Glacial Legacies Influence Riparian Vegetation Expansion on Subarctic Alaska Floodplains

Subarctic floodplains are “hotspots” of biological productivity, diversity, and cross‐trophic habitat quality embedded in otherwise relatively unproductive ecosystems. Broad‐scale and rapid changes in the discharge regime, biogeochemistry, and seasonality of subarctic rivers are occurring, with many changes related to watershed‐scale permafrost thaw. However, the consequences for subarctic riparian ecosystems remain largely unexplored, particularly for glacierized catchments which encompass large areas of riparian habitat in the North American subarctic. To assess differences between floodplain types and their relative importance for landscape‐level change, we quantified spatiotemporal patterns of riparian vegetation and floodplain characteristics in 9 glacierized and 8 nonglacierized catchments in Denali National Park and Preserve, Interior Alaska, using paired circa 1980–2020 high‐resolution images. Overall, ecotype transitions linked to vegetation colonization and succession exceeded disturbance by fourfold, and the total cover of nonvegetated ecotypes decreased by 22.5%. Change has been more rapid on glacial floodplains, which are wider and tend to have extensive young, barren surfaces available for primary succession. Nonglacial floodplains support a higher proportion of late‐successional spruce forest compared with glacial floodplains, where contemporary vegetation is dominated by early and mid‐successional shrub ecotypes. Riparian ecotypes serve as bioindicators of intertwined changes in regional climate, catchment‐scale permafrost and hydrology, and riparian‐scale disturbance and flow regime. The increases in vegetation cover and structural complexity we observed have wide‐ranging implications within and beyond subarctic floodplains, especially in northwestern North America, where there is a high potential for future change due to ongoing glacial retreat and associated successional processes.