Climate Warming May Contribute to Shrub Expansion and Lichen Decline in the Western Canadian Arctic
A 2014 study by Robert H. Fraser et. al attempts to understand the reasons behind shrub expansion into more northern climates and the decline in lichen patches in the Low Arctic region. The researchers argue that climate warming around the Arctic region has caused northern regions in Canada to go through important temperature driven changes “in the form of increased thermokarst, larger tundra fires, and enhanced vegetation productivity” (1154).
The researchers evaluated the vegetation changes using a tool known as normalized difference vegetation index (NDVI). This tool uses different types of sunlight reflected off of vegetation to determine the breadth and density of vegetation through satellite imaging. NDVIs can provide information about “shrub cover, leaf area, and biomass in tundra vegetation.” (1155) The researchers also studied aerial photos from 1980 and 2013 of the region to compare with the NDVI results.
The study focused on the Tuktoyaktuk Coastal Plain (TCP) ecoregion, which lies on ice-rich permafrost, retrogressive thaw slumps, and small lakes. The vegetation in this ecoregion consists mostly of low shrubs, that is, woody plants no taller than 2 meters. NDVI change was compared with broad-scale climate data from Inuvik, 50km south of the TCP, to understand vegetation activity.
Analysis of the photos and NDVIs indicate an increase in shrub cover in 97% of photo pairs from 1980 to 2013, and “a decrease in lichen cover… in 94% of pairs” (1157). The researchers suggest climate warming is the best explanation for the trends observed in the study. Although land disturbances due to animal use explain some of the results, the most significant contribution found was warming air and ground temperatures. Mild winters and modest expansion of shrubs allow snowpack retention, insulating the soil and preventing heat loss. This process creates a positive feedback loop that allows more shrubs to expand in the area, explaining the significant increase in shrub coverage.
The researchers predict that future climate change will shift the vegetation state of the region, with more woody vegetation growth as the colder regions become more temperate. In the 1980 data, some areas were almost fully enveloped in lichen cover, whereas the 2013 data rarely showed long stretches of lichen. They also believe that at least some of the lichen loss was due to the expansion of shrub cover in lichen areas. The data also suggests that “shrubs will out-compete lichen in a warming climate.” (1159)
The trends identified in this study will affect “wildlife, permafrost, hydrologic regimes, wildfire, and infrastructure in the region.” (1165) With lichen reduction, wildlife that requires lichen as a food source will have to migrate further north to find viable food resources or convert to a “lower energy winter diet containing more shrubs” (1165). Shrub expansion will lead to soil retaining more heat and causing permafrost thaw. The increased temperature resulting from expanded shrub presence could affect water flow rate in rivers and water presence in soil from snow melting, “changing the timing of spring green-up” (1165). A dry climate in addition to shrub expansion increases the likelihood of fire spread and more intense wildfires Permafrost thaw destabilizes the infrastructures built on top of them, and an increase in wildfire places many of those infrastructures at risk of damage.
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