Forests that live in the clouds
Cloud forests are tropical and subtropical mountain forests that sit in the zone where clouds frequently touch the ground. They typically occur at middle to high elevations, often somewhere between about 1,000 and 3,500 meters, though the exact band depends on the mountain, its distance from the sea and local climate. At these heights, moist air rising up a slope cools and condenses, so the forest is wrapped in cloud or mist for much of the year.
You can recognize one quickly. Trees are shorter and more twisted than in lowland rainforest, branches are thick with moss, and everything drips. Light is dim and diffuse, and the air feels cool and damp.
How they catch water from fog
In ordinary rain, water falls from above. In a cloud forest, much of the water arrives sideways. Tiny fog droplets, too small to fall as rain, are carried by wind and stick to leaves, twigs, mosses and epiphytes. The droplets merge, run down branches and trunks, and drip to the ground. This is often called fog drip or cloud interception.
The amount varies widely, but in some cloud forests this extra water is a significant part of the total moisture, especially in dry seasons when little rain falls. Measuring it is tricky, because a rain gauge placed in the open often misses it. That is one reason estimates differ and should be treated cautiously.
Why the forest looks the way it does
Several conditions shape the vegetation.
- Frequent cloud cover reduces sunlight, so photosynthesis is slower, and trees grow more slowly.
- Cool temperatures slow decomposition. Dead plant material builds up into a thick, spongy layer of organic matter.
- Wind and thin or waterlogged soil keep many trees short and gnarled, particularly on exposed ridges.
- Constant moisture allows mosses, liverworts, ferns and orchids to cover almost every surface.
Epiphytes are especially abundant. Their thick mats act like sponges, soaking up fog and rain and releasing it slowly. They also create small habitats, because water collects in the leaf bases of plants such as bromeliads in the Americas, supporting tiny aquatic animals. For more on these plants, see the article on canopy epiphytes.
Species adapted to the mist
Cloud forests often hold many species found nowhere else. Because they occur on separate mountain ranges, like islands in a lowland sea, populations are isolated, and over time can diverge into distinct species. Many amphibians, orchids, small mammals and birds have very small ranges. Some regions, such as the Andes and parts of Central America, are known for high endemism in these forests, which contributes to the status of certain mountain regions as biodiversity hotspots.
Common adaptations include:
- Leaves that shed water quickly, often with pointed tips, to limit growth of algae and fungi on the surface.
- Small, thick leaves in exposed areas, which resist wind and cold.
- Hummingbirds and other nectar feeders that pollinate flowering plants in cool, cloudy conditions where insects are less active.
- Amphibians that rely on constant moisture and often breed in water held by plants.
Specific examples vary by continent, and the cloud forests of Africa, Asia and the Pacific have their own distinctive species.
Why they matter for people
Cloud forests often grow in the upper parts of river catchments. By catching fog and storing water in moss and soil, they can help maintain stream flow in dry periods and feed springs used by towns and farms downhill. The mossy soil and dense roots also help hold steep slopes together, supporting slope stability. These benefits depend on the forest remaining intact; the strength of the effect varies by site, and researchers continue to study it.
Threats
Cloud forests face a mix of pressures:
- Clearing for farms and pasture, including coffee, potatoes and cattle, as agriculture moves up mountains.
- Fragmentation, as roads and fields divide forest into smaller pieces.
- Climate change. If the cloud base rises in a warming climate, forests may be left with less fog than before. Species adapted to cool, damp conditions may need to move upslope, but near the summit there is nowhere higher to go.
Because many cloud forest species have tiny ranges, the loss of even a small area can threaten a species with extinction. Protection, restoration of cleared slopes, and sustainable land use in surrounding areas are commonly proposed responses, though cloud forests regrow slowly after disturbance.