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Ecological site AX001X01X406

Frigid Udic Forest

Last updated: 5/15/2025
Accessed: 05/20/2025

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General information

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

MLRA notes

Major Land Resource Area (MLRA): 001X–Northern Pacific Coast Range, Foothills, and Valleys

This area consists of a long and narrow range of mountains with associated foothills and valleys that parallels the Pacific Ocean. This area is entirely within the Pacific Border Province of the Pacific Mountain System in Oregon and Washington. MLRA 1 is bounded on the north by the highest elevations of the Olympic Mountains and the strait of Juan de Fuca, and by the Klamath Mountains on the south. The Washington portion of this MLRA is primarily composed of young Tertiary sedimentary rocks (siltstone and sandstone) mixed with some volcanic rocks of the same age. Glacial till and outwash deposits are also found in the northern half of this area in Washington. Much of this area is accreted terrane formed by tectonic processes. The average annual precipitation ranges from 60 to 200 inches (1,525 to 5,580 millimeters), increasing with elevation. Most of the precipitation in this area occurs during low-intensity, Pacific frontal storms and is evenly distributed throughout fall, winter, and spring.

The dominant soil orders in this MLRA are Andisols, Inceptisols, and Ultisols. Soil depths broadly range from shallow to very deep. Soils are primarily well drained, however poorly drained soils may be found in depressional areas and on alluvial floodplains. Surface textures are typically medial and loamy or clayey. Soils in this area dominantly have a mesic or frigid temperature regime and a udic moisture regime. Soils with aquic moisture regimes and cryic temperature regimes also occur.

Ecological site concept

Frigid Udic Forest sites occur on less stable landscape positions on glacial valley walls, ridges, and colluvial aprons in the frigid temperature zone. These sites are located on the windward side of the Olympic mountains where precipitation is abundant, and on some protected aspects on the leeward side of the mountains where evaporative water loss is low. Relatively high slope gradients limit water infiltration on these sites, generating runoff to more stable Frigid Udic Moist Forests, Frigid Aquic Forests, and Temperate Wet Meadows. As a result, Frigid Udic Forest sites characteristically favor the growth and production of moist mesophytic understory species, such as twinflower (Linnaea borealis) and western swordfern (Polystichum munitum).

Frigid Udic Forest sites are characterized by a dense overstory of Pacific silver fir (Abies amabilis) and Western hemlock (Tsuga heterophylla), accompanied by an understory shrub community of oval-leaf blueberry (Vaccinium ovalifolium) and red huckleberry (Vaccinium parvifolium). Salal (Gaultheria shallon) is often present in the shrub layer. Western redcedar (Thuja plicata) may be present as a minor overstory component. The herbaceous layer is dominated by twinflower, sidebells wintergreen (Orthilia secunda), and western swordfern. Bride’s bonnet (Clintonia uniflora), pipissewa (Chimaphila umbellata), and deerfoot vanilla leaf (Achlys triphylla) are other common forbs on this site.

Associated sites

AX001X01X200	Temperate Wet Meadow Temperate Wet Meadow sites may be found adjacent to or surrounded by Frigid Udic Forest sites. Temperate Wet Meadows occur on depressions and seeps. Temperate Wet Meadow sites lack tree cover.
AX001X01X108	Frigid Aquic Forest Frigid Aquic Forest sites may occur downslope or in mosaic with Frigid Udic Forest sites. Frigid Aquic Forest sites occur on depressions and seeps and are subject to ponding.
AX001X01X407	Frigid Udic Moist Forest Frigid Udic Moist Forest sites may occur downslope of Frigid Udic Forest sites and generally have more stable slope gradients. Frigid Udic Moist Forest sites have higher productivity and typically support salmonberry (Rubus spectabilis).

AX001X01X200

Temperate Wet Meadow

Temperate Wet Meadow sites may be found adjacent to or surrounded by Frigid Udic Forest sites. Temperate Wet Meadows occur on depressions and seeps. Temperate Wet Meadow sites lack tree cover.

