The floodway zone of the Hoh River exhibits four terrace levels of different ages, formed by erosional activity of the river on valley fills. The vegetation in this valley is in a long—term seral sequence as shown by the zonal pattern in relation to aging and development of these land surfaces. Succession starts on gravel bars, which are dominated by Alnus rubra and Salix scouleriana. The following sequential forest communities, and associated ages of land surfaces, are found: Alnus rubra on alder flats (80—100 yr); Picea sitchensis—Acer macrophyllum—Populus trichocarpa on first terraces (400 yr); Picea sitchensis—Tsuga heterophylla on second terraces (750 yr); and Tsuga heterophylla on third terraces. The latter represents the climax community for the river terrace sere, and it occurs on surfaces exposed by retreating Pleistocene alpine glaciers. The first three terraces are derived from Neoglacial alluvial fills. There is a strong correlation among zonation patterns, forest succession, age of terraces, soil moisture, and soil profile development. Available soil moisture is an important factor governing the zonal sequence. The younger land surfaces are significantly drier than the older terraces. Plants on alder flats and first terraces must withstand greater moisture stress than those of second and third terraces. As the land surface ages, the soil profile develops; deeper, more mature soils are found away from the river. The term "Olympic rain forest" is inappropriately applied to this vegetation; "temperate moist coniferous forest" is more appropriate not only for forests in the Hoh Valley, but also for the rest of the Olympic Mountains and vegetation along the northern Pacific coast.
The relationships among community pattern, date of snowmelt, and microclimate were examined on Excelsior Ridge in the North Cascades. Nine communities were delineated and placed into three groups based on date of snowmelt and topographic location. The Veratrum viride, Valeriana sitchensis, Lupinus latifolius, and Carex spectabilis communities, all found on windward slopes, were released from snow from June to mid-July. The Phyllodoce empetriformis, Vaccinium deliciosum, and Saxifraga tolmiei communities were located on leeward slopes where snow melted in July. Sheltered basins, which retained snow until late July or early August, supported the Carex nigricans and Luetkea pectinata communities. The influence of time of snowmelt generally was reflected in soil temperature, so that earlier sites were warmer than later sites. A soil moisture gradient was not apparent among any of the communities.
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