MOUNT RAINIER The Forest Communities of Mount Rainier National Park

Mount Rainier National Park occupies 96,797 ha (241,922 acres) on the western slopes of the Cascade Range in Washington. The diversity of climate, geology, and soils found in this rugged mountain region is outlined in this chapter. Much variability in both vegetation and environment is related to the geography of the Park, making its division into four quadrats of major drainage basins (Carbon, Nisqually-Puyallup, Ohanapecosh-Cowlitz, and White) (Fig. 1) useful in much of the discussion that follows.

Figure 1. Outline of Mount Rainier National Park showing major features and subdrainages.

Climate

Mount Rainier is situated within a temperate, maritime climate. A high pressure region over the north Pacific Ocean shifts southward during fall and winter, and warm, moist air moves from a southwesterly direction into the Cascade Range. Condensation of this cooling air as it rises along the mountain slopes results in a rainy season during late fall and winter and continues almost without break until March or April (Phillips n.d.). At the higher elevations, snow begins accumulating in early November and builds to depths of 5-7 m (18-23 ft) or more by March or April. These wet seasons end when high pressure again develops over the region, and July and August are usually comparatively dry.

Seasonal trends of mean monthly precipitation and temperature are given in Figure 2. Several climatic zones exist within the forested region around the massive ice-laden volcano. The mean monthly temperatures are 2-4°C (4-9°F) lower at Paradise (parkland subzone of Tsuga mertensiana Zone) than at Longmire (Tsuga heterophylla Zone). In addition to the gradient in temperature with elevation, the precipitation varies both with elevation and with position around the volcanic cone. Greatest precipitation is recorded at the high-elevation Paradise station, often in the form of snowfall. Maximum snow accumulation occurred there in March 1956 when the depth reached 9.3 m (367 in). At lower elevations, the Parkway station in the White River drainage is in the driest sector and the Carbon River station is in the wettest sector (especially during spring and summer months).

Because of the dry summers at low to intermediate elevations and the forest vegetation's high transpiration rate, soil water supply is low by August and September; summer months can result in strong contrasts in soil water supply for various forest types (Zobel et al. 1976). At higher elevations, however, snow melt, runoff, and shorter growing seasons considerably lessen the duration and intensity of water stress in forest vegetation. Also, fog or cloud condensation at elevations below dew-point further lower the gradient in transpiration during summer months. Therefore, forests within the climate of the Tsuga mertensiana Zone appear to be influenced and differentiated by topographic variations in snow accumulation and duration, soil water drainage, and heat budgets. Growing seasons are comparatively short, and opportunities for tree seedling germination and survival are limited by snow depths, low temperatures, and restricted periods of photosynthesis.

Geology

The geology of the Park has been clearly summarized by Crandell (1969). In the forested landscape, four general geological processes have influenced forest development: (1) Tertiary volcanism and sedimentation; (2) Pleistocene and Holocene eruptions that produced the Rainier volcanic cone and showered the region with volcanic ash; (3) lahars, landslides, rockfalls, and other mass wasting and removal processes; and (4) glacial and fluvial events of the Pleistocene and Holocene eras.

Figure 2. Temperature and precipitation regimes in Mount Rainier National Park: A, mean monthly temperatures from Paradise station (x) at 1665 m (5,550 ft) and Longmire station (&bul;) at 829 m (2,762 ft); B, mean monthly precipitation from Paradise station (x, 31-39 years of record, 1665 m or 5,550 ft elevation), Longmire (&bul;, 48-50 years of record. 829 m or 2,762 ft elevation), Ohanapecosh (—, 24-28 years of record, 577 m or 1,925 ft elevation), Carbon River (c, 21-22 years of record, 608 m or 2,026 ft elevation), and Parkway (Δ, 13-17 years of record, 945 m or 3,150 ft elevation). (Adapted from U.S. Department of Commerce, Weather Bureau, 1965.)