AX001X01X108

Frigid Aquic Forest

Frigid Aquic Forest sites may occur downslope or in mosaic with Frigid Udic Forest sites. Frigid Aquic Forest sites occur on depressions and seeps and are subject to ponding.

AX001X01X407

Frigid Udic Moist Forest

Frigid Udic Moist Forest sites may occur downslope of Frigid Udic Forest sites and generally have more stable slope gradients. Frigid Udic Moist Forest sites have higher productivity and typically support salmonberry (Rubus spectabilis).

Similar sites

AX001X01X401	Mesic Udic Forest Mesic Udic Forest sites occur at lower elevations and do not support Pacific silver-fir (Abies amabilis).
AX001X01X408	Frigid Udic Dry Forest Frigid Udic Dry Forests receive less effective precipitation. Frigid Udic Dry Forests have significantly lower productivity.
AX001X01X410	Low Cryic Udic Forest Low Cryic Udic Forest sites are found at higher elevations and support mountain hemlock (Tsuga mertensiana).

AX001X01X401

Mesic Udic Forest

Mesic Udic Forest sites occur at lower elevations and do not support Pacific silver-fir (Abies amabilis).

AX001X01X408

Frigid Udic Dry Forest

Frigid Udic Dry Forests receive less effective precipitation. Frigid Udic Dry Forests have significantly lower productivity.

AX001X01X410

Low Cryic Udic Forest

Low Cryic Udic Forest sites are found at higher elevations and support mountain hemlock (Tsuga mertensiana).

Table 1. Dominant plant species

Tree	(1) Abies amabilis (2) Tsuga heterophylla
Shrub	(1) Vaccinium ovalifolium (2) Vaccinium parvifolium
Herbaceous	(1) Linnaea borealis (2) Orthilia secunda

Legacy ID

F001XA406WA

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Physiographic features

This site primarily occurs on glacial valley walls, ridges, and colluvial aprons on mountains. These upland forest sites are strongly influenced by slope gradient. Frigid Udic Forest sites are typically found on less stable slopes and generate run-off that is collected by more stable Frigid Udic Moist Forest, Frigid Aquic Forest, or Temperate Wet Meadow sites. Frigid Udic Forest sites lose a moderate amount of moisture from run-off and display an intermediate character between the Frigid Udic Moist Forest and Frigid Udic Dry Forest sites.

Table 2. Representative physiographic features

Landforms	(1) Mountains (2) Glacial-valley wall (3) Colluvial apron (4) Ridge
Flooding frequency	None
Ponding frequency	None
Elevation	1,476 – 3,773 ft
Slope	100%
Water table depth	59 in
Aspect	W, NW, N, NE, E, SE, S, SW

Climatic features

This site occurs in a frigid temperature and udic moisture regime. Precipitation arrives mostly via low-intensity, Pacific frontal storms. Precipitation is unevenly distributed, with the lowest amounts on the leeward side of the Coast Range mountains. Precipitation falls largely as snow in higher elevations. Precipitation is evenly distributed throughout the fall, winter, and spring, while summers are dry. Air temperatures vary significantly along the elevation gradient.

Table 3. Representative climatic features

Frost-free period (characteristic range)	90-150 days
Freeze-free period (characteristic range)
Precipitation total (characteristic range)	79-118 in

Soil features

The soils are moderately deep to very deep, have very high or high Ksat throughout, and are well drained. The soils are formed from residuum or colluvium derived from metasedimentary rock. Soil surface textures are loam to sandy loam texture classes, with gravelly to extremely gravelly texture modifiers. Clay content is eight to 17 percent throughout. The soil series for this ecological site are Skookumcreek, Charlialakes, and Beecreek. Although representative of this site, these soils may exist across multiple ecological sites because of naturally variable slope, texture, rock fragments, and pH. An on-site soil pit and the most current ecological site key are necessary to classify a site.

