A forest that expects the river
Most trees do their best when their roots are not underwater. Floodplain forests are an exception. They grow on low ground beside rivers, in the area that a river covers when it overflows its banks, and they depend on that flooding happening at roughly regular times. The pattern of rising and falling water is sometimes called a flood pulse, and it shapes which trees grow, where animals feed and how the soil is built.
Floodplain forests are found in many climates, from temperate river valleys to the great tropical rivers. Some areas flood for a few days in spring, and others are underwater for months each year. The same word can therefore describe very different places.
What the flood delivers
A floodplain receives several things from the river that a hillside forest does not.
- Fresh sediment. Floodwater slows as it spreads out across the forest, so the silt it carries settles. Each flood can lay down a thin layer of fertile soil, which is one reason many floodplains have been farmed.
- Nutrients. Dissolved and particulate nutrients arrive with the water, and leaf litter and other plant material washed in from upstream add to the supply.
- Moisture. Even after the water drains away, the soil can stay damp, which helps trees get through dry spells in the growing season.
- Seeds and plant pieces. Water carries seeds, fruits and broken twigs and drops them in new places.
- Fish and other animals. Fish may move into the flooded forest to feed and spawn, then retreat to the river as the water falls.
How trees time their lives to floods
Many floodplain trees have life cycles that match the river’s calendar. Willows and cottonwoods, which are poplars, are classic examples in temperate regions. They release large numbers of light seeds in late spring or early summer, often as floods recede. The seeds germinate best on bare, damp sediment that a flood has just left behind, and the seedlings need moist ground for their first growing season. A flood that arrives at the wrong time, or a river that no longer floods at all, can mean few new trees get started.
Other species are better at standing in water for long periods. Some have roots with air spaces, trunks that sprout new roots when buried, or seeds that float. In the Amazon basin, forests flooded by sediment-rich rivers and those flooded by clear or dark “blackwater” rivers differ in their plants. In these flooded forests, fish are known to eat fruit and seeds that fall into the water and may carry some seeds to new places, an unusual form of seed dispersal.
Habitat across the year
A floodplain forest changes through the seasons. At high water it is part of the river, with fish moving among the trunks. At low water it is a woodland, with birds, mammals and amphibians using the damp ground, pools and side channels. Old river channels cut off from the main stream, known as oxbows, often hold still water that supports amphibians and wetland plants.
Large trees and fallen logs add structure. When trees topple into the water, they create shade, cover and places for aquatic insects, and a dead log on the forest floor becomes deadwood habitat for fungi and beetles. Because floodplains connect water, land and the stretch of river corridor that stretches for miles, they can also act as routes for wildlife corridors between larger blocks of habitat.
Floodplains and flood control
Floodplain forests are not just passive victims of high water. Dense vegetation slows water down and allows it to spread out, which can reduce the height of a flood downstream. Floodplains also store water that later seeps back to the river, which can help maintain flows between rains. This is part of how a catchment moves water through the landscape. The effect depends on the size of the flood and the shape of the valley, and a floodplain cannot stop a very large flood, but intact ones tend to soften the peaks.
What changes the pattern
Many of the world’s floodplain forests have changed because the rivers beside them have been altered.
- Dams can reduce spring peaks or change the timing of releases, so the flood that triggers seed germination no longer comes at the right time. They can also trap sediment.
- Levees and embankments keep the river in its channel and separate it from the forest, which may dry out or be converted to farmland.
- Drainage and clearing for crops and settlements remove the trees altogether.
- Channel deepening can lower the water table so that roots no longer reach groundwater.
- Invasive species such as non-native shrubs and trees can take advantage of altered flows, a situation that overlaps with the wider problem of invasive species in forests.
Where flows are less natural, older trees may survive for decades but few young ones replace them, leaving a forest of one age that slowly thins out.
Managing and restoring floodplain forests
Restoration of floodplain forests often focuses on the river as much as on the trees. Common measures include reconnecting side channels, setting back levees so that the river has more room, and releasing water from dams in patterns that imitate natural spring floods. Planting may help in some places, but where seeds and floods still operate, natural regeneration can be effective and cheaper. Success depends on land ownership and local needs, since floodplains are often valuable farmland and homes are built on them. Many projects look for ways to combine flood storage, wildlife and agriculture on the same land.