Much of the forested landscape further away from the Rainier volcanic cone occurs on ridges of Tertiary rocks radiating from it in spokelike fashion. Major geologic formations include the Ohanapecosh (sandstone and breccias), Steven's Ridge (welded ash-flow tuffs), and Fifes Peak Formations (andesite). Granodiorites intruded the Mount Rainier area about 12 million years ago; they outcrop in various places along the White, Carbon, and Nisqually valleys, and include the Tatoosh Range. The distribution of these Tertiary formations is shown on a map by Fiske et al. (1963). Many of our study plots of steep colluvial slopes or ridges (such as Backbone Ridge) occur on these formations that predate the Rainier volcanic cone.

Mount Rainier is a complex composite volcano that probably originated half a million years ago. Ridgetops such as Rampart Ridge and Klapatche Ridge were formed by andesitic lava flows that moved down and filled old river valleys. Similar flows, piling on top of one another through time, built up the central cone until its summit was some thousands of feet higher than Mount Rainier is at present. During Holocene time (the past 10,000 years), eruptions from Mount Rainier and other volcanoes, particularly Mount St. Helens, have showered the area with a variety of pyroclastic deposits (Mullineaux 1974). These deposits are particularly important because they form part or all of the soil materials in most forested areas.

Many lahars (volcanic mudflows) originating on Mount Rainier during the last 10,000 years have moved down valley floors and buried them under meters of rock debris. These lahars devastated existing forests and their deposits provided new substrate for primary succession. Many lahar surfaces on Mount Rainier have been dated (Crandell 1971). Recent lahars occurred in all the valleys, but did not reach lower, forested areas of the Ohanapecosh and Carbon River valleys. The Osceola mudflow in the White and West Fork valleys evidently originated in avalanches of hydrothermally altered rock near the summit of Mount Rainier about 5,700 years ago. It is one of the largest lahars known in the world; its tremendous volume might represent the former summit of Mount Rainier (Crandell 1971). Effects of smaller lahars on forests are shown by the 1947 Kautz lahar in the Nisqually drainage on which forests are now in early successional stages (Fig. 3).

Other recent mass wasting processes that start forest succession include large landslides, and more localized rock debris flows, talus slides, and floods. Some of the major landslides are ancient relative to the ages of the oldest forests we studied (Fiske et al. 1963) and are covered by layers of pyroclastic deposits. On steeper slopes, however, where colluvial processes are active, soils commonly consist of mixtures of volcanic ash and rock fragments. Important, too, are the relief and drainage features created by these mass wasting or slump processes on forested slopes (Swanston and Swanson 1976). Slow drainage on closed depressions (such as benches) and very rapid drainage of taluses are good examples.

Figure 3. Mudflows have periodically destroyed forests in some of the major river valleys. Kautz Creek mudflow, pictured here in 1978, occurred in October of 1947 and killed most of the trees by burying their root systems.

Glacial, glacial-outwash, and alluvial landforms are common at all forested elevations on Mount Rainier. Rates of forest establishment on high elevation moraines have been measured by Sigafoos and Hendricks (1972). Complex interbeddings of alluvial and lahar deposits are found along the lower valleys (the lahar assemblages of Crandell 1971). Glacial outburst floods (Richardson 1968) may cut away existing river terraces and create new ones as substrates for new forest successions. Surficial deposits of glacial origin include a wide variety of drift materials (Crandell and Miller 1974). Many of these are buried under recent volcanic ash, and are not important as soil substrates. But some of our forest plots are located in landscapes topographically or edaphically controlled by these older drifts. At limited locations in the Tsuga mertensiana Zone, more recent drifts (McNeeley and Garda Drifts) are found, but most of the Holocene deposits are within the subalpine park land outside the closed forest region of this study.

Topography

The topography of the forested valleys and slopes around Mount Rainier is very rugged because the landscape is comparatively youthful (Fig. 4). Steep runoff and erosion gradients exist along the flanks of this 4394-m (14,410-ft) volcano. The variety of geomorphic processes shaping the landscape include volcanic, glacial, fluvial, mass wasting, and other erosion processes described earlier. The resultant topography exists in scales ranging in magnitude from kilometers (for major landform features) to meters (microrelief).

Figure 4. Forests typically occupy rugged mountain slopes and ridgetops in Mount Rainier National Park. A portion of the White River valley.

At elevations below 1000 m (3,500 ft), broad valleys radiating from the volcanic cone are separated by steep to vertical canyon walls. Nearly flat or very gentle slopes characterize the alluvial and lahar deposits on these valley floors, with hummocky microrelief at the smaller scale. Lower valley sideslopes are of moderate to severe steepness, especially where glaciers or streams exerted intense, downcutting activity.

Cliffs and ledgy canyon sidewalls are particularly conspicuous, for example, in the Ohanapecosh drainage. At smaller scales, hillslope erosion and mass wasting have created systems of ridges and depressions and local, benchy topography on these sideslopes (Swanston and Swanson 1976).

The forested midelevations of 1000 to 1350 m (3,500 to 4,500 ft) present certain topographic contrasts to the lowlands. Streambed gradients of valleys are steeper; in places, spectacular, entrenched rivers are illustrated by such features as the Box Canyon of the Cowlitz. Valley profiles within major drainages may present more abrupt contrast between valley flows, narrow toeslopes, and steep sidewalls. Cliffs and falls are common. The canyon slopes present a complex focal relief of draws and downsloping ridges, benchy and uneven topography.

The upper elevation forests above 1350 m (4,500 ft) are commonly found on the upper surfaces of volcanic flows and within cirques of upper glacial basins. Relief varies from nearly flat (for example, Grand Park) to gentle, undulating topography. The steep and very steep to pography of midelevations is less prevalent. Upper slopes and ridges are common. Glacial basins and morainal features are dissected by snowmelt and runoff channels. Other local topographic features include rock outcrops (along Pinnacle Peak trail in the Tatoosh Range, for example), rock avalanche and talus slopes, and rough broken land along upper valley slopes.

The distribution of topographic relief features according to elevation determines the shape of the forest-environment diagrams provided in Chapter 6 (see, e.g., Fig. 34). Upper slopes and ridgetops are more characteristic of upper elevations, whereas broad valley bottoms, streamside terraces, and river bars are most important at the lower. But each of the major forested drainages around Mount Rainier present differences in topography which account for the shape of each diagram.

Soils

The soils of the Park have not been described in published literature. Franklin (1966) recognized the podzolic nature of many soil profiles and described numerous buried soil horizons resulting from successive volcanic ash deposits. Data from similar soils in the northern Washington Cascade Range indicate that such soils fit all but one criterion for classification into the Spodosol order (Singer and Ugolini 1974). The accumulations of surface organic horizons, development of iron pans, and particle movement from eluvial to illuvial horizons are typical features of soil profiles (Franklin 1966, Hobson 1976).

All soil parent materials in the Park are of Holocene age. Profiles have developed in materials of glacial, alluvial, colluvial, or pyroclastic origin. Topography and geographic locations relative to eruptive sources of pyroclastic particles are major determinants of soil parent material composition. The most common soil parent materials are the pyroclastic deposits (tephras)¹ erupted from Mount Rainier, Mount St. Helens, and, less commonly, Mount Mazama. Mullineaux (1974) and Hobson (1976) describe the distribution of these pyroclastics in our study area and their importance as soil parent materials. Hobson (1976) classified Mount Rainier soils on the basis of geological origin, relief, and drainage features. His system was adopted for this report.

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¹Tephra refers to all types of volcanic ejecta (e.g., ash or pumice) that are aerially deposited on the landscape.

