What Threatens the U.S. Temperate Rainforests?

U.S. temperate rainforests face an unusual convergence of threats, from rising temperatures and vanishing coastal fog to the lingering effects of decades of industrial logging, invasive pests enabled by warmer winters, and atmospheric pollution that quietly strips the forest canopy of its lichen communities. These forests, concentrated along the Pacific coast from Northern California through Southeast Alaska with fragments in the southern Appalachians, store a wildly disproportionate share of the continent’s carbon and harbor species found nowhere else. What makes the threats so concerning is that many of them reinforce each other, and the forests themselves recover on timescales measured in centuries.

A Globally Rare, Carbon-Dense Ecosystem

To understand what is at stake, it helps to appreciate how unusual these forests are. The Pacific coastal temperate rainforest covers a thin strip of land, accounting for roughly 0.3% of the world’s total forest area. Yet it stores somewhere between 0.6% and 1.1% of all global aboveground forest carbon as live tree biomass, with tree biomass carbon stocks averaging around 211 to 218 megagrams of carbon per hectare.¹ Among all western U.S. ecoregions, the Pacific Northwest ranks as the most carbon-dense.² That carbon density is the product of centuries of slow, steady accumulation. Old-growth Sitka spruce, western red cedar, and coast redwood take hundreds of years to reach full stature. When these forests are damaged or removed, the carbon they release cannot be recaptured on any timescale that matters for current climate targets.

The evergreen conifers that dominate these forests have a competitive advantage tied directly to the regional climate. Summer in the Pacific Northwest is warm and dry, with high evaporative demand that limits photosynthesis. Deciduous hardwoods, which can only photosynthesize during the growing season when water stress is worst, lose that competition repeatedly. Conifers, which keep their needles year-round, can photosynthesize during the cool, wet winters when conditions are more favorable.³ This arrangement has worked for millennia. But it depends on a climate pattern that is now shifting.

Heat, Drought, and the Disappearance of Coastal Fog

Climate change hits temperate rainforests differently than it hits drier forests. The threat is not simply that temperatures are rising; it is that the moisture regime these forests evolved under is changing in ways that erode their fundamental character. One of the most striking examples came in late June 2021, when a record-shattering heat dome settled over the Pacific Northwest. Multiple days of extreme heat caused unprecedented, widespread death of tree canopy foliage. Healthy green foliage turned red or orange almost overnight, visible in satellite imagery and confirmed by aerial surveys and ground observations as heat scorch.⁴ These are forests that rarely experienced temperatures above the mid-30s Celsius historically. The 2021 event pushed well past that, and the trees had no physiological defense against it.

A slower but arguably more consequential shift involves coastal fog. Coast redwoods and other species along the U.S. west coast depend on summer fog to reduce drought stress during the dry season. Fog drip supplies moisture directly to roots and soil, and fog cover lowers evaporative demand so trees lose less water through their needles. Research has documented a long-term decline in summer fog frequency along the California and Oregon coasts, and physiological data suggest that coast redwood and associated ecosystems are becoming increasingly drought-stressed as fog retreats and evaporative demand rises.⁵ For a tree that can live over a thousand years, a few decades of shifting fog patterns might not look catastrophic yet. But the trajectory is concerning, and younger trees establishing in a drier fog regime may never reach the size of their predecessors.

The Lingering Effects of Industrial Logging

Logging is the most physically obvious threat, and its consequences ripple through every other issue facing these forests. Across the Pacific Northwest, vast swaths of old-growth temperate rainforest were cleared during the twentieth century. What has grown back is structurally and ecologically different from what was removed. Young-growth stands are dense, even-aged, and lack the layered canopy architecture, standing dead trees, and fallen logs that characterize old-growth forests. Research on Prince of Wales Island in Southeast Alaska found that several bird species were significantly more abundant in old-growth forest (trees 150 years or older) than in 20-year-old young growth, and at least one uncommon species was detected almost exclusively in old growth during both breeding and winter seasons.⁶ Other species did better in young growth, but the point is that the two forest types are not interchangeable. Losing old growth means losing the habitat that particular species require.

The damage extends well beyond the trees themselves. A quantitative synthesis of paired catchment studies across the Pacific salmon range found that forestry activities elevated peak stream flows by about 20%, reduced low flows by roughly 25%, and increased maximum summer water temperatures by around 15% on average.⁷ Those numbers represent averages across dozens of catchments; individual watersheds varied widely. But the pattern is consistent: removing forest cover destabilizes the hydrology that salmon and other cold-water species depend on. Higher peak flows scour streambeds, lower summer flows concentrate fish in shrinking pools, and warmer temperatures push water past the thermal limits many salmon populations can tolerate.

