Washington State spans an extraordinary range of biomes for its size, from dripping coastal forests on the Olympic Peninsula to arid shrub-steppe receiving barely 30 centimeters of rain a year in the Columbia Basin. The Cascade Range acts as a dramatic climatic wall, and crossing it east to west is like traveling through several climate zones on different continents. The result is a patchwork of temperate rainforest, montane conifer forest, alpine tundra, grassland-shrubland steppe, riparian corridors tied to salmon runs, coastal estuaries, and even urban ecosystems with their own ecological dynamics.
Temperate Moist Coniferous Forest
The lush forests on the western slopes of the Olympic Mountains are among the most iconic landscapes in the Pacific Northwest. They receive enormous amounts of rainfall, and the valleys of rivers like the Hoh, Queets, and Quinault are draped in moss, ferns, and towering conifers. The common label “Olympic rain forest” is actually somewhat misleading. A detailed study of forest succession in Olympic National Park argued that “temperate moist coniferous forest” better describes the vegetation not just in the Hoh Valley but across the Olympic Mountains and the northern Pacific coast in general.1Ecology. Forest Succession in Relation to River Terrace Development in Olympic National Park, Washington The distinction matters because these forests are defined less by rainfall alone than by the interaction of moisture, soil development, and centuries-long successional sequences.
On the river terraces of the Hoh, succession begins on bare gravel bars colonized by red alder and willows, then moves through stages dominated by Sitka spruce, bigleaf maple, and black cottonwood before eventually reaching old-growth western hemlock stands on the oldest and highest terraces. These oldest surfaces date back to the retreat of Pleistocene glaciers, and their soils are deep and hold moisture well. Younger surfaces closer to the river are drier and more stressful for plants, which is counterintuitive for a “rainforest.”1Ecology. Forest Succession in Relation to River Terrace Development in Olympic National Park, Washington
Sitka spruce, one of the signature trees of these wet forests, turns out to be particularly sensitive to environmental shifts. Research on tree growth in the Olympic Mountains found that the Sitka spruce forest type showed the highest growth variability and the strongest response to changes in temperature and moisture among the forest types studied. This challenges the older assumption that low-elevation coastal forests are ecologically stable and sluggish. They may actually play an outsized role in carbon storage if the climate warms, because their growth can spike in favorable years.2Canadian Journal of Forest Research. Spatial and temporal variability in forest growth in the Olympic Mountains, Washington: sensitivity to climatic variability
The Cascade Gradient and Montane Forests
The Cascade Range is the single most important feature shaping Washington’s biome map. Moisture-laden air from the Pacific rises over the mountains, drops most of its rain and snow on the western slopes, and arrives on the eastern side dry and warm. This rain-shadow effect creates two fundamentally different ecological worlds within a few dozen kilometers of each other. West-side slopes support dense forests of Douglas fir, western red cedar, and western hemlock, while east-side slopes transition into drier ponderosa pine and mixed-conifer forests before giving way to grassland and shrub-steppe.
Climate projections for the North Cascades suggest that higher temperatures will stress lower-elevation tree species on both sides of the crest, reducing their growth and productivity, while high-elevation species may actually benefit from longer growing seasons. The east side, already fire-prone, faces particular risk from increased wildfire, insect outbreaks, and invasive species, all of which can reshape ecosystem structure across large areas in a short time.3Treesearch (USDA Forest Service). Climate change vulnerability and adaptation in the North Cascades region, Washington
The southeast corner of the state includes the northern edge of the Blue Mountains ecoregion, a landscape of forested uplands and canyon grasslands that differs from the Cascades in its geology, elevation patterns, and species mix. USDA Forest Service researchers have subdivided the Columbia Plateau and Blue Mountains into finer-scale ecological units, partly to better understand anadromous fish habitat, since salmon and steelhead depend on the cold mountain streams draining these highlands.4Treesearch (USDA Forest Service). Hierarchical subdivisions of the Columbia Plateau and Blue Mountains ecoregions, Oregon and Washington
Alpine and Subalpine Zones
Above the treeline on Washington’s major volcanoes and high ridges, the alpine zone is a world of rock, snow, and small but remarkably diverse plant communities. Mount Rainier, Mount Baker, Glacier Peak, and the high ridges of the North Cascades all host alpine ecosystems where plants have adapted to short growing seasons, extreme cold, intense ultraviolet radiation, and thin, rocky soils.
What grows on a given peak depends on a surprising mix of factors. A study of alpine plant communities across Cascade peaks found that the age of the underlying rock formation, precipitation, latitude, and temperature all played major roles in shaping which species were present. Simple distance between peaks and the timing of field surveys mattered less than these geological and climatic drivers.5PubMed Central. Geology and climate drive alpine plant compositional variation among peaks in the Cascade Range of Washington In other words, two nearby peaks can have noticeably different plant communities if their bedrock geology differs.
On individual peaks, strong environmental gradients play out over short distances. Research on Grouse Ridge near Mount Baker documented a sharp transition from exposed fellfields on ridgetops, where soils are hot and dry, down through grassy slopes to moist meadows at the base. Snowmelt timing controls almost everything: plants bloom and set seed in a compressed schedule dictated by when their patch of ground emerges from snow. During droughts, plants near the base of slopes can actually suffer more moisture stress than those on the ridge, because denser vegetation draws more water out of the soil through evapotranspiration.6Ecological Monographs. Alpine and High Subalpine Plant Communities of the North Cascades Range, Washington and British Columbia
Just below the true alpine zone, the subalpine harbors one of Washington’s most visually striking trees: alpine larch, a deciduous conifer that turns brilliant gold in autumn before dropping its needles. Unlike its evergreen neighbors, alpine larch thrives on cold, rocky, exposed sites that are too harsh for shade-tolerant species like subalpine fir. It often grows in tree form at elevations where its associates survive only as stunted, wind-battered shrubs. Its understory companions, including white mountain-heather and yellow mountain-heather, confirm the snowy, cold character of these habitats.7Ecological Monographs. Ecology of Alpine Larch (Larix lyallii Parl.) in the Pacific Northwest
These high-elevation ecosystems are among the most vulnerable to climate change in the state. Modeling of eight high-elevation endemic plant species in Washington predicted that seven would lose substantial habitat under continued high emissions, with four expected to lose virtually all of their suitable range. Even under a lower-emissions scenario, losses averaged around a quarter for each species, though reduced from the worst case.8PubMed. High-elevation endemic plants predicted to lose habitat from changing climate in Washington State For species that are already restricted to narrow elevational bands, there is nowhere higher to go.
Shrub-Steppe of the Columbia Basin
East of the Cascades, Washington’s landscape opens into a broad, rolling terrain of sagebrush, bunchgrass, and basalt that bears no resemblance to the forests on the other side of the mountains. The Columbia Basin’s shrub-steppe is part of one of the largest dryland ecosystems in North America, dominated by sagebrush species and native perennial grasses. The climate is semi-arid, with a mean annual temperature around 8°C and average precipitation of roughly 32 centimeters, most of which falls between October and March.9Ecosphere. Cascadia Burning: The historic, but not historically unprecedented, 2020 wildfires in the Pacific Northwest, USA
This biome once covered vast stretches of eastern Washington, but conversion to agriculture has been devastating. Before European settlement, shrub-steppe was the dominant habitat type across much of the region. Since then, over 60 percent of Washington’s shrub-steppe has been lost, mostly to cropland.10Washington Department of Fish and Wildlife. Patterns in biological soil crust recovery in Conservation Reserve Program fields, Washington State What remains faces ongoing pressure from invasive annual grasses like cheatgrass, which alter fire regimes by providing continuous fine fuel that burns readily and frequently.
Healthy shrub-steppe depends heavily on biological soil crusts: thin, living layers of cyanobacteria, mosses, and lichens on the soil surface that stabilize against erosion, fix nitrogen, and help retain moisture. These crusts are fragile and slow to recover after disturbance, so the combination of agricultural conversion and increased fire frequency can leave soils bare and degraded for decades.
