Patterns of conifer establishment and vigor on montane river floodplains in Olympic National Park, Washington, USA

Abstract

In the Pacific Coastal Ecoregion, coniferous trees are often prescribed for riparian restoration, yet little is known about their establishment on floodplains under natural conditions. In this study, 10- to 50-year-old floodplain surfaces of six rivers were surveyed to (1) quantify conifer distribution along study reaches, (2) describe relationships between conifer presence and selected biological and environmental variables, and (3) compare growth rates and relative vigor of conifers among sites. We found conifers on 17%–36% of the plots we sampled. Sitka spruce ( Picea sitchensis (Bong.) Carrière) was most common on the wetter sites, while Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) was most common on the drier sites. Other conifers common to adjacent terraces were extremely rare. Douglas-fir preferred elevated sites with shallower soils and fewer hardwood competitors (e.g., Alnus rubra Bong. and Salix spp.) than similar plots without conifers. For Sitka spruce, the variables examined revealed no statistical differences between conifer and non-conifer plots. Our findings suggest that the tolerance of Douglas-fir to drier conditions allows it to survive on relatively higher, drier sites where more moisture-demanding competitors fail. For Sitka spruce, factors other than those measured appear to be more important in spruce establishment and survival.