Multiple Scales of Influence on Wetland Vegetation Associated with Headwater Streams in Alaska, USA

Vegetation of wetlands adjacent to headwater streams on the Kenai Lowlands was dominated by , indicating that it is a keystone species that influences stream-wetland interactions across a wide range of geomorphic settings from which headwater streams have their origin. We sampled 30 sites as part of a project to determine the relationships between landscape features and the biological and chemical characteristics of headwater streams and their associated wetlands. In this paper we consider vegetation in wetlands adjacent to headwater streams. was the only species that occurred at all sites and only a few species were widespread and abundant across the range of sites sampled. Nonmetric multidimensional scaling of species importance values indicated that the distribution of sites and species was primarily related to stream-reach scale environmental and biological factors. Sixteen stream-reach factors were significantly correlated with the distribution of sites and species on one axis of the ordination. Headwater streams that were located in relatively flat areas with extensive wetlands had species characteristic of nutrient poor wetlands and sites located in steep valleys with narrow wetlands had species characteristic of uplands and wetlands on mineral soils. The distribution of sites and species on the second ordination axis was interpreted to be a response to biological interactions; primarily the negative relationship between and the diversity of other species. We concluded that large-scale watershed features of the landscape are less important than local scale factors in determining the characteristics of vegetation in headwater stream-wetland complexes in the Kenai Lowlands. There was no evidence, however, that differences in the stream-reach scale conditions across the study sites resulted in distinct plant communities associated with the headwater wetlands even though the headwater streams had their origin in different landscape settings.