Joint Secretariat.
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