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

Abstract

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 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

Tree and shrub expansion at treeline drive contrasting responses in a subarctic passerine community

“…In Denali National Park and Preserve (Denali) within subarctic Alaska, heterogeneity in the expansion of woody vegetation is evident along multiple topographic and edaphic gradients. For example, formerly barren areas such as alluvial floodplains have experienced conspicuous and widespread expansion of vegetation over recent decades (Frost et al, 2023; Roland et al, 2016) while comparisons of modern and historical photos of Denali landscapes indicate relatively infrequent transitions from shrubland to forest and low‐statured tundra to shrubland (Brodie et al, 2019).…”

“…erect shrubs and trees). Advances in tree-and shrubline (Harsch et al, 2009;Myers-Smith & Hik, 2018) and colonization by tall shrubs along riparian corridors and on hillslopes are evident across the circumpolar Arctic (Myers-Smith et al, 2011;Tape et al, 2006) and Subarctic (Brodie et al, 2019;Frost et al, 2023;Roland et al, 2013Roland et al, , 2016Stueve et al, 2011). Such changes in vegetation structure and composition are expected to affect the distribution of vertebrates (Tape, Christie, et al, 2016;Tape, Gustine, et al, 2016), including passerine birds (Mizel et al, 2016(Mizel et al, , 2021.…”