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Recent widespread increases in vegetation cover on subarctic floodplains of Interior Alaska

A collaborative team of ecologists led by ABR’s Gerald V. Frost and including Carl A. Roland and Joshua H. Schmidt of the National Park Service recently published a study in which they compared 1980s-era and recent high-resolution imagery of 12 floodplains located in Denali National Park and Preserve, Wrangell–St. Elias National Park and Preserve, and Yukon-Charley Rivers National Preserve. Imagery from the different periods was georeferenced and compared to examine changes in vegetation cover at a total of almost 18,000 sample points distributed on sections of floodplain from above treeline, to lower elevations. Riparian corridors support mosaics of forest, shrub, and partially vegetated areas that change through time due to the interplay of floodplain disturbance and ecological succession. Contrary to expectations, the team found a widespread pattern of “greening” on Interior Alaska floodplains, with a general decrease in the extent of riverine barrens and an increase in tall willow shrublands and other early successional ecotypes. Between about 1981 and 2010, the extent of active channels and barrens had decreased by 22%, while the extent of tall shrub, low shrub, and seral meadow all increased substantially. The most dramatic vegetation increases occurred in glaciated watersheds of Denali and Wrangell/St. Elias, while changes in the older landscapes of Yukon-Charley were more subtle. The observed changes have implications for the structure and function of important riparian habitats of Interior Alaska and have occurred amid changes in river discharge regime, climate, and permafrost extent in subarctic watersheds of Alaska and beyond.

From the paper:

Boreal forest and tundra ecosystems are undergoing rapid climatic and environmental changes with consequences for ecosystem structure, function, and services. Although riparian zones occupy a small footprint within subarctic landscapes, they have disproportionately high value as foci of hydrological processes, biogeochemical cycling, ecological disturbance, biodiversity, and wildlife activity. Recent observations of increased winter discharge, reduced peak flows, and increased connectivity between catchments, streams, and groundwater in subarctic riparian zones have prompted predictions of altered riverine disturbance regimes, increased channelization, and a decline in the extent of active floodplains. However, few observational data exist concerning the spatiotemporal dynamics of subarctic floodplain vegetation, which can serve as a bioindicator to corroborate such predictions.

We observed a strong tendency toward increasing cover, stature, and density of vegetation.

We analyzed the distribution and extent of riparian ecotypes across a network of streams in 12 Interior Alaska watersheds using high-resolution image pairs from circa 1981–2010. All stream reaches encompassed pronounced elevational gradients and included elevational forest–tundra ecotones. We classified riparian ecotypes using an image-based point-intercept sampling approach, calculated the probability of ecotype transitions, and evaluated relationships between ecotype transitions and environmental covariates. Our results reveal widespread increases in the stature, density, and extent of

riparian vegetation spanning gradients of elevation, floodplain morphology, and climate. Ecotype transitions occurred at >20% of sample points, and there was a strong imbalance toward forward successional transitions (16.5%) versus backward transitions (4.0%). That is, we observed a strong tendency toward increasing cover, stature, and density of vegetation communities across our extensive sampling domain across our approximately 30-year sample period. This relatively rapid riparian “greening” signal tended to be most pronounced in our glaciated watersheds. Although the streams we studied displayed high local variability in ecotype transitions, our results support hypotheses of increasing channelization and reduced extent of unvegetated surfaces on subarctic floodplains. They also likely reflect a trend toward more rapid and extensive plant recruitment and growth due to processes associated with conspicuous warming in northern ecosystems, consistent with greening documented in other subarctic landscape segments along gradients of elevation and latitude.


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