Riparian willow communities on the Arctic Slope of Alaska and their environmental relationships: A classification and ordination analysis

Abstract

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 Anal- ysis. We also examined synecological differences in terms of community characteristics (e.g. growth form distributions, species richness, soil pH and climatic affinities, phyto- geographic 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 eco- systems. 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 com- munities. Classification resulted in three associations and four subassociations, each occu- pying 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 thermo- philous species and comparatively higher cover percentage of forbs. Higher terraces show the paradoxon of better developed soils and decreasing produc- tivity of the shrub layer. With decreasing river influence and the transition to finer-tex- tured, more nutrient-rich, less basic soils, the tall willow community is replaced by the Anemono-Salicetum richardsonii ass. nov. (subass. lupinetosum arctici). De- creasing active layer depth, caused by the insulation of a thick moss layer and conse- quently 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 predomi- nantly autogenic processes resulting inter alia in an uniquely arctic soil thermal regime.