Alaska’s southeastern panhandle and south-central coast are home to one of the largest temperate rainforests on Earth. The Tongass and Chugach National Forests together cover roughly 97,000 square kilometers of dripping, moss-draped woodland that receives as much as five to six meters of rain a year in its wettest pockets. Combined with the adjacent forests of coastal British Columbia, this region holds about a quarter of the world’s remaining temperate rainforest, a fact that surprises people who picture Alaska as tundra and glaciers. The ecosystem is distinct from tropical rainforests in almost every way except sheer wetness, and it plays an outsized role in carbon storage, salmon habitat, and coastal ocean chemistry.
Size and Geography
The Tongass National Forest is the largest national forest in the United States, spanning about 69,000 square kilometers across most of southeastern Alaska and the Alexander Archipelago, a chain of more than a thousand islands running along the panhandle. The Chugach National Forest, the second largest, covers roughly 28,000 square kilometers around the Copper River delta, Prince William Sound, and part of the Kenai Peninsula. Most of both forests lie within the temperate rainforest ecoregion.1North American Journal of Fisheries Management. Quantifying the Monetary Value of Alaska National Forests to Commercial Pacific Salmon Fisheries Together they account for a disproportionate share of high-integrity forest landscape in the entire National Forest system: the Tongass and Chugach hold about 31 percent of all high landscape-integrity area found on National Forest lands nationwide.2AGU Advances. Southern Alaska’s Forest Landscape Integrity, Habitat, and Carbon Are Critical for Meeting Climate and Conservation Goals
The scale is hard to grasp in the abstract. The Tongass alone is roughly the size of West Virginia. From the southernmost tip near Ketchikan to the northern reaches near Yakutat, the forest stretches over 800 kilometers, stitched together by saltwater channels, fjords, and mountain passes rather than roads. Much of it is accessible only by boat or floatplane, which is part of the reason it has stayed relatively intact compared to temperate rainforests farther south along the Pacific coast.
Why It Rains So Much
Temperate rainforests exist in a narrow set of conditions: mild maritime temperatures, persistent moisture, and topography that squeezes water out of the air. Southeast Alaska checks all three boxes. The region sits in the path of the prevailing midlatitude westerlies, which push moisture-laden air off the North Pacific directly into steep coastal mountains. When that air is forced upward, it cools and dumps enormous amounts of rain and snow, a process called orographic lifting. Precipitation ranges from about 1,500 to 5,000 millimeters per year across the Tongass, and from 500 to 6,000 millimeters on the Chugach, depending on elevation and exposure.1North American Journal of Fisheries Management. Quantifying the Monetary Value of Alaska National Forests to Commercial Pacific Salmon Fisheries
A major driver of the heaviest downpours is atmospheric rivers, narrow corridors of concentrated water vapor that stream across the Pacific from the subtropics. Along the coast of British Columbia and southeastern Alaska, atmospheric rivers contribute up to about a third of total annual precipitation in the wettest coastal zones. Their contribution to extreme precipitation events is even larger: along the western arc of the Coast Mountains and the adjacent shoreline, atmospheric rivers account for more than 90 percent of extreme rainfall.3Journal of Geophysical Research: Atmospheres. Contribution of Atmospheric Rivers to Annual, Seasonal, and Extreme Precipitation Across British Columbia and Southeastern Alaska When upper-level atmospheric patterns over the northeastern Pacific amplify into large wave patterns, they steer these moisture plumes directly into southeast Alaska, where the steep terrain wrings them dry.4Journal of Geophysical Research: Atmospheres. Atmospheric Rivers in Southeast Alaska: Meteorological Conditions Associated With Extreme Precipitation
The result is a climate that is cool, cloudy, and relentlessly wet for much of the year, but rarely bitterly cold at low elevations thanks to the moderating influence of the ocean. Winter temperatures along the coast typically hover around freezing or just above it, and summers are mild. That combination keeps the growing season relatively long for such a high latitude and favors evergreen trees that can photosynthesize whenever the temperature cooperates.
What the Forest Looks Like
If you have seen photographs of old-growth rainforest in the Pacific Northwest and mentally scaled them up, you are close. The dominant trees are Sitka spruce and western hemlock. In riparian areas along streams, Sitka spruce tends to dominate; on drier upland slopes, it shares the canopy with western hemlock.5Canadian Journal of Forest Research. Species composition of old-growth and riparian Sitka spruce–western hemlock forests in southeastern Alaska Alaska yellow-cedar and red cedar also appear, especially at higher elevations or in bogs. The number of tree species is low compared to tropical rainforests or even temperate deciduous forests farther south. But low species richness does not mean structural simplicity. Old-growth stands in southeast Alaska are highly structurally diverse, with wide variation in tree sizes, complex spatial arrangements of trunks, and canopy gaps created by windthrow and decay that let light reach the forest floor.6Canadian Journal of Forest Research. Spatial aspects of structural complexity in Sitka spruce – western hemlock forests, including evaluation of a new canopy gap delineation method
The understory is thick with mosses, ferns, and blueberry. Fallen logs can take centuries to decompose in the cool, wet conditions, and as they slowly rot, they serve as nursery beds for the next generation of trees. In some stands, you can see neat rows of young spruce and hemlock growing along the top of a decomposing log that fell decades or centuries earlier. Every surface that holds moisture hosts bryophytes, giving the forest its characteristic look of dripping green.
Temperate rainforests rank among the most biomass-dense terrestrial ecosystems on Earth. Unlike tropical rainforests, where nutrients cycle rapidly through living tissue because soils tend to be nutrient-poor, temperate rainforests accumulate massive amounts of organic material both above and below ground. The cool temperatures and persistent dampness slow decomposition, so dead wood, leaf litter, and organic soil layers build up over time.7Geoderma. Soils of temperate rainforests of the North American Pacific Coast
Wildlife and Island Endemism
Southeast Alaska’s island geography has produced a surprising amount of evolutionary distinctiveness among its mammals. Of the 107 mammalian species or subspecies recorded in the region, 24 are considered endemic to southeast Alaska, and another 12 are mostly confined to the area.8Biological Conservation. A phylogeographic perspective on endemism in the Alexander Archipelago of southeast Alaska The Alexander Archipelago wolf, several subspecies of brown and black bear, and distinct populations of Sitka black-tailed deer and marten have all diverged on individual islands or island groups over thousands of years of isolation.
The mechanism is straightforward: islands separated by deep cold-water channels act like miniature continents. Populations that were cut off at the end of the last ice age have been evolving in relative isolation ever since, accumulating genetic differences from their mainland relatives. The dense, productive rainforest supports these populations with plentiful food, especially salmon, berries, and the invertebrates that thrive in old-growth canopy and forest floor habitats. For bears in particular, the fall salmon runs are the caloric engine that makes surviving winter possible, and the forest surrounding the streams is where bears rest, den, and deposit the nutrient-rich remains of the fish they catch.
How Salmon Feed the Trees
The relationship between salmon and this rainforest runs deeper than bears dragging fish into the woods. Pacific salmon spend years at sea accumulating marine nutrients, then return to freshwater streams to spawn and die. When they do, they deliver a pulse of marine-derived nitrogen to the watershed. Trees and shrubs growing near spawning streams in Alaska derive roughly a quarter of their leaf nitrogen from salmon.9PubMed. Salmon and alder as nitrogen sources to riparian forests in a boreal Alaskan watershed Spruce growing near streams that receive these nitrogen inputs also show enhanced growth, measured by wider annual rings.
The nutrient transfer happens through several pathways. Bears, eagles, and other scavengers haul carcasses away from the stream, distributing nutrients across the forest floor. Carcasses that stay in the water decompose and release nutrients into the streambed, where riparian tree roots take them up. Insects that feed on salmon carcasses become food for birds and other animals, spreading the marine signal further. The result is a feedback loop: the forest provides shade and fallen wood that create the cool, structured stream habitat salmon need to spawn, and the salmon fertilize the forest in return. This is one of the best-documented examples of a marine-terrestrial nutrient subsidy in ecology, and it is one reason the conservation of old-growth riparian forest and healthy salmon runs are treated as a single management problem in southeast Alaska.
