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Ecological site AX001X01X003

Mesic Udic Alluvial Terrace Forest

Last updated: 5/15/2025
Accessed: 10/06/2026

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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 flood plains. 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

Mesic Udic Alluvial Terrace Forest sites occur on high terraces and colluvial aprons in river valleys. This site occurs is in the transitional zone between flood plain forests and upland forests. Unlike the flood plain forests, flooding occurs very rarely on this site. Mesic Udic Alluvial Terrace sites are characterized by a dense overstory of western hemlock (Tsuga heterophylla) and Douglas-fir (Pseudotsuga menziesii). Western hemlock is a shade-tolerant species, and may continually regenerate in the shaded understory, while Douglas-fir is shade-intolerant and will not regenerate in the absence of adequate sunlight. The understory shrub layer is dominated by salmonberry (Rubus spectabilis), red huckleberry (Vaccinium parvifolium), and vine maple (Acer circinatum). The forb layer is typically composed of western swordfern (Polystichum munitum), twinflower (Linnaea borealis), youth on age (Tolmiea menziesii), pacific trillium (Trillium ova tum), common ladyfern (Athyrium filix-femina), false lily of the valley (Maianthemum dilatatum), and redwood-sorrel (Oxalis oregana).

Associated sites

AX001X01X002	Mesic Udic Flood Plain Forest Mesic Udic Flood Plain Forest sites occur on lower alluvial terraces and are subject to more frequent flooding.
AX001X01X401	Mesic Udic Forest Mesic Udic Forests are upland forest sites found above Mesic Udic Alluvial Terrace Forest sites.
AX001X01X402	Mesic Udic Moist Forest Mesic Udic Moist Forests are upland forest sites found above Mesic Udic Alluvial Terrace Forest sites.
AX001X01X200	Temperate Wet Meadow Temperate Wet Meadows are found on depressions and seeps adjacent to or surrounded by Mesic Udic Alluvial Terrace Forest sites. Temperate Wet Meadow sites lack tree cover.

Similar sites

R004AA005WA	Bog or Fen Isomesic Udic Alluvial Terrace Forest sites occur at lower elevations in the Sitka Spruce belt and are readily distinguished by the presence of Sitka spruce (Picea sitchensis).
AX001X01X107	Frigid Udic Alluvial Terrace Forest Frigid Udic Alluvial Terrace Forest sites occur at higher elevations and are distinguished by the presence of Pacific silver-fir (Abies amabilis).

R004AA005WA

Bog or Fen

Isomesic Udic Alluvial Terrace Forest sites occur at lower elevations in the Sitka Spruce belt and are readily distinguished by the presence of Sitka spruce (Picea sitchensis).

AX001X01X107

Frigid Udic Alluvial Terrace Forest

Frigid Udic Alluvial Terrace Forest sites occur at higher elevations and are distinguished by the presence of Pacific silver-fir (Abies amabilis).

Table 1. Dominant plant species

Tree	(1) Tsuga heterophylla (2) Pseudotsuga menziesii
Shrub	(1) Rubus spectabilis (2) Acer circinatum
Herbaceous	(1) Tolmiea menziesii (2) Polystichum munitum

Legacy ID

F001XA003WA

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Physiographic features

This site primarily occurs on terraces and colluvial aprons on river valleys. Mesic Udic Alluvial Terrace Forest sites are found on higher alluvial terraces and do not receive frequent floodwater. As a result of their transitional landscape position, these forests display some character of both Mesic Udic Flood Plain Forests and Upland Mesic Udic Forest sites.

