Patterns of tree dominance in coniferous riparian forests

Abstract

This research quantified patterns of riparian tree dominance in western Oregon, USA and then compared the observed patterns with the expected patterns defined from the literature. Research was conducted at 110 riparian sites located on private and public lands. The field sites were selected by probability surveys that were post-stratified by stream order and riparian forest type. An importance value index (IVI), which was the summation of tree relative density, relative frequency, and relative basal area, was used to define tree dominance. The IVI patterns of 19 riparian tree species with respect to ecoregion, geomorphic landform, stream order, and management practice were evaluated. In addition, the riparian trees were assigned to growth habit, life history strategy, and wetland indicator guilds depending on their taxonomy, response to flood disturbance, and wetland ecology, respectively. Douglas-fir, alder, western hemlock, and bigleaf maple were the trees with the highest IVI scores in western Oregon. The general IVI pattern of tree importance on the geomorphic landforms was hardwoods dominating the floodplain, hardwood and coniferous trees co-dominating the terrace, and conifers dominating the transition slope. However, there was considerable variability in the mix of the tree species because of differences in ecoregion, stream order, and management practice. The ecological importance of the dominant species was illustrated by their influence in determining the IVI patterns of the different guilds. For example, Douglas-fir, alder, and western hemlock were one, two, and three in their IVI rankings in western Oregon. The avoider, invader, and resister guilds were also ranked one, two, and three in importance because Douglas-fir, alder, and western hemlock were respectively classified in these groups. Such information is meaningful to developing conservation policy and plans to maintain or restore properly functioning riparian forests for aquatic and terrestrial biota, and to characterize reference condition and biological indicators for long-term ecological monitoring.