The Channeled Scablands and Their Ecological Legacy
No discussion of eastern Washington’s landscapes is complete without the Channeled Scablands, a vast network of interlocking rock channels, dry waterfalls, gravel bars, and scoured basalt left behind by some of the largest floods ever documented on Earth. During the last ice age, the Cordilleran Ice Sheet periodically dammed a massive lake in western Montana. When the ice dams broke, catastrophic floods roared across eastern Washington, stripping away soil and carving deep channels into the basalt bedrock.11Annual Review of Earth and Planetary Sciences. The Channeled Scabland: A Retrospective
Dozens of these megafloods occurred over thousands of years, and the routing of floodwaters was not static. Research has shown that deformation of the Earth’s crust under the weight of ice sheets changed the regional topography by hundreds of meters, redirecting where the floodwaters could go at different times during the deglaciation.12PubMed Central. Glacial isostatic adjustment directed incision of the Channeled Scabland by Ice Age megafloods The ecological result is a landscape of thin, patchy soils surrounded by bare rock, where shrub-steppe vegetation competes on scattered pockets of windblown silt (loess) between basalt outcrops. The Scablands support a distinctive mosaic of plant communities shaped by where the floods left or removed soil.
Riparian Corridors and the Salmon Connection
Washington’s rivers thread through every biome in the state, and the riparian zones along them function as distinct ecosystems in their own right. In the Pacific Northwest, these corridors get an unusual nutrient subsidy: marine-derived nitrogen carried upstream by spawning salmon. When salmon return from the ocean to reproduce and die, their decomposing bodies release nitrogen and other nutrients into stream gravels, banks, and adjacent forest soils, fertilizing the vegetation along the river.
The removal of two large dams on the Elwha River on the Olympic Peninsula reopened roughly 60 kilometers of river habitat that had been blocked to anadromous salmon for a century. Researchers began tracking stable nitrogen isotopes in riparian vegetation along Elwha tributaries to monitor how quickly marine-derived nitrogen returns to the ecosystem as salmon recolonize.13Northwest Science. Monitoring the Return of Marine-Derived Nitrogen to Riparian Areas in Response to Dam Removal on the Elwha River, Washington Red alder, a tree that also fixes atmospheric nitrogen, complicates the picture somewhat, since both salmon and alder enrich riparian soils but through completely different pathways. Disentangling the two sources is one of the challenges of studying nutrient cycling in these systems.
Riparian zones also serve as corridors connecting otherwise isolated biomes. In the semi-arid east, a cottonwood gallery along a river can look wildly different from the sagebrush just a few hundred meters away, supporting everything from songbirds to amphibians that could not survive in the surrounding steppe.
Coastal and Estuarine Ecosystems
Washington’s Pacific shoreline and the protected waters of Puget Sound host a range of coastal biomes, from rocky intertidal zones to mudflats to eelgrass meadows. Willapa Bay, on the southern coast, contains some of the most extensive eelgrass beds in the Pacific Northwest. Eelgrass meadows serve as nursery habitat for commercially important fish and shellfish species, stabilize sediments, and sequester carbon. The extent and density of these meadows vary from year to year and from one part of a bay to another, driven by light availability, sediment conditions, and water temperature.14Springer / Estuaries. Factors influencing spatial and annual variability in eelgrass (Zostera marina L.) meadows in Willapa Bay, Washington, and Coos Bay, Oregon, estuaries
Puget Sound itself is a complex inland sea with fjord-like channels, rocky shorelines, and river deltas. Its kelp forests, mudflats, and saltmarshes provide habitat for marine mammals, seabirds, and the salmon populations that are central to the region’s ecology and economy. Coastal ecosystems in Washington face pressure from development, sea-level rise, ocean acidification (which threatens shellfish), and nutrient runoff from agriculture and urban areas.
Fire as a Biome-Shaping Force
Fire plays very different roles on opposite sides of the Cascades. In the dry forests and steppe of eastern Washington, fire has been a regular ecological force for millennia, and many plant species are adapted to periodic burning. Ponderosa pine, for instance, has thick bark that protects it from low-intensity surface fires. Sagebrush, by contrast, is killed by fire and can take decades to reestablish from seed, which makes the increasing fire frequency driven by invasive grasses a genuine threat to the shrub-steppe biome.