Table 2. Representative physiographic features

Landforms	(1) River valley > Terrace (2) River valley > Colluvial apron
Flooding duration	Brief (2 to 7 days)
Flooding frequency	Very rare to occasional
Ponding frequency	None
Elevation	656 – 1804 ft
Slope	0 – 15 %
Water table depth	0 – 59 in
Aspect	W, NW, N, NE, E, SE, S, SW

Climatic features

This site occurs in a mesic temperature regime, 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)	180-240 days
Freeze-free period (characteristic range)
Precipitation total (characteristic range)	70-110 in

Soil features

The soils that support this ecological site occur in the mesic soil temperature regime and the udic soil moisture regime. Stukey and Haydenpass soils are well drained, very deep, formed from mixed geology alluvium, and are found on alluvial terraces in river valleys. Saturated hydraulic conductivity is high or very high throughout. These soils have a seasonally high water table that is below 60 inches of the surface during the growing season (March through October). These soils have very rare or occasional flooding potential in January, February, March, and April. Stukey has greater than 35 percent rock fragments and Haydenpass has less than 35 percent rock fragments in the control section. Flooding frequency is the limiting factor for plant growth on these soils. Most soils contain a thin organic layer that protects the soil from wind and water erosion. 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) Alluvium
Surface texture	(1) Silt loam (2) Loam
Drainage class	Well drained
Soil depth	59 – 0 in
Surface fragment cover <=3"	0 – 5 %
Surface fragment cover >3"	Not specified
Available water capacity (0-40in)	2.8 – 7.7 in
Soil reaction (1:1 water) (0-10in)	4.5 – 5.5
Subsurface fragment volume <=3" (0-20in)	0 – 50 %
Subsurface fragment volume >3" (0-20in)	0 – 15 %

Ecological dynamics

Mesic Udic Alluvial Terrace sites are subject to disturbance from windstorm events, wildfires, floods, and mass movement events. Windthrow may create small gaps in the overstory canopy that favor understory regeneration. Frequent windthrow creates varied-age patches of trees. Root and butt rot fungi exacerbate trees’ susceptibility to windthrow. Wildfires are an additional source of disturbance, though they occur much less frequently. The fire regime of this forest type is characterized by infrequent, severe fire events. The fire return interval is very high, 100 to nearly 1,000 years (FEIS, 2012). Flooding events may occur very rarely. Since these sites are generally located on high stream terraces, a large volume of water is required for streams to overflow to this level. Alluvial terrace forest sites experience less than one percent probability of flooding in any year. When they do occur, intense flood events may cause tree mortality, scour the mineral soil surface, and can remove existing vegetation from the site.

Any stand-replacing disturbance that removes overstory tree species will generally favor the regeneration of shade-intolerant species in its aftermath. Douglas-fir is a highly shade-intolerant tree species and generally requires a stand-replacing wildfire event to clear canopy for successful recruitment and regeneration. In the absence of large-scale disturbance, more shade-tolerant western hemlock will continually regenerate and gradually succeed Douglas-fir. The longevity of Douglas-fir preserves evidence of historical high-intensity wildfire events. Sufficiently destructive windthrow or flood events may create conditions favorable for Douglas-fir recruitment, but not as reliably as do wildfire events.

Red alder recruits quickly regenerate in the aftermath of stand-replacing disturbance, accompanied by Sitka willow (Salix sitchensis) and coastal hedgenettle (Stachys chamissonis). Bigleaf maple will be the next tree species to establish on the site, while salmonberry (Rubus spectabilis) and California blackberry, locally known as trailing blackberry, (Rubus ursinus) typically establish. As time passes, shrub and forb layers develop discrete from the overstory layers, dominated by salmonberry, vine maple (Acer circinatum), and western swordfern (Polystichum munitum). By this point, Douglas-fir has likely established on the site. Western hemlock will slowly regenerate in the shaded understory and will gradually replace bigleaf maple, and eventually will become the dominant tree species on the site. At this point, the forest 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 a shade-tolerant 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

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

State 1
Reference

The Reference state is comprised of five communities is varying stages of regeneration. Communities in the Reference state range from a sparse, emergent understory of shrubs and forbs that is found in the early aftermath of disturbance, to mature stands of conifers.

