Recent Shrub Proliferation in the Mackenzie Delta Uplands and Microclimatic Implications

Abstract

Local observations, repeat photos, and broad-scale remote sensing suggest that tall shrubs are becoming an increasingly dominant component of Low Arctic ecosystems. This shift has the potential to alter the surface energy balance through changes to the surface albedo, snow accumulation and melt, and ground thermal regimes. However, to date there have been few quantitative estimates of the rate of tall shrub expansion. We used soft copy stereo visualization of air photos to map fine-scale changes in tall shrub tundra and green alder density in the upland tundra north of Inuvik, NT between 1972 and 2004. We also used 2004 photos to map tall shrub tundra in areas affected by fires that occurred between 1960 and 1968. To assess the potential impact of vegetation change on microclimate, we used pyranometers to measure albedo and net solar radiation, thermistors attached to data loggers to record ground temperatures, and field surveys to record winter snow conditions in three common vegetation types. Fine-scale mapping shows that green alder stem density has increased by 68% (±24.1) since 1972. Average tall shrub tundra cover has also increased by 15% (±3.6) since 1972. Historical tundra fires had the highest proportion of tall shrub cover of all areas mapped using 2004 photos, ranging from 92 to 99%. Based on these results, we suggest that predicted increases in the size and frequency of tundra fire are likely to drive rapid shrub proliferation in the Low Arctic. Shrub-dominated sites have decreased albedo, increased net solar radiation, deeper snow pack, and elevated near-surface ground temperatures, indicating that continued increases in shrub cover will affect regional climate, hydrology, permafrost temperatures, and terrain stability.