Spatio-temporal Trends in Tree and Shrub Cover in the Circumpolar Low Arctic: Evidence from 1960s Satellite Photography and Ground Observations

Expansion of trees and shrubs into tundra-dominated areas is one of the principal changes to Arctic landcover expected with climatic warming, and there is evidence that ecological state-shifts are currently occurring in ecotones of the North American Low Arctic. The ubiquity of these state-shifts across the circumpolar Low Arctic is unclear, however, because few data exist for the vast Eurasian continent. This study is quantifying state-level vegetation change in tundra ecotones at ~20 sites in northern Eurasia and Alaska using comparisons of circa 1965 Corona and contemporary high-resolution satellite photography. Corona was the world's first operational satellite surveillance system and offers a readily available data source for land-surface change studies over a ~40 year temporal interval. The degree to which patterns of vegetation change are shared among sites will indicate the ubiquity of ecological state-shifts in the Low Arctic, as well as the relative influence of large-scale forcing mechanisms (e.g., climate change) and local environmental controls (e.g., disturbance regime, geomorphology) on tree and tall shrub dynamics.

Preliminary findings indicate that tall shrublands have expanded at several sites in northwestern and eastern Siberia. Recent expansion is most apparent on floodplains, uplands, and drained lake basins. Ground data indicate that dramatic expansion of alder shrubs at a tree-line site near Kharp, northwest Siberia has occurred in areas affected by an antecedent high-intensity wildfire that removed the surface organic layer. Additionally, alder recruitment is concentrated on disturbed mineral soils associated with frost boils. On the southern Yamal Peninsula, comparison of 1968 Corona and 2009 aerial photographs indicate that alders have colonized retransported sands derived from barren uplands near Ozero Yaroto. Additionally, alders and willows have become more abundant on terraces and point bars on the Tanlova River floodplain. In northeast Siberia, larch woodlands have expanded since 1965 on the lower Kolyma River floodplain, although some forest die-back has occurred in association with rapid degradation of ice-rich permafrost. These findings indicate that local-scale disturbance events that create mineral-dominated edaphic conditions have promoted growth of shrubs and trees and enhanced productivity in parts of the Low Arctic. These changes have occurred against a backdrop of recent climatic warming at all sites.