Dominant plant species

western hemlock (Tsuga heterophylla), tree
Douglas-fir (Pseudotsuga menziesii), tree
red alder (Alnus rubra), tree
bigleaf maple (Acer macrophyllum), tree
salmonberry (Rubus spectabilis), shrub
vine maple (Acer circinatum), shrub
Sitka willow (Salix sitchensis), shrub
California blackberry (Rubus ursinus), shrub
youth on age (Tolmiea menziesii), other herbaceous
western swordfern (Polystichum munitum), other herbaceous
coastal hedgenettle (Stachys chamissonis), other herbaceous

Community 1.1
Western Hemlock, Douglas-fir, Salmonberry, Vine Maple, Youth on Age, and Western Swordfern

Structure: multistory with small gap dynamics
Western hemlock and Douglas-fir are the dominant tree species in the reference community. Individual mortality of mature trees creates small canopy gaps. Western hemlock tolerates shade and will continually regenerate in the understory. Deciduous regeneration is generally limited to canopy gaps. The understory consists of diverse shrub, forb, and grass layers.

Community 1.2
Red Alder, Sitka Willow, and Coastal Hedgenettle

Structure: sparse understory of emergent shrubs, forbs, and grasses
This community follows a stand-replacing disturbance event. Red alder, Sitka willow, and coastal hedgenettle quickly establish where the canopy has been removed.

Community 1.3
Red Alder, Bigleaf Maple, Salmonberry, California Blackberry, Youth on Age, and Western Swordfern

Structure: shrubby single story with scattered understory of trees, shrubs, forbs, and grasses
This community follows the initial phase of regeneration post-disturbance. As time progresses, bigleaf maple successfully establishes. The shrub and forb layers gradually diversify as slow-growing species begin to regenerate.

Community 1.4
Douglas-fir, Bigleaf Maple, Salmonberry, Vine Maple, Youth on Age, and Western Swordfern

Structure: deciduous forest with a mix of shrubs, forbs, and grasses, with conifer regeneration in the understory
Douglas-fir recruits have established on the site and largely replaces red alder. Vine maple is a significant understory shrub component in this community.

Community 1.5
Douglas-fir, Western Hemlock, Salmonberry, Vine Maple, Youth on Age, Western Swordfern

Structure: single story with few small openings
Western hemlock begins to replace bigleaf maple as the subdominant tree species and will continually regenerate in the shaded understory. The canopy consists mainly of a single, dense layer. The dense canopy results in lowered understory productivity relative to the reference community phase. As time progresses, individual tree mortality and small-scale disturbance will act on the site to create canopy gaps and increase vertical stratification.

Pathway 1.1A
Community 1.1 to 1.2

Stand-replacing disturbance events such as high-intensity fire, catastrophic windstorms, and floods open the forest canopy and lead to the stand initiation phase of development.

Pathway 1.2A
Community 1.2 to 1.3

Time without disturbance allows regeneration, growth, and progression to a later seral stage.

Pathway 1.3B
Community 1.3 to 1.2

Stand-replacing disturbance events such as high-intensity fire, catastrophic windstorms, and floods open the forest canopy and lead to the stand initiation phase of development.

Pathway 1.3A
Community 1.3 to 1.4

Time without disturbance allows regeneration, growth, and progression to a later seral stage.

Pathway 1.4B
Community 1.4 to 1.2

Stand-replacing disturbance events such as high-intensity fire, catastrophic windstorms, and floods open the forest canopy and lead to the stand initiation phase of development.

Pathway 1.4A
Community 1.4 to 1.5

Time without disturbance allows regeneration, growth, and progression to a later seral stage. Individual tree mortality creates small canopy openings.

Pathway 1.1A
Community 1.5 to 1.1

Time without disturbance allows regeneration, growth, and progression to a later seral stage. Vertical stratification increases and forb diversity increases. Individual tree mortality creates varied-age patches.

Pathway 1.5B
Community 1.5 to 1.2

Stand-replacing disturbance events such as high-intensity fire, catastrophic windstorms, and floods open the forest canopy and lead to the stand initiation phase of development.