Yellow-Cedar Decline

Yellow-cedar, also called Alaska yellow-cedar, is one of the longest-lived trees in the coastal temperate rainforest, capable of surviving over a thousand years. It is dying across large areas of Southeast Alaska and adjacent British Columbia, and the cause is one of the stranger consequences of climate change. The trees are not being killed by heat or drought directly. They are being killed by the loss of snow.

Yellow-cedar roots are remarkably cold-sensitive, tolerant only to about minus five degrees Celsius even in midwinter. In a colder climate, deep snowpack insulates the soil and keeps root temperatures from dropping to lethal levels. As winters warm, snowpack at lower elevations diminishes. Without that insulating blanket, roots are exposed to freeze-thaw cycles that cause cumulative damage. The injury starts below ground as root freezing, then shows up as foliar browning after spring warming begins, and ultimately kills the tree. Experimental work confirmed this sequence: seedlings whose soil was left fully exposed to winter temperatures suffered complete fine-root destruction and died, while seedlings with insulated soil survived.⁸

Dendroclimatological analysis across Southeast Alaska supports this mechanism at landscape scale. Declining yellow-cedar forests appeared soon after the end of the Little Ice Age and are concentrated at low elevations, where snow loss has been greatest, while higher-elevation forests remain healthy. Warming winter trends have created conditions for frequent severe thaw-freeze events, and the pattern suggests that cedar decline will expand as warming continues.⁹ On the Haida Gwaii archipelago, research suggests the mechanism may be less about isolated extreme freeze events and more about reduced cold hardening throughout the winter, exposing roots to repeated low-grade freezing damage across multiple cycles.¹⁰ Either way, the underlying driver is the same: warmer winters paradoxically make conditions more lethal for a tree that evolved under reliable snow cover.

Hemlock Woolly Adelgid and Spruce Beetles

Yellow-cedar decline is not the only pest or disease threat reshaping temperate rainforest composition. In the Appalachian temperate rainforests of the southeastern United States, eastern hemlock has been devastated by hemlock woolly adelgid, a tiny invasive insect from East Asia. Mortality has been rapid in invaded stands, with 50% of hemlock trees dying within about six years of invasion in southern Appalachian riparian forests.¹¹ Hemlocks are keystone species in these ecosystems, shading streams and maintaining the cool, moist microclimate that defines the rainforest-like character of Appalachian coves. Their loss transforms the forest from the canopy down.

In the western mountains, spruce beetles pose a growing threat to spruce forests that form a major component of the temperate rainforest belt in Alaska and British Columbia. Modeling of outbreak distribution found that minimum winter temperature, precipitation as snow, and mean growing season temperatures are the best predictors of where spruce beetle outbreaks occur.¹² As winters warm across northern latitudes, conditions become more favorable for beetle survival and reproduction, potentially expanding outbreak zones into forests that were historically too cold for sustained infestations. The combination of stressed trees and thriving beetles is exactly the kind of feedback loop that accelerates forest loss.

Nitrogen Pollution and the Loss of Lichens

Old-growth temperate rainforests are draped in epiphytic lichens and mosses that play roles the casual observer might not appreciate. Lichens fix nitrogen from the atmosphere, provide forage for wildlife, regulate canopy moisture, and serve as sensitive indicators of air quality. They are also among the first casualties of elevated nitrogen and sulfur deposition from industrial and agricultural pollution carried on prevailing winds.

A study assessing ecological risks from atmospheric deposition across U.S. forests found that forage lichen diversity and abundance dropped by 80% at deposition levels of around 10 kilograms of nitrogen and 13 kilograms of sulfur per hectare per year.¹³ Those are not extreme deposition levels; many areas in the western U.S. approach or exceed them, particularly near agricultural valleys and urban corridors. Research into the specific mechanisms confirms that rising nitrogen levels affect lichens at the cellular, organism, and community level, with effects that interact with other stressors in ways that are still being worked out.¹⁴ The practical result is that forests downwind of pollution sources can look intact from a distance while their canopy ecosystems are quietly impoverished.

The Marbled Murrelet’s Habitat Squeeze

Few species illustrate the compounding nature of temperate rainforest threats as clearly as the marbled murrelet, a small seabird that nests on moss-covered limbs high in old-growth trees and feeds in nearshore ocean waters. It depends on two habitats simultaneously, and both are deteriorating.

Population monitoring across the Pacific Northwest has documented recent declines, with nesting habitat losses greatest in Washington (9 to 11% from 1996 to 2006) and Oregon (5 to 17% over the same period), driven by logging, fire, and windstorms. There is a strong positive correlation between murrelet population size and area of high-quality nesting habitat, suggesting that the species’ numbers may be directly limited by how much old forest remains.¹⁵ At the same time, research analyzing long-term occupancy patterns found that landscapes with more old forest and closer proximity to the ocean showed reduced rates of local extinction. With accelerated ocean warming and increased global timber demand projected, the murrelet faces simultaneous erosion of both its terrestrial and marine habitats.¹⁶ Protecting old-growth forest alone will not save the murrelet if ocean conditions continue to degrade, and improving ocean conditions would not help a bird with nowhere to nest.

