The United States contains both temperate and tropical rainforests spread across at least six distinct regions, from the moss-draped valleys of Washington’s Olympic Peninsula to the cloud-wrapped peaks of Hawaii and Puerto Rico. These are not remnants on the verge of disappearing from maps; they collectively span millions of acres and include the largest temperate rainforest on the planet. Most Americans picture rainforests as exclusively equatorial, but several of these ecosystems sit within a short drive of major cities, and each one runs on a different set of climatic engines that keep it saturated enough to qualify.
The Pacific Northwest Temperate Rainforest
The most famous American rainforest stretches along the western slopes of the Olympic Mountains in Washington State, where annual rainfall regularly exceeds 140 inches. The Hoh, Quinault, and Queets river valleys are the poster children. Trees here, primarily Sitka spruce, western hemlock, western red cedar, and bigleaf maple, are draped in thick layers of mosses, ferns, and lichens so dense that the forest canopy develops its own soil. Research on the Olympic Peninsula has documented that this temperate rainforest biome is defined by unusually high epiphyte biomass and the canopy soils that form around it, essentially a second forest growing on top of the first.
1Botany. Patterns of moisture and temperature in canopy and terrestrial soils in a temperate rainforest, WashingtonWhat keeps it so wet is a combination of geography and atmospheric plumbing. The Pacific Ocean sits just to the west, sending moisture-laden air inland. When that air hits the Olympic Mountains, it is forced upward and cools rapidly, wringing out rain on the windward slopes. This orographic effect is supercharged during atmospheric river events, corridors of concentrated moisture that sweep in from the tropics and can dump extraordinary volumes of water in short periods. Research on coastal steeplands has connected these atmospheric rivers directly to landslide patterns, finding that slides concentrate on windward, steep slopes at mid-slope positions, exactly where orographic forcing and wind-driven precipitation are strongest.
2Geophysical Research Letters. Shallow Landslides Align With Atmospheric Rivers in Coastal SteeplandsOne of the quieter but ecologically critical features of these forests is what happens on the ground after a tree falls. In temperate rainforests, fallen logs serve as “nurse logs,” elevated platforms where the next generation of trees germinates. Tree seedling density on nurse logs runs about 4.6 times higher than on the surrounding forest floor. The reason is competition: the forest floor is dominated by deep carpets of the moss Hylocomium splendens, which can grow more than 20 centimeters thick and effectively locks out seedlings. Nurse logs host 17 bryophyte species compared to just six on the forest floor, making them not just nurseries for trees but biodiversity hotspots for mosses and liverworts.
3PubMed Central. Plant-plant interactions change during succession on nurse logs in a northern temperate rainforestAlaska’s Tongass and the World’s Largest Temperate Rainforest
Southeast Alaska is home to the Tongass National Forest, which at roughly 17 million acres is the largest national forest in the country. It sits within a broader coastal temperate rainforest belt that stretches from northern California up through British Columbia and into Alaska, and this belt collectively forms the largest temperate rainforest on Earth. The dominant tree species here are western hemlock and Sitka spruce, with forests that regenerated after past logging now expected to be the primary source of future timber value as the region transitions away from old-growth harvesting.
4Treesearch. Commercial thinning strategies in southeast Alaska: establishment and effects of the Prince of Wales Commercial Thinning StudyWind is the dominant natural disturbance here rather than fire. Studies of southeast Alaska’s forest dynamics have shown that wind disturbance operates along a continuum tied to landscape features like exposure and topography. Exposed landscapes experience recurrent large-scale blowdowns, while sheltered valleys are shaped by small canopy gaps where individual trees snap or uproot. This means the Tongass is not a static cathedral of old trees; it is a patchwork of age classes stitched together by centuries of windstorms.
