Known for the great display of subalpine meadow flowers, the terrestrial vegetation of Mount Rainier National Park is diverse as a result of the co-occurrence of climatic gradients and topographic diversity over relatively short distances across the park's 12,700 foot elevation gradient. Diversity is observed in terms of numbers of species, as well as spatial variation in distribution and abundance. Vegetative assemblages vary across an elevation gradient and somewhat east - west with a precipitation gradient. High annual snowfall is the limiting factor to plant distribution and growth at higher elevations. Patterns of vegetation distribution are also temporally dynamic. Large-scale disturbances (fire, cyclonic winds, insects, avalanches, lahars) can remove the forest overstory over thousands of hectares, resulting in a new successional sequence of overstory and understory species. The size and frequency of these disturbances vary greatly among ecosystems. Fire, although relatively infrequent in the park, is the major disturbance creating diverse successional stages on a large scale. Avalanches and lahars are small to medium scale disturbances. Disturbance processes also act on a small scale, with windthrow, pathogens, and insects causing small gaps and affecting local successional dynamics over time.
Climatic variation is always an overarching factor that affects species regeneration and distribution on long time scales, resulting in non-equilibrium systems with unique assemblages of species co-occurring over centuries to millennia. Botanical exploration has occurred over the past two centuries in the park, with many citations from the late 1800's and early 1900's. There are over 890 vascular species and more than 260 non-vascular plant species and fungi in the park. There are more than 100 exotic plant species, especially along transportation corridors, near trails, and in riparian areas. Approximately 58 percent of the park is covered by forests. Low elevation forests are distributed from the park's boundary at 1700 ft to about 2700-ft elevation and are dominated by western hemlock, Douglas fir, and western red cedar. Mid-elevation forests extend upward to 4000 to 6000 ft elevation and contain Pacific silver fir, Alaska yellow cedar, western white pine, and noble fir. Above 4500 ft, trees become less dense as the forest grades in the subalpine parkland, Forest ages range from young stands (less than 100 years old) invading on moraines left by receding glaciers and on burned areas to old-growth stands 1000 or more years old. The subalpine parkland covers approximately 23 percent of the park; vegetation in this zone is a mosaic of tree clumps and herbaceous meadows extending from 5000 ft to about 7000-ft elevation. Tree cover in this zone is limited by the depth and duration of the snow pack.
Meadow vegetation of this zone is categorized as five broad vegetation types (Henderson 1974): (1) heath shrub, dominated by ericaceous species such as heather or huckleberry; (2) lush herbaceous vegetation, dominated by tall perennials including Sitka valerian, subalpine lupine, and green hellebore; (3) low herbaceous vegetation, dominated by fanleaf cinquefoil and pussytoes, often with lesser amounts of black sedge; (4) wet sedge, in low, wet areas dominated by sedges and sometimes with alpine aster and pussytoes; (5) and dry grass, found on well-drained sites common on the east side of the park and dominated by green fescue and subalpine lupine. Dominant tree species in the subalpine zone are subalpine fir, mountain hemlock, and Alaska yellow cedar. Whitebark pine and Englemann spruce are present on drier sites on the east side of the park. The alpine zone extends from treeline to the mountain's summit. Approximately 50 percent of the zone is covered by permanent snow and ice and the remainder by alpine vegetation, which can be described by four broad vegetation types (Edwards 1980): fellfields, talus slopes, snowbeds, and heather communities. Talus slopes and snow beds have small, well-spaced groups of plants that are often overlooked by park visitors and casual observers.
Vegetation types and species distribution in the alpine zone are controlled by length of the growing season, slope, and aspect. Talus slopes and ridge tops are among the first areas free of snow and thus have the longest growing season. Snow beds have the shortest growing season and may not be snow-free every year. Fellfields and heather communities have an intermediate growing season. Fellfields are areas with gentle slopes covered by small rocks and small, dispersed groups of plants such as sedges, penstemons, and asters. The heather types are the oldest known communities in the park. Some heather communities have persisted in the park for up to 10,000 years.