Table 4. Representative soil features

Parent material	(1) Colluvium (2) Residuum – metasedimentary rock
Surface texture	(1) Loam (2) Sandy loam (3) Silt loam
Drainage class	Well drained
Soil depth	8 – 31 in
Surface fragment cover <=3"	5%
Surface fragment cover >3"	5%
Available water capacity (0-40in)	1 – 3.5 in
Soil reaction (1:1 water) (0-10in)	4.5 – 5.5
Subsurface fragment volume <=3" (0-20in)	35 – 65%
Subsurface fragment volume >3" (0-20in)	5 – 20%

Ecological dynamics

Frequent, small-scale disturbances from windthrow events create a mosaic fabric of early-seral patches within late-seral communities. Windthrow events create canopy gaps that favor the establishment of deciduous tree species such as vine maple (Acer circinatum). As conifers succeed and close the canopy gap, vine maple fails to regenerate. Dominant conifers on this site are susceptible to a variety of root and butt rot fungi. Wood decay fungi exacerbate mature trees’ vulnerability to windthrow events.

Infrequent, large-scale disturbances may occur in the form of stand-replacing wildfires, cataclysmic wind events, or large mass movement events. High-force windstorms are a major source of large-scale disturbance in these forests. These blowdown events may be stand replacing and conditions tend to favor shade-intolerant species in their aftermath (Van Pelt, 2007). The fire regime of the western half of the Olympic Peninsula is characterized by high-intensity, stand replacing fires with a long return interval of greater than 100 years (Agee, 1987). Though infrequent, wildfires have a profound influence on the composition of these forests. Douglas-fir is a shade-intolerant species and generally requires a stand-replacing wildfire to clear canopy for successful recruitment and regeneration. In the absence of large-scale disturbance, more shade-tolerant western hemlock and pacific silver fir regenerate successfully and gradually succeed Douglas-fir. The longevity of Douglas-fir preserves evidence of historical high-intensity fire events. Large mass-movement events also provide conditions for the initiation of primary succession. Landslides are a significant source of disturbance, owing to the steep terrain and sedimentary geology of the park. (Gavin, 2014). As with stand-replacing wildfires, bare patches created by landslides favor the establishment of non-shade tolerant Douglas-fir (Geertsema & Pojar, 2007).

Red alder and Douglas-fir recruits regenerate in the aftermath of stand-replacing disturbance, accompanied by red huckleberry (Vaccinium parvifolium) and fireweed (Chamerion angustifolium). Western hemlock gradually establishes post-disturbance, and along with Douglas-fir, begins to form a discrete canopy. As this happens, the understory community diversifies. Shrub and forb layers develop, dominated by red huckleberry and Cascade barberry (Mahonia nervosa), and bride’s bonnet (Clintonia uniflora) and sweet after death (Achlys triphylla), locally known as deerfoot vanilla leaf, respectively. As the canopy begins to mature, western hemlock begins to replace Douglas-fir as the dominant tree species. Given enough time, extremely shade-tolerant Pacific silver fir will establish on the site. At this point, the canopy is composed of a single, dense stratum. Canopy stratification will be encouraged by tree maturation, as well as by mortality induced by small-scale disturbances. Increased vertical stratification promotes diversity and abundance of shrubs and forbs, allowing a return to reference community conditions.

Changes in disturbance frequency or intensity through human alteration of the natural fire regime and a changing climate may promote shifts in community structure and function. For example, fire suppression efforts will likely shift communities further towards shade-tolerant Pacific silver fir and western hemlock-dominated seral stage. (Reed, 2010) Alternatively, increased fire frequency in some areas due to human activity will encourage Douglas-fir recruitment and establishment. (Gavin, 2015)

State and transition model

More interactive model formats are also available. View Interactive Models
Click on state and transition labels to scroll to the respective text

1.1B	-	Minor disturbance
1.1A	-	High-intensity disturbance
1.2A	-	Time without disturbance
1.2B	-	High-intensity disturbance
1.3A	-	Time without disturbance
1.4B	-	High-intensity disturbance
1.4A	-	Time without disturbance
1.5B	-	High-intensity disturbance
1.5A	-	Time without disturbance
1.6A	-	Time without disturbance
1.6B	-	High-intensity disturbance

State 1
Reference

The Reference state is comprised of six communities in varying stages of regeneration following either small-scale or large-scale disturbance.