Additional community tables

Table 5. Community 1.1 plant community composition

Group	Common name	Symbol	Scientific name	Annual production ()	Foliar cover (%)

Table 6. Community 1.2 plant community composition

Group	Common name	Symbol	Scientific name	Annual production ()	Foliar cover (%)

Table 7. Community 1.3 plant community composition

Group	Common name	Symbol	Scientific name	Annual production ()	Foliar cover (%)

Table 8. Community 1.4 plant community composition

Group	Common name	Symbol	Scientific name	Annual production ()	Foliar cover (%)

Table 9. Community 1.5 plant community composition

Group	Common name	Symbol	Scientific name	Annual production ()	Foliar cover (%)

Supporting information

Other references

Breemen, Nico van. “How Sphagnum Bogs down Other Plants.” Trends in Ecology & Evolution 10, no. 7 (July 1, 1995): 270–75. https://doi.org/10.1016/0169-5347(95)90007-1.

Dwire, K. and Kauffman, J. 2003. Fire and Riparian Ecosystems in Landscapes in the Western United States. Forest Ecology and Management, Vol. 178 pg. 61-74.

Franklin, Jerry F.; Cromack, Kermit, Jr.; Denison, William; [and others]. 1981. Ecological characteristics of old-growth Douglas-fir forests. Gen. Tech. Rep. PNW-118. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 48 p. [7551]

Gavin, D., Brubaker, L. (2015). The Modern Landscape of the Olympic Peninsula. In: Late Pleistocene and Holocene Environmental Change on the Olympic Peninsula, Washington. Ecological Studies, vol 222. Springer, Cham. https://doi.org/10.1007/978-3-319-11014-1_1

Geertsema, Marten & Pojar, James. (2007). Influence of landslides on biophysical diversity — A perspective from British Columbia. Geomorphology. 89. 55-69. 10.1016/j.geomorph.2006.07.019.

Goheen, E.M. and Willhite, E.A. 2006. Field Guide to Common Diseases and Inspect Pests of Oregon and Washington Conifers. Portland, Oregon: USDA Forest Service, Pacific Northwest Region R6-NR-FID-PR-01-06.

Hanley, D.P and D.M. Baumgartner. 2002. Forest Ecology in Washington. Washington State University Extension Publishing. Technical Report EB 1943.

Hanson, E.J., D.L. Azuma and B.A. Hiserote. 2002. Site Index Equations and Mean Annual Increment Equations for Pacific Northwest Research Station Forest Inventory and Analysis Inventories, 1985-2001. USDA Forest Service Pacific Northwest Research Station, Research Note PNW-RN-533.

Hemstrom, M., Franklin, J. 1982. Fire and Other Disturbances of the Forests in Mount Rainier National Park. Quaternary Research, Vol 18 pp 32-61.

James K. Agee and Mark H. Huff. 1987. Fuel succession in a western hemlock/Douglas-fir forest. Canadian Journal of Forest Research. 17(7): 697-704. https://doi.org/10.1139/x87-112

Nielsen, E. M., R. L. Brunner, C. Copass and L. K. Wise, 2021. Olympic National Park map class descriptions. National Park Service, Fort Collins.

Pojar J., and MacKinnon. 1994. Plants of the Pacific Northwest Coast. Lone Pine, Vancouver, British Columbia. 528 pages.

PRISM Climate Group, Oregon State University, http://prism.orgeonstate.edu, visited October 2023.

Reed Wendel, Darlene Zabowskl "Fire History within the Lower Elwha River Watershed, Olympic National Park, Washington," Northwest Science, 84(1), 88-97, (1 January 2010)

Rochefort, R.M. and Peterson, D.L. 1996. Temporal and Spatial Distribution of Trees in Subalpine Meadows of Mount Rainier National Park. Arctic and Alpine Research, Vol. 28, No. 1 pp 52-59.

Seastedt, T.R., Adams, G.A. 2001. Effects of Mobile Tree Islands on Alpine Tundra Soils. Ecology, Vol 82 pp 8-17.

Smith, K., G. Kuhn, and L. Townsend. 2008. Culmination of Mean Annual Increment for Indicator Tree Species in the State of Washington. USDA-NRCS Technical Note Forestry-9.

United States Department of Agriculture, Forest Service, 2015. Silvics Manual Vol 1. http://na.fs.fed.us/spfo/pubs/silvics_manual/Volume_1/vol1_Table_of_contents.htm, visited December 2015.

