Forests with wet feet
Forested wetlands are areas where trees grow in soil that is saturated or flooded for a meaningful part of the growing season. Many people call them swamps, but the term covers a surprisingly varied group of ecosystems. What links them is water, which fills the spaces in the soil and leaves little oxygen for roots. What separates them is how long the water stays, where it comes from, how acidic or nutrient-rich it is and what kind of soil it creates.
Names for these forests differ among countries and scientists, and the boundaries are not sharp. The sections below describe three widely recognized groups.
Swamp forests
In common usage, a swamp forest is a wooded wetland with standