Hierarchical Organization of Ecosystems at Multiple Spatial Scales on the Yukon-Kuskokwim Delta, Alaska, U.S.A.

Abstract

I conducted an ecological land survey near Hazen Bay, on the Yukon-Kuskokwim Delta of Alaska during 1994–1998 in order to assess potential effects of sea-level rise on coastal ecosystems in this region. Independent classification of three landscape components grouped ecological characteristics of the area into 10 geomorphic units (e.g., tidal flats, abandoned floodplain cover deposits), 9 surface forms (e.g., levees, basins), and 18 plant associations (e.g., Carex rariflora–Salix fuscescens). I then used hierarchical associations among these landscape components to derive an ecosystem classification at three levels of organization that included 10 ecosections (based on geomorphology), 11 ecoseries (based on surface forms and geomorphology), and 27 ecotypes (primarily based on vegetation).

The nature and distribution of ecosystems at all levels showed a strong influence from geomorphic processes. The active floodplain, with frequent flooding and sedimentation, had brackish ecotypes that were dominated by graminoids and forbs. The inactive floodplain, where flooding and sedimentation were infrequent, had slightly brackish ecotypes with a wide diversity of species and growth forms. In contrast, the abandoned floodplain, which lacked flooding and sediment deposition, but was strongly affected by permafrost aggradation, had ecotypes that were dominated by evergreen shrubs, mosses, and lichens that were intolerant to salts, but tolerated acidic, nutrient-poor conditions. Ecotypes with similar vegetation generally had similar environmental properties, including surface elevation, soil morphology, sedimentation, organic matter accumulation, thaw depths, water depths, pH, and electrical conductivity. When similar ecotypes were aggregated into ecosections based on geomorphic similarities, differences in ecological properties increased.