The United States is home to diverse ecosystems, including rainforests. These environments exist within US territories, encompassing both warm tropical and cooler temperate types. They support an extraordinary array of life.
What Defines a Rainforest?
Rainforests are characterized by consistently high annual rainfall and a dense, multi-layered canopy. These areas typically receive over 80 inches (200 cm) of rain per year. This constant moisture supports lush vegetation and a complex forest structure.
The canopy, formed by closely spaced trees, creates a ceiling of branches and leaves 100 to 130 feet off the ground. This upper layer supports much of the forest’s plant and animal life. Tropical rainforests are found in consistently warm climates, often near the equator, while temperate rainforests occur in regions with milder temperatures.
Tropical Rainforests in the US
Tropical rainforests within US jurisdiction are primarily found in Puerto Rico and Hawaii. El Yunque National Forest in Puerto Rico is the sole tropical rainforest within the US National Forest System. Situated in the rugged Luquillo Mountains, this forest receives over 200 inches of rainfall annually in its higher elevations.
El Yunque is known for its remarkable biodiversity, harboring over 240 native tree species, 88 rare types, and 150 fern species. The forest is also home to the Coquí frog and the endangered Puerto Rican Parrot. Its average temperature of around 73°F (21°C) fosters a year-round tropical environment.
Hawaii also contains significant tropical rainforest areas, particularly on the windward sides of its islands. They are characterized by high rainfall, with some areas like Mount Waiʻaleʻale receiving an average of 373 inches (9,500 mm) per year, making it one of the wettest places globally.
Hawaiian rainforests exhibit unique biodiversity, shaped by millions of years of isolation. The canopy is often dominated by ʻōhiʻa lehua and acacia koa trees, with an understory rich in ferns, orchids, and mosses. Many endemic species, including fungi, snails, and various birds, have evolved here due to this isolation.
Temperate Rainforests in the US
Temperate rainforests are found along the Pacific Northwest coast of North America, extending from northern California through Washington and into Alaska. These forests experience cooler temperatures than their tropical counterparts but still receive substantial rainfall and maintain a dense, evergreen canopy.
The Olympic National Park in Washington is a prime example, showcasing towering conifers like Sitka spruce, Western hemlock, and Douglas fir. These trees can reach immense heights, forming a multi-layered structure that supports a rich understory of mosses, ferns, and various shrubs. Constant moisture creates a distinct ecosystem where decomposition enriches the soil, supporting rapid tree growth.
Further north, Alaska’s Tongass National Forest represents the largest temperate rainforest in the world. Covering nearly 17 million acres, it is dominated by Sitka spruce and Western hemlock. This vast forest also includes Western red cedar and yellow-cedar.
The Tongass ecosystem is characterized by its rugged coastline, fjords, and a profusion of wildlife, including brown and black bears, wolves, Sitka black-tailed deer, and numerous bird species like bald eagles. The forest floor is often covered with a lush carpet of mosses and ferns, and many trees are draped with lichens.