United States Department of Agriculture, Natural Resources Conservation Service, and United States Department of the Interior, National Park Service. 2014. Ecological Site Descriptions for North Cascades National Park Complex, Washington.

United States Department of Agriculture, Natural Resources Conservation Service, and United States Department of the Interior, National Park Service. Ecological Site Descriptions for Mount Rainier National Park, Washington.

Van Pelt, R. 2007. Identifying Mature and Old Forests in Western Washington. Washington State Department of Natural Resources, Olympia, WA. 104 p

Washington Department of Natural Resources, Natural Heritage Program. 2015. Ecological Systems of Washington State. A Guide to Identification.

Wood, David, and Moral, Roger del. “Mechanisms of Early Primary Succession in Subalpine Habitats on Mount St. Helens.” Ecology 68, no. 4 (1987).

Zhao, Yunpeng, Chengzhu Liu, Simin Wang, Yiyun Wang, Xiaoqing Liu, Wanqing Luo, and Xiaojuan Feng. “‘Triple Locks’ on Soil Organic Carbon Exerted by Sphagnum Acid in Wetlands.” Geochimica et Cosmochimica Acta 315 (December 2021): 24–37. https://doi.org/10.1016/j.gca.2021.09.028.

Contributors

Alec Haulotte
Erin Kreutz
Max Ross
Abigail Field
Erik Dahlke

Approval

Grant Petersen, 5/15/2025

Rangeland health reference sheet

Interpreting Indicators of Rangeland Health is a qualitative assessment protocol used to determine ecosystem condition based on benchmark characteristics described in the Reference Sheet. A suite of 17 (or more) indicators are typically considered in an assessment. The ecological site(s) representative of an assessment location must be known prior to applying the protocol and must be verified based on soils and climate. Current plant community cannot be used to identify the ecological site.

Author(s)/participant(s)
Contact for lead author
Date	10/06/2026
Approved by
Approval date
Composition (Indicators 10 and 12) based on	Annual Production

Indicators

Number and extent of rills:
Presence of water flow patterns:
Number and height of erosional pedestals or terracettes:
Bare ground from Ecological Site Description or other studies (rock, litter, lichen, moss, plant canopy are not bare ground):
Number of gullies and erosion associated with gullies:
Extent of wind scoured, blowouts and/or depositional areas:
Amount of litter movement (describe size and distance expected to travel):
Soil surface (top few mm) resistance to erosion (stability values are averages - most sites will show a range of values):
Soil surface structure and SOM content (include type of structure and A-horizon color and thickness):
Effect of community phase composition (relative proportion of different functional groups) and spatial distribution on infiltration and runoff:
Presence and thickness of compaction layer (usually none; describe soil profile features which may be mistaken for compaction on this site):
Functional/Structural Groups (list in order of descending dominance by above-ground annual-production or live foliar cover using symbols: >>, >, = to indicate much greater than, greater than, and equal to):

Dominant:

Sub-dominant:

Other:

Additional:
Amount of plant mortality and decadence (include which functional groups are expected to show mortality or decadence):
Average percent litter cover (%) and depth ( in):
Expected annual annual-production (this is TOTAL above-ground annual-production, not just forage annual-production):
Potential invasive (including noxious) species (native and non-native). List species which BOTH characterize degraded states and have the potential to become a dominant or co-dominant species on the ecological site if their future establishment and growth is not actively controlled by management interventions. Species that become dominant for only one to several years (e.g., short-term response to drought or wildfire) are not invasive plants. Note that unlike other indicators, we are describing what is NOT expected in the reference state for the ecological site:
Perennial plant reproductive capability:

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Ecological site AX001X01X003

Mesic Udic Alluvial Terrace Forest

Last updated: 5/15/2025 Accessed: 10/06/2026

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General information

MLRA notes

Ecological site concept

Associated sites

Similar sites

Table 1. Dominant plant species

Legacy ID

Natural Resources
Conservation Service

Last updated: 5/15/2025
Accessed: 10/06/2026