5Pacific Northwest Research Station. The effects of wind disturbance on temperate rain forest structure and dynamics of southeast AlaskaThe Tongass matters enormously for carbon storage. A study mapping soil organic carbon across the North Pacific coastal temperate rainforest estimated stocks of about 4.5 billion metric tons of carbon to one meter depth, with roughly a fifth of that locked in organic soil layers alone. The researchers concluded that coastal temperate rainforest ecosystems represent a moisture-dependent hotspot for soil carbon storage at mid-latitudes, and they extrapolated that temperate rainforest soils globally might hold between 11 and 33 billion metric tons of carbon.
6Environmental Research Letters. Large, climate-sensitive soil carbon stocks mapped with pedology-informed machine learning in the North Pacific coastal temperate rainforestThe policy implications of that carbon are concrete. Analysis of the Tongass found that inventoried roadless areas account for just over half the forest’s carbon, with old-growth stands holding about 48 percent. Nearly two centuries of historical and projected logging would release an estimated 69.5 million metric tons of COâ‚‚ equivalent, roughly equal to the cumulative emissions of 15 million cars. The case for protecting old growth and allowing previously logged areas to regrow rests heavily on these numbers.
7Land. The Tongass National Forest, Southeast Alaska, USA: A Natural Climate Solution of Global SignificanceCalifornia’s Fog-Fed Redwood Forests
The coast redwood belt running along California’s northern and central coastline operates on a fundamentally different moisture strategy than the Olympic or Alaskan rainforests. Summer rainfall here is almost nonexistent, so the forest depends on marine fog rolling in from the Pacific. Redwood trees are not just passive recipients of that fog; they actively capture it. Research in redwood forests found that about 34 percent of the total annual water input came from fog dripping off the trees themselves, and when trees were removed from the landscape, fog water input dropped to roughly 17 percent. The trees literally amplify the moisture they need to survive.
8PubMed. Fog in the California redwood forest: ecosystem inputs and use by plantsDuring summer, when rain is absent, understory plants got about two-thirds of their water from fog that had dripped from redwood canopies into the soil. For the redwoods themselves, fog water made up between 13 and 45 percent of annual transpiration. This dependency makes the coast redwood ecosystem sensitive to changes in fog frequency. A study analyzing fog trends along the California coast found evidence that summer fog has been declining, and the researchers presented physiological data suggesting that redwoods and other west-coast ecosystems could face increasing drought stress under a climate with less frequent fog and higher evaporative demand.
9PubMed Central. Climatic context and ecological implications of summer fog decline in the coast redwood regionWhether you call the redwood belt a “rainforest” is partly a question of definitions. It does not get the 80-plus inches of annual rainfall some classification systems require for temperate rainforests. But when you add fog drip to rainfall totals and consider the lush, layered canopy and the thick carpet of ferns and sorrel on the forest floor, it fits ecologically. Most ecologists treat it as a temperate rainforest variant, and its biological structure, including the tallest trees on the planet, is a product of the same general principle: an extraordinary and reliable supply of moisture.
The Appalachian Temperate Rainforest
This is the one that surprises people. Portions of the southern Appalachian Mountains, particularly in western North Carolina and the Great Smoky Mountains, receive enough rainfall (sometimes more than 80 inches a year at higher elevations) and harbor sufficiently lush forest ecosystems to qualify as temperate rainforests. The term “Appalachian temperate rainforest” has gained traction among ecologists, though it is less widely recognized than the Pacific Northwest version.
These high-elevation forests, dominated by red spruce and Fraser fir, also benefit from fog and cloud immersion. Measurements in the Great Smoky Mountains documented fog interception gains averaging between 0.24 and 0.69 millimeters per day across spruce-fir and mixed hardwood forests, adding about 3 to 8 percent on top of already heavy rainfall.
10Ecohydrology & Hydrobiology. Fog interception in spruce-fir and mixed northern hardwood forests of Great Smoky Mountains National Park, Southeast USAThe Appalachian spruce-fir forests have a vulnerability the western rainforests do not share: the balsam woolly adelgid, an invasive insect introduced in the 1950s. Fraser fir forests in the Great Smoky Mountains experienced widespread die-off as a result, fundamentally reshaping stand dynamics and affecting both plant and animal communities throughout the ecosystem.