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Tephra soils can be identified by individual ash layers (Mullineaux 1974) differentiated by color, texture, and presence or absence of lapilli (Fig. 5). Many possible combinations of ash layers are found in profiles because of the nature of tephra distribution in this region, the presence of lithic layers between tephras, and various mixing and local redistribution processes associated with topographic relief (Hobson 1976). Furthermore, any particular pyroclastic deposit might bury the existing forest soil profile in either intact or altered condition. Colluvial material might also be found within or between various tephra layers, but generally comprise less than about 2 percent of the soil volume. At wetter sites internal drainage might be moderately or strongly impeded by iron pan development. On relatively hot, dry sites drainage is usually good, and development of podzolic A2—B2 horizon sequences may be weak.

Figure 5. Many of the soils in Mount Rainier National Park are formed in pyroclastic materials such as the conspicuous tephra W. the whitish deposit, underlain by coarse-textured lapilli of the tephra C.

Tephra soils are very common in the forests. The following profiles indicate typical features of these soils. The first profile was found beneath an Abies amabilis/Rhododendron albiflorum community near Narada Falls (Franklin 1966):

This profile, taken from a river terrace along Fryingpan Creek (see Fig. 5), shows the relationships between pyroclastic deposits (Mullineaux 1974) and soil horizons in even more detail:

Colluvial soils are the dominant soil group in the Park. Hobson (1976) describes these as unstable soils, rapidly drained, and consisting of coarse, unconsolidated, mixed parent materials. They are found on slopes at all elevations, but especially the steeper slopes and south-facing aspects. Colluvial soils intergrade with tephra soils, but are identified primarily by the mixing of pyroclastic and nonpyroclastic materials. The pyroclastic materials are minimally layered, and where several ash deposits exist they are generally well mixed. Notable exceptions occur in colluvial soils having an unmixed tephra W surface layer or tephra Y sublayer. The following profile was considered by Hobson (1976) to be representative of colluvial soils in the Park. It is found under an Abies amabilis/Xerophyllum tenax community on a steep mid-slope on Rampart Ridge:

Alluvial soils occur in major river valleys, along lesser streamsides, on wet benches where fine-textured water-deposited materials are often mixed or interbedded with tephras, and on alluvial slopes and fans. Soil materials are often in stratified, water-laid layers on depositional landforms. Along the floodplains of broader valleys, centuries of deposition and subsequent downcutting by rivers produced terraces well above the present-day stream channels. Elsewhere, alluvial deposits have been formed by glacial outburst floods or by ephemeral streams carrying snowmelt discharge from upper slopes. Depositional sequences of alluvial materials vary considerably in thickness and textures. Various surface or subsurface strata may consist of skeletal, cobbly sands, coarse undifferentiated sand, and fine or very fine sands. Deposition may also have resulted in patterns of mounds and depressions, whose microrelief presents strong contrasts in soil moisture and drainage. Some alluvial soils exhibit A1 horizon, others may have subsurface gleyed horizons. Many can be classified as Fluvents (Soil Survey Staff 1975). The following example of an alluvial soil is from a Tsuga heterophylla/Oplopanax horridum community on a river bar at the Grove of the Patriarchs:

Mudflow soils are Hobson's (1976) fourth group. Surface or subsurface parent materials within the rooting zone are of laharic origin. The soils may also contain tephra W or alluvial or colluvial surface deposits. In such cases, the presence of rounded rocks or boulders on or beneath the surface help identify mudflow soils. Soil profiles range from totally undifferentiated to displaying well-developed horizons, as given in the example below. Old lahar surfaces in major valleys often have surficial alluvial deposits, so that soils may closely resemble the group of alluvial soils described above. The following profile was found under an Abies amabilis/Vaccinium alaskaense community on an upper slope near Round Pass and belongs to Hobson's group of mudflow soils:

Hobson's classification of soils did not include all the soil types we encountered in this study. Miscellaneous soils included Entosolic profiles of recent moraines and drifts, and occasional peaty or deep humic soils along streams within the Tsuga mertensiana Zone.