Fire in Forests That Rarely Burned

Temperate rainforests are often assumed to be essentially fireproof, and under historical conditions that was close to true. These forests are too wet for most of the year to carry fire. But “most of the year” is not all of it, and the dry window is widening. Research on fire history in coastal temperate rainforests has complicated the old assumption of thousand-year fire return intervals. On Hecate Island in British Columbia, charcoal records revealed that fire occurrence was roughly 25 times more likely than those previous estimates suggested, with fires recurring at multidecadal intervals during certain periods. The spatial pattern of fires relative to habitation sites, combined with the absence of fire after Indigenous peoples left the area, points to intentional use of fire as a resource management tool.¹⁷

In the Pacific Northwest Cascades, a research framework for assessing pre-contact fire patterns concluded that Indigenous fire stewardship is the most parsimonious explanation for historically high fire frequency in western Cascade montane forests.¹⁸ The implication is that these forests are not simply wet enough to ignore fire. They coexisted with fire under specific conditions, often shaped by human management. The removal of that management, followed by a century of fire suppression and now a warming climate that produces longer dry spells, has created forests with heavy fuel loads and no recent fire history. When fire does arrive under extreme drought conditions, it burns with an intensity these forests were never adapted to withstand.

Invasive Species Reshaping the Forest Floor

Invasive plants are a quieter threat but one that alters forest ecosystems from the ground up. When invasive species replace native vegetation along streams and forest edges, they change the chemistry and pace of nutrient cycling. Research comparing leaf litter decomposition rates found that litter from invasive herbs broke down about 2.6 times faster than litter from native herbs, and invasive tree litter decomposed about 1.3 times faster than native tree litter. The difference was partly explained by the lower structural carbohydrate content of invasive leaves.¹⁹ Faster decomposition might sound harmless, but aquatic invertebrates and the fish that eat them are adapted to the timing and nutritional profile of native leaf litter entering streams. When that input changes, the base of the food web shifts in ways that cascade upward through the ecosystem.

Old logging roads and recreational trails serve as corridors for invasive species to penetrate deep into otherwise intact forest. The road networks left behind by decades of timber harvest crisscross many temperate rainforest landscapes, providing sunny, disturbed edges where invasive plants establish and spread inward. Eradicating these species once they are established in a wet, productive environment is extremely difficult, and the cost of inaction compounds over time.

Ghost Forests Where Land Meets Sea

At the lowest-elevation margins of temperate forests along U.S. coastlines, rising sea levels are creating what researchers call ghost forests: standing dead trees killed by saltwater intrusion. A study of unmanaged public lands along the coast found a net loss of about 152 square kilometers of forest between 2001 and 2014, with roughly 167 square kilometers converting from living coastal forest to transition or ghost forest during that period, representing approximately 15% of the study area.²⁰ This process plays out primarily on low-lying Atlantic coastal margins rather than the mountainous Pacific coast where most temperate rainforest is found. But it underscores a broader principle: the edges of temperate forest ecosystems are often the most vulnerable, and the losses at the margins can be rapid once a threshold is crossed. In parts of the Olympic Peninsula and the Oregon coast where temperate rainforest reaches low-elevation coastal terraces, even modest sea-level rise combined with storm surge could begin to squeeze forest habitat from below while climate stress presses from above.

Why These Threats Compound

What makes the situation particularly difficult for U.S. temperate rainforests is that few of these threats operate in isolation. A forest stressed by drought is more vulnerable to beetle outbreaks. A watershed degraded by logging warms faster under climate change. A murrelet population squeezed by old-growth loss has less resilience to ocean warming. Nitrogen pollution weakens lichen communities that otherwise contribute to nutrient cycling and canopy water retention. Invasive species colonize the gaps left by logging roads and tree die-offs. Each stressor lowers the threshold at which the next one becomes damaging.

The forests’ own biology makes recovery slow. A coast redwood or Sitka spruce takes centuries to reach old-growth stature. The structural complexity that supports the full web of associated species, including deep bark fissures, large branches draped in moss, standing dead snags, and massive fallen logs, accumulates on timescales that dwarf human planning horizons. A clearcut replanted with the same tree species is not ecologically equivalent to the old-growth forest it replaced for generations, and under a changing climate, it may never become equivalent at all. The gap between the speed of degradation and the pace of recovery is the central challenge for anyone trying to protect what remains.