11Plant Ecology. Spectral response and spatial pattern of Fraser fir mortality and regeneration, Great Smoky Mountains, USAHawaii’s Tropical Rainforests
Hawaii is where the United States has genuine tropical rainforest, the kind most people picture when they hear the word. The windward slopes of the major islands, especially Maui and the Big Island, receive staggering quantities of rain. Mount Waialeale on Kauai averages more than 450 inches a year and has been called one of the wettest spots on Earth. The mechanism is familiar: moist trade winds blow in from the northeast, hit volcanic mountains, and drop their moisture on the windward side. During certain climate phases, weakened trade winds can redistribute that rainfall, with anomalous westerly winds creating a dipole pattern that shifts moisture toward the normally drier leeward coasts.
12Journal of Climate. Impact of the Pacific Meridional Mode on Hawaiian Rainfall VariabilityThe biological character of Hawaiian rainforests is unlike anywhere else in the United States. Geographic isolation gave evolution free rein, and the result is extreme endemism. Many of Hawaii’s plant lineages trace back to ancestors that arrived from North America via rare, often bird-mediated long-distance dispersal. Molecular evidence has confirmed North American ancestry for several of Hawaii’s most species-rich plant groups, including the silversword alliance, native mints, violets, and spurges. Ecological opportunities associated with the islands’ diverse elevation zones, from sea level to alpine conditions, drove the kind of rapid diversification that produced hundreds of species found nowhere else on the planet.
13Annals of Botany. Hawaiian angiosperm radiations of North American originThese forests face serious pressure from invasive species. The nitrogen-fixing tree Myrica faya, introduced from the Azores and Canary Islands, invades montane Hawaiian rainforests and grows at rates 16 to 44 percent higher than the dominant native tree Metrosideros polymorpha, with that growth advantage closely tied to climate conditions. The concern is that Myrica alters soil nitrogen cycling, shifting the competitive balance away from native species that evolved in low-nutrient volcanic soils.
14Ecosystems. Vegetation-Climate Interactions among Native and Invasive Species in Hawaiian RainforestInvasive animals compound the problem. The Puerto Rican coqui frog has established large populations in Hawaiian forests. While early fears centered on the frog consuming native invertebrates, data suggest coquis mostly eat non-native invertebrates, and there is currently little habitat overlap between coquis and Hawaii’s endangered endemic birds. The ecological picture remains uncertain, though, because indirect effects on food webs are hard to predict, and the coqui’s impact on endemic invertebrate populations could grow as the frog spreads into new areas.
15Diversity and Distributions. Potential consequences of the coqui frog invasion in HawaiiPuerto Rico’s El Yunque
El Yunque National Forest in northeastern Puerto Rico is the only tropical rainforest in the US National Forest system. It covers about 28,000 acres and receives upward of 200 inches of rain annually at its highest elevations. The forest is organized by altitude into four distinct zones: tabonuco forest at the base, palo colorado and sierra palm in the middle elevations, and elfin cloud forest at the summits where wind-stunted trees are wrapped in dripping moss.
One of the stranger ecological stories from El Yunque involves Saharan dust. Tank bromeliads, plants that collect water in their rosette of leaves, act as passive samplers of atmospheric chemistry. Analysis of the material collected in these tanks found rare earth element concentrations up to five-fold higher than expected, with a chemical signature closely matching dust sampled over Jordan and Barbados, tracing the source back across the Atlantic to the Sahara Desert. African dust delivers both nutrients and pollutants to Caribbean rainforests, connecting ecosystems separated by an entire ocean.
16Elsevier. Tank bromeliads capture Saharan dust in El Yunque National Forest, Puerto RicoEl Yunque has been hit hard by hurricanes, most recently Maria in 2017, which stripped canopy across much of the forest. Recovery studies are ongoing, but the forest has a long evolutionary history with cyclones and generally bounces back within decades, a pattern that distinguishes it from temperate rainforests where fire and wind operate on longer timescales.
How Logging Intensity Affects Rainforest Carbon
One of the more policy-relevant findings from temperate rainforest research concerns how different logging strategies affect greenhouse gas budgets. A study tracking carbon fluxes over 26 years in north temperate rainforest found that emissions scaled linearly with logging intensity. Unharvested stands acted as net carbon sinks, absorbing about 2.2 metric tons of COâ‚‚ equivalent per hectare per year. Clearcuts flipped to net sources, releasing roughly 21 metric tons per hectare per year. Light partial cuts came close to recovering their pre-harvest carbon, losing only about 2.9 metric tons per hectare per year.
17Elsevier (Forest Ecology and Management). Reducing logging intensity in north temperate rainforests for climate and economic benefitsThe practical message is that light selective harvesting can maintain most of a rainforest’s carbon sink function while still producing timber, whereas clearcutting converts the forest into a carbon source for years or decades. This finding feeds directly into the debate over the Tongass and other publicly managed rainforest lands, where balancing economic use with climate goals is a live policy question.
Climate Change and the Future of American Rainforests
Each of these rainforest types faces a different set of climate risks. The Pacific Northwest forests are projected to experience more frequent and severe wildfire, which historically has been rare in the wettest coastal zones. Research on changing fire regimes in the region concluded that interactions between fire, drought, and insect outbreaks will be the primary drivers of ecosystem change under warming. Reburns, where fire returns to a site before it has fully recovered, are expected to increase, with hotter and drier sites at particular risk for regeneration failure where young trees cannot reestablish after repeated burns.
18Fire Ecology. Changing wildfire, changing forests: the effects of climate change on fire regimes and vegetation in the Pacific Northwest, USAFor the California redwoods, the threat is subtler but arguably more existential. If summer fog continues to decline, the moisture subsidy that sustains these forests shrinks. Redwoods are long-lived and resilient, but seedling establishment depends on fog, and a drier future could slowly constrict the range even if mature trees persist for centuries.
Hawaiian rainforests face the double hit of changing precipitation patterns and accelerating invasive species spread, both of which interact with warming temperatures to shift competitive advantages toward non-native plants and animals. In the Appalachians, warming may push spruce-fir forests further uphill, reducing their total area on mountaintops that offer no higher ground to retreat to.
Soil and Below-Ground Ecology
Rainforest soils are not just dirt beneath the trees; they are active biological systems that regulate carbon storage, nutrient cycling, and water filtration. In the coastal temperate rainforests of southeast Alaska, dissolved organic carbon in forest and wetland soils ranged from 9 to 32 milligrams of carbon per liter, with 23 to 42 percent of that carbon proving biodegradable. That biodegradable fraction matters because it determines how quickly carbon exported to streams and rivers gets consumed by microbes, affecting water chemistry and downstream ecosystems all the way to the ocean.
19Biogeochemistry. Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast AlaskaIn tropical rainforests, the below-ground story involves mycorrhizal fungi, the symbiotic networks that connect tree roots to soil nutrients. Research in tropical rainforest soils found that mycorrhizal tree characteristics and soil nutrients together explained the majority of variation in soil organic carbon content, with soil nutrients alone accounting for about 42 percent and mycorrhizal tree traits contributing another 10 percent. The balance between different types of mycorrhizal associations shifted depending on nutrient availability, meaning that changes in soil chemistry from pollution, invasive species, or climate shifts could ripple through the whole carbon storage system.
20Elsevier. Mycorrhizal and nutrient controls of carbon sequestration in tropical rainforest soilThis below-ground complexity helps explain why rainforests are hard to restore once lost. You can replant trees, but rebuilding the fungal networks, soil organic layers, and microbial communities that took centuries to develop is another matter entirely. It also explains why intact rainforest soils store so much more carbon than degraded ones: the biological machinery that locks carbon into stable forms depends on an ecosystem that is functioning as a whole, not just on trees standing above the surface.