Carbon Storage Underground
One of the less visible but globally significant features of Alaska’s temperate rainforest is the amount of carbon locked in its soils. The combination of heavy rainfall, cool temperatures, and waterlogged ground slows the breakdown of organic matter, allowing thick organic soil layers to develop over millennia. Across the North Pacific coastal temperate rainforest region, researchers have estimated total soil carbon stocks at about 4.5 billion metric tons to a depth of one meter, with around 22 percent stored in organic soil layers alone. Precipitation and terrain features that control soil wetness are the dominant factors determining how much carbon accumulates in any given spot.10Environmental Research Letters. Large, climate-sensitive soil carbon stocks mapped with pedology-informed machine learning in the North Pacific coastal temperate rainforest
At the individual site level, upland soils in southeast Alaska’s temperate rainforest hold an average of roughly 198 metric tons of carbon per hectare, with accumulation surprisingly consistent across different underlying rock types.11Soil Science Society of America Journal. Spodosol development and soil organic carbon distribution along a lithosequence in perhumid coastal temperate rainforest Scaled globally, the researchers estimate that temperate rainforest soils worldwide could hold between 11 and 33 billion metric tons of carbon, making these ecosystems a moisture-dependent hotspot for carbon storage at middle latitudes.10Environmental Research Letters. Large, climate-sensitive soil carbon stocks mapped with pedology-informed machine learning in the North Pacific coastal temperate rainforest
What makes this finding consequential for climate policy is that the carbon stocks are “climate-sensitive,” meaning they depend on continued high moisture. If warming shifts precipitation patterns or dries out soils even modestly, decomposition could accelerate and release stored carbon as carbon dioxide. The forest looks permanent, but the soil carbon beneath it is maintained by the same wet conditions that define the ecosystem in the first place.
What Flows to the Sea
All that rain does not just grow trees and saturate soil. It washes enormous quantities of dissolved organic carbon into the coastal ocean. Watersheds draining the temperate rainforests, peatlands, and glaciers of southeast Alaska deliver an estimated 1.17 million metric tons of dissolved organic carbon per year into the adjacent marine system, along with roughly 430 cubic kilometers of freshwater.12Journal of Geophysical Research: Biogeosciences. Riverine Dissolved Organic Carbon and Freshwater Export in the Eastern Gulf of Alaska Researchers estimate that somewhere between a quarter and two-thirds of this dissolved organic carbon is bioavailable, meaning marine microorganisms can use it as an energy source.
The geography of the coastline matters here. Southeast Alaska’s shoreline is extraordinarily complex, with deep fjords, enclosed sounds, and hundreds of channels that slow the mixing of freshwater runoff with open ocean water. That gives marine organisms more time and opportunity to metabolize the organic carbon pouring out of the rainforest. Small coastal watersheds, rather than a few large rivers, dominate the total export, which means the carbon input is spread across a vast stretch of near-shore habitat rather than concentrated at a handful of river mouths.13Geophysical Research Letters. Small, Coastal Temperate Rainforest Watersheds Dominate Dissolved Organic Carbon Transport to the Northeast Pacific Ocean This distributed delivery system likely fuels microbial food webs across the entire inner coast, connecting the productivity of the rainforest to the productivity of the fisheries just offshore.
Yellow-Cedar Decline and What Warming Means
Alaska yellow-cedar is one of the more commercially and culturally valued trees in the region, prized for its rot-resistant wood and used for centuries by Indigenous peoples for carving, building, and weaving. Beginning in the early twentieth century, stands of yellow-cedar across southeast Alaska started dying in large numbers. The dieback was puzzling because it did not match the pattern of a disease or insect outbreak. Trees were dying from the roots up, across wide areas, and preferentially at certain elevations.
Decades of research have linked the decline to warmer winters. Yellow-cedar has shallow, fine roots that rely on consistent snowpack for insulation. When warming reduces snow cover, those roots lose their protective blanket and become vulnerable to freeze damage during cold snaps that still occur. In southeast Alaska, warming winter trends have resulted in reduced snowfall and more frequent thaw-freeze cycles, exactly the conditions that injure yellow-cedar roots.14Canadian Journal of Forest Research. Twentieth-century warming and the dendroclimatology of declining yellow-cedar forests in southeastern Alaska The pattern shows a clear elevational gradient: trees at mid-elevations where snow cover has become unreliable are dying, while trees at higher elevations that still receive enough snow, or at low elevations near the coast where temperatures stay mild enough to avoid severe freezing, remain healthy.