West of the Cascades, fire has historically been far less frequent, but the September 2020 fire season was a stark reminder that it is not absent. In just two weeks, fires burned roughly 340,000 hectares of forest west of the Cascade crest, nearly equaling the total area burned on the west side over the previous five decades (around 406,000 hectares).9Ecosphere. Cascadia Burning: The historic, but not historically unprecedented, 2020 wildfires in the Pacific Northwest, USA Researchers investigating these fires noted that while the 2020 event was extreme in the modern record, it was not unprecedented in the longer historical and paleoecological record. The temperate rain forests of the Pacific Northwest have burned before, just not within living memory.
Post-Glacial Origins of Washington’s Biomes
The biomes you see in Washington today are geologically young. At the end of the last ice age, roughly 10,000 years ago, the landscape looked radically different. Pollen and plant fossil records from high-elevation lakes on the Olympic Peninsula show that immediately after deglaciation, the subalpine zone supported sparse tundra vegetation with no close modern analogue. The forests, meadows, and heathlands that characterize the subalpine today assembled gradually over thousands of years as the climate warmed and species migrated upslope.15The Holocene. Postglacial history of subalpine forests, Olympic Peninsula, Washington, USA
Go further back to the Eocene, around 50 million years ago, and the Okanogan Highlands of northeastern Washington hosted a forest that would be unrecognizable today. Fossil plant assemblages from this region resemble the modern mixed mesophytic forests of the southeastern United States and eastern Asia, with oaks, walnuts, birches, and the dawn redwood Metasequoia alongside more familiar northwestern conifers like fir and spruce.16Canadian Journal of Earth Sciences. Fossil biotas from the Okanagan Highlands, southern British Columbia and northeastern Washington State: climates and ecosystems across an Eocene landscape The state’s modern biome distribution is a product of tectonic uplift of the Cascades, millions of years of climate change, and the repeated glaciations of the Pleistocene.
Specialized Habitats and Edaphic Islands
Scattered across Washington are small, specialized habitats driven not by regional climate but by unusual soils or rock types. Serpentine soils, derived from ultramafic rocks rich in magnesium and heavy metals, create harsh growing conditions that most plants cannot tolerate. Where serpentine outcrops occur, the vegetation is strikingly different from adjacent areas: sparse, often stunted, and home to a disproportionate number of endemic species found nowhere else. Serpentine-tolerant plants have evolved distinct strategies for handling toxic metal concentrations and nutrient imbalances in the soil.17Annual Review of Ecology, Evolution, and Systematics. Evolutionary Ecology of Plant Adaptation to Serpentine Soils These serpentine patches function as ecological islands, isolated by the very different surrounding soils, and they contribute disproportionately to the state’s botanical diversity.
Other edaphic oddities include the ash-rich soils near Mount St. Helens, where ecological succession has been studied intensively since the 1980 eruption, and the pumice flats of the eastern Cascades, where porous volcanic soils create unusually dry microclimates even in areas with moderate rainfall.
Urban Ecosystems as a New Biome Category
Seattle and the greater Puget Sound metropolitan area represent something ecologists increasingly recognize as a distinct ecosystem type: the urban biome. Cities are not ecological voids. They support a mix of native and non-native plant species, maintain tree canopies that provide cooling and habitat, and harbor surprising biodiversity in parks, yards, and greenbelts. The mild, moist climate of western Washington makes urban areas particularly productive. A study of urban foraging in Seattle found that 26 percent of residents surveyed had gathered plant or fungal products in city parks, reflecting how accessible wild food remains even in a major metropolitan area.18Springer / Estuaries. Urban non-timber forest products stewardship practices among foragers in Seattle, Washington (USA)
Urban ecosystems interact with the surrounding native biomes in complex ways. Stormwater runoff from developed areas affects salmon streams. Urban heat islands shift microclimates, altering which plants thrive. Invasive species often get their start in urban landscapes before spreading into wildlands. At the same time, urban parks and restored wetlands can function as refugia for native species, particularly when connected by green corridors that allow wildlife to move between larger natural areas. In a state where the population is concentrated along the Interstate 5 corridor, how cities manage their ecological footprint has real consequences for every adjacent biome.