Hierarchial organisation of ecosystems at multiple spatial scales on the Yukon-Kuskokwim Delta, Alaska, USA

I conducted an ecological land survey near Hazen Bay, on the Yukon-Kuskokwi m Delta of Alaska during 1994-1998 in order to assess potential effects of sea-level rise on coastal ecosystems in this region. Independent classifica tion of three landscape components grouped ecological characteristics of th e area into 10 geomorphic units (e.g., tidal flats, abandoned floodplain co ver deposits), 9 surface forms (e.g., levees, basins), and 18 plant associa tions (e.g., Carex rariflora-Salix fuscescens). I then used hierarchical as sociations among these landscape components to derive an ecosystem classifi cation 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 inf luence from geomorphic processes. The active floodplain, with frequent floo ding and sedimentation, had brackish ecotypes that were dominated by gramin oids and forbs. The inactive floodplain, where flooding and sedimentation w ere infrequent, had slightly brackish ecotypes with a wide diversity of spe cies and growth forms. In contrast, the abandoned floodplain, which lacked flooding and sediment deposition, but was strongly affected by permafrost a ggradation, had ecotypes that were dominated by evergreen shrubs, mosses, a nd Lichens that were intolerant to salts, but tolerated acidic, nutrient-po or conditions. Ecotypes with similar vegetation generally had similar envir onmental properties, including surface elevation, soil morphology, sediment ation, organic matter accumulation, thaw depths, water depths, pH, and elec trical conductivity. When similar ecotypes were aggregated into ecosections based on geomorphic similarities, differences in ecological properties inc reased.