Riparian tree seedling distribution on Wisconsin River sandbars: controls at different spatial scales

We investigated the influence of physical characteristics at the local (quadrat) and landscape (sandbar, channel, and river cross-section) scale on the distribution of pioneer tree seedlings (Acer saccharinum, Betula nigra, Populus deltoides, and Salix spp.) on 30 sandbars along a 16-km reach of the Wisconsin River in 1998. At the landscape scale, new seedlings were most frequent in side channels that wore inactive (stagnant) at low river flow, older seedlings were most frequent in narrower side channels, and saplings were most frequent on higher sandbars and in narrower channels. At the local scale, seedling occurrence in individual 1 x 2 m quadrats (n = 692) was significantly related to the horizontal and topographic position and the vegetative cover on the quadrat. Landscape relationships for species that dispersed prior to a small midsummer flood (Acer and Betula) were stronger than models for later dispersers (Populus and Salix), and models for old seedlings and saplings were stronger than those for new seedlings. Both local and landscape variables were significant for explaining local seedling occurrence, and significant landscape x local scale interactions suggested that the influences of some local variables on seedling occurrence were conditional on landscape context. All in all, our results suggest that both local and landscape variables influence the distribution of riparian vegetation, that the influence of local variables may be constrained by landscape context, and that the influence of landscape variables may increase with seedling age. Key words: Acer saccharinum; Betula nigra; fluvial geomorphology; hierarchy; landscape ecology; multiple spatial scales; plant demography; Populus deltoides; riparian vegetation; Salix exigua; Salix nigra; succession.