Dominant plant species

Pacific silver fir (Abies amabilis), tree
western hemlock (Tsuga heterophylla), tree
red alder (Alnus rubra), tree
Douglas-fir (Pseudotsuga menziesii), tree
oval-leaf blueberry (Vaccinium ovalifolium), shrub
red huckleberry (Vaccinium parvifolium), shrub
Cascade barberry (Mahonia nervosa), shrub
vine maple (Acer circinatum), shrub
salal (Gaultheria shallon), shrub
twinflower (Linnaea borealis), other herbaceous
sidebells wintergreen (Orthilia secunda), other herbaceous
sweet after death (Achlys triphylla), other herbaceous
western swordfern (Polystichum munitum), other herbaceous
bride's bonnet (Clintonia uniflora), other herbaceous
fireweed (Chamerion angustifolium), other herbaceous

Community 1.1
Pacific Silver Fir, Western Hemlock, Oval-Leaf Blueberry, Red Huckleberry, Twinflower, and Sidebells Wintergreen

Structure: Multistory with small gap dynamics Pacific silver fir and western hemlock are the dominant overstory species in the reference community. The dense canopy encourages continual regeneration of the extremely shade-tolerant reference conifer species. Common understory species include oval-leaf blueberry, red huckleberry, salal, twinflower, sidebells wintergreen, western swordfern, bride’s bonnet, and deerfoot vanillaleaf. Vine maple may be found in small canopy gaps. High vertical stratification in the canopy and the presence of small gaps favors an abundant understory.

Community 1.2
Pacific Silver Fir, Western Hemlock, Vine Maple, Red Huckleberry, Sweet After Death, and Twinflower

Structure: Mosaic of overstory and openings in varying states of regeneration This community is initiated in the wake of small-scale disturbance which creates small canopy openings. Vine maple establishes in regenerating canopy gaps. Pacific silver fir and western hemlock recruits will eventually establish in the canopy openings and regenerate the canopy. Twinflower is diminished in these less-shaded gaps, while sweet after death frequently establishes in small canopy gaps.

Community 1.3
Red Alder, Douglas-fir, Red Huckleberry, and Fireweed

Structure: open forest with shrubby regeneration and snags This community is an early seral stage, occurring after a stand-replacing disturbance. Nearly all trees have been removed. In the case of high-intensity fire, few large fire-resistant trees may remain. Red alder establishes quickly in recently opened areas. Douglas-fir germinates and establishes in the absence of canopy cover post-disturbance. Bigleaf maple may also establish. Shrubs and forbs are often able to outcompete tree saplings for several years post-disturbance. Red huckleberry and fireweed are frequently the first to establish in early seral communities on this site.

Community 1.4
Douglas-fir, Western Hemlock, Red Huckleberry, Cascade Barberry, Bride's Bonnet, and Sweet After Death

Structure: Dense single-story As a canopy layer establishes, western hemlock seedlings begin to establish. Salal, cascade barberry, and vine maple often establish on this site, accompanied by bride’s bonnet and sweet after death.

Community 1.5
Western Hemlock, Douglas-fir, Cascade Barberry, Red Huckleberry, Twinflower, and Sweet After Death

Structure: Dense single stratum canopy with diminished understory Additional time without disturbance allows western hemlock to gradually replace Douglas-fir. The understory layer continues to develop, with twinflower establishing.

Community 1.6
Western Hemlock, Pacific Silver Fir, Oval-Leaf Blueberry, Red Huckleberry, Twinflower, and Sidebells Wintergreen

Structure: Dense single stratum canopy with diminished understory Pacific silver fir recruits will establish in shaded conditions under the dense canopy. With continued understory diversification, the species composition begins to resemble the reference community. In this community, however, the understory productivity is diminished due to low canopy stratification and dense tree growth which limits available sunlight at the forest floor. Additional time will be required for the stand to develop vertical stratification. Individual tree mortality will promote age class diversity and Pacific silver fir will become either dominant or codominant with western hemlock.

Pathway 1.1B
Community 1.1 to 1.2

Minor disturbances, often caused by individual tree mortality, create small gaps in the forest canopy.