The same mechanism has been documented on Haida Gwaii, the island archipelago off the coast of British Columbia. There, the issue is not necessarily single extreme freeze events but rather less cold hardening throughout the winter, leaving roots exposed to varying degrees of freeze damage over multiple cycles.15Ecosphere. Climate‐induced yellow‐cedar decline on the island archipelago of Haida Gwaii The geographic breadth of the decline suggests it will expand if warming continues, potentially shifting the species out of large portions of its current range. Yellow-cedar decline is one of the clearest examples anywhere of a tree species being killed not by warming directly, but by the loss of winter snow that warming causes.
Landslides as a Forest Process
In a landscape this steep and this wet, landslides are not occasional disasters. They are a regular part of how the forest works. Saturated soils on steep slopes give way, sending debris flows down hillsides and into stream channels. Over a 55-year study period, landslides in southeast Alaska mobilized a total of about 4.7 million metric tons of carbon, an average rate of 2.5 metric tons of carbon per square kilometer per year.16Journal of Geophysical Research: Biogeosciences. Estimated Amounts and Rates of Carbon Mobilized by Landsliding in Old‐Growth Temperate Forests of SE Alaska The slides strip away centuries-old trees and the organic soil they grew in, depositing the material in stream valleys or the ocean.
From a forest ecology perspective, landslides create openings in the canopy and expose bare mineral soil, resetting the successional clock in patches. New trees colonize the scar, mosses move in, and organic soil begins accumulating again. In a forest with few large-scale fire events (the climate is simply too wet for fire to play a major role), landslides and windstorms are the primary disturbance agents. They create the mosaic of stand ages and canopy gaps that keeps the forest structurally complex over time. The tradeoff is that landslides also redistribute carbon from stable upland soils into streams and the ocean, where it enters entirely different biogeochemical cycles. Understanding that balance matters for any accounting of how much carbon the forest actually holds onto over the long term.
Managing Second-Growth Forests
Not all of southeast Alaska’s forest is old growth. Decades of logging, particularly from the 1950s through the 1990s, created large tracts of second-growth forest, dense young stands that grew back after clearcutting. These second-growth stands present a management puzzle. Left alone, they grow into thick, even-aged canopies that shade out understory vegetation and offer poor habitat for deer, birds, and other wildlife that depend on the structural variety of old growth. Thinning them can speed their development toward old-growth-like conditions, improve deer habitat, and eventually produce some commercial timber, but those goals do not always align neatly.
Current forest planning in the Tongass is wrestling with exactly this kind of tradeoff. Researchers have laid out frameworks for evaluating second-growth management across three ecosystem services simultaneously: timber production, carbon sequestration, and deer habitat.17Treesearch (USFS). Managing second-growth timber in southeast Alaska for deer habitat and carbon sequestration—informing multiobjective forest planning The tension is real. Maximizing carbon storage means leaving trees standing as long as possible. Maximizing deer habitat means creating openings and structural diversity sooner. Maximizing timber revenue means harvesting on a schedule that may compromise both. The shift in Tongass management over the past decade has been away from old-growth logging and toward working with second-growth stands, but the details of how to do that well are still being worked out, and the answers depend heavily on what local communities need from their forests.
How It Differs from a Tropical Rainforest
People hear “rainforest” and picture parrots and bromeliads. Alaska’s version has none of that. Tropical rainforests have staggering species diversity but nutrient-poor soils. Temperate rainforests flip that pattern: they have relatively few tree species but build up enormous stores of nutrients and organic matter in the soil. Tropical rainforests cycle nutrients rapidly through living tissue because the soil cannot hold them. Temperate rainforests hoard nutrients in thick organic horizons and slowly decomposing wood because the cold slows everything down.7Geoderma. Soils of temperate rainforests of the North American Pacific Coast
Both types of rainforest are defined by heavy precipitation and the biological productivity that follows from it, but the character of that productivity is fundamentally different. A tropical rainforest stores most of its carbon in living biomass. A temperate rainforest stores a huge fraction in dead wood, litter, and soil organic matter. The trees in southeast Alaska may not be as species-rich as those in Borneo, but a single Sitka spruce can live for 700 years and reach over 60 meters in height, and the soil beneath it may hold more carbon per hectare than almost any other terrestrial ecosystem at similar latitudes. The forest’s value to the global carbon budget is disproportionate to its modest species count, which is part of why conservation attention to this region has intensified in recent years.