Riparian willow communities on the Arctic Slope of Alaska and their environmental relationships: A classification and ordination analysis
Schickhoff, Udo; Walker, Marilyn D.; Walker, Donald A.
We present the first analysis of riparian vegetation types on the Arctic Slope of Alaska including classification and ordination. We classified riparian willow communities according to the Braun-Blanquet approach, and analysed environmental relationships of associations to complex environmental gradients using Detrended Correspondence Analysis. We also examined synecological differences in terms of community characteristics (e.g. growth form distributions, species richness, soil pH and climatic affinities, phytogeographic patterns) to better understand ecological alteration processes and changing species compositions along successional gradients. Data were collected along a transect from the Brooks Range to Prudhoe Bay, primarily in the watersheds of Sagavanirktok River and Kuparuk River. Riparian vegetation in arctic Alaska mainly consists of willow shrub communities which are functionally important components of arctic landscape ecosystems. A combination of edaphic conditions (soil pH, soil moisture) and factors related to topography, disturbance regime and landscape evolution (river terrace/stream bank development) controls spatial patterns and floristic compositions of riparian plant communities. Classification resulted in three associations and four subassociations, each occupying distinct riparian habitats, and, thus, indicating distinct environmental conditions. The Epilobio-Salicetum alaxensis ass. nov. is a true pioneer community along mountain creeks (subass. polemonietosum acutiflori) and on gravel bars, floodplains and lower terraces of rivers (subass. parnassietosum kotzebuei). This tall willow association indicates sites with frequent disturbances and coarser-textured, relatively dry, initial alluvial soils with basic reaction that have deep active layers and relatively high soil temperatures. The association may persist on river banks as long as erosion and deposition of new increments of alluvium occurs, i.e. as long as predominantly allogenic processes are operative in succession cycles. It is characterized by relatively low species richness, considerable percentage of North American endemics, higher amount of more thermophilous species and comparatively higher cover percentage of forbs. Higher terraces show the paradoxon of better developed soils and decreasing productivity of the shrub layer. With decreasing river influence and the transition to finer-textured, more nutrient-rich, less basic soils, the tall willow community is replaced by the Anemono-Salicetum richardsonii ass. nov. (subass. lupinetosum arctici). Decreasing active layer depth, caused by the insulation of a thick moss layer and consequently lower soil temperatures, as well as lower root activity and rooting space, and higher soil moisture reduce the competitiveness of Salix alaxensis-stands and are thus key factors for this successional replacement with low willows. The Anemono-Salicetum richardsonii characterizes later stages of succession on river alluvium with predominantly autogenic processes resulting inter alia in an uniquely arctic soil thermal regime. Shrubs and mosses are dominant growth forms in this association, which is further characterized by comparatively highest species richness. It also occurs on upland tundra stream banks (subass. salicetosum pulchrae), where environmental conditions and floristic composition point to a certain affinity to the Valeriano-Salicetum pulchrae ass. nov. The latter association is distributed on older, long-deglaciated land surfaces with paludified, loamy, acid soils with massive ground ice and thick moss layers, resulting in cold soils, decreased depth of thaw, and increased soil moisture. It typically occupies the immediate margins of smaller tundra streams and creeks with a specific streamflow regime (low disturbance level), originating in the gentle topography of the foothills. Water saturation of the soils lead to reduced decomposition of organic residues and thus to high contents of organic matter. Percentage of acidophilous species and of more cold-adapted species as well as moss cover and bryophyte species richness are comparatively very high. This terminal riparian vegetation type seems to be connected to long-established hydrologic patterns and associated riparian ecosystem evolution along headwaters in upland tundra, and has developed in other time scales compared to the above associations. Due to synecological and physiognomic correspondences and floristic similarities in supraregional and circumpolar perspective, North Alaskan riparian willow associations can be assigned to existing higher syntaxonomic units of the Braun-Blanquet system, established in Europe and Greenland. Strong affinities do exist to alluvial willow associations of the Salicetea purpureae. We propose to extend the range of this class to the North American Arctic.