The United States contains diverse rainforest biomes, though they are not exclusively the hot, humid environments typically associated with the word. A rainforest is defined by the sheer volume of precipitation it receives, generally requiring a minimum of 1,700 millimeters (about 67 inches) of annual rainfall. This supports a dense, multi-layered forest canopy and moisture-dependent vegetation. The US landscape supports these conditions in several distinct geographic regions, creating unique ecosystems. While the iconic tropical variety exists in US territories, the most extensive rainforest area within the nation is characterized by a cooler, more moderate climate.
Distinguishing US Rainforest Classifications
The rainforests found within the US fall into two major categories, separated by their thermal profiles: tropical and temperate. Tropical rainforests, situated closer to the equator, maintain consistently warm temperatures that exceed 18°C (64°F) year-round. They experience heavy, predictable rainfall without a substantial dry season, resulting in constant biological activity.
Temperate rainforests are located in cooler coastal and mountainous regions, experiencing a distinct winter season with periodic freezing temperatures. These forests receive high annual precipitation, often over 2,500 millimeters (100 inches), supplemented by persistent coastal fog. This fog mitigates summer drought and sustains the enormous biomass characteristic of these northern ecosystems. The difference in temperature dictates the types of flora, with tropical forests favoring broadleaf species and temperate forests dominated by conifers.
The Temperate Rainforests of the Pacific Coast
The largest contiguous expanse of rainforest in the US is the coastal temperate rainforest, stretching along the Pacific Northwest from Northern California to Southeast Alaska. This biome is fueled by moist air masses from the Pacific Ocean colliding with the steep slopes of the Coast Ranges, forcing the air to cool and release its moisture in the form of heavy rain and fog. The Tongass National Forest in Alaska and Olympic National Park in Washington contain spectacular examples of this ecosystem.
These forests are distinguished by an immense accumulation of biomass, featuring some of the world’s tallest and largest trees. Dominant species include the massive Sitka spruce, Western Hemlock, and Western Red Cedar, which grow to tremendous heights and support a dense, dark canopy. The forest floor is characterized by thick layers of mosses, ferns, and epiphytes like lichens, which thrive on the high humidity and moderate temperatures. Constant moisture supports complex nutrient cycling, where fallen trees often become “nurse logs” that provide the foundation for new saplings.
The Tropical Rainforests of the Islands
The US also contains true tropical rainforests, confined to its low-latitude island regions and territories. These areas meet the criteria for a tropical climate, with consistent high temperatures and year-round heavy precipitation driven by trade winds and mountainous topography. El Yunque National Forest in Puerto Rico, the only tropical rainforest in the US National Forest System, is a prime example of this biome.
Parts of the Hawaiian Islands, such as the windward slopes of the Big Island, also receive sufficient rainfall to support tropical rainforests. These island rainforests exhibit a different biological composition than their temperate counterparts, featuring a higher density of species, including unique flora like tree ferns and a greater variety of epiphytes. The fauna is distinct, with species like the Puerto Rican parrot and several types of coquí frogs found exclusively in the warm, humid environment of El Yunque.
Conservation Status of US Rainforests
US rainforests, both temperate and tropical, are natural resources largely under federal protection. Key areas are preserved within National Parks and National Forests, which helps safeguard them from immediate destruction. However, these ecosystems face ongoing threats that require continuous preservation efforts.
Historically, the temperate forests of the Pacific Coast were heavily impacted by commercial logging, leading to a significant reduction in old-growth stands. Current challenges include the effects of climate change, which can alter precipitation patterns and increase the frequency of severe weather events. Invasive species pose a threat to the endemic biodiversity of the island rainforests, necessitating dedicated management to maintain the unique ecological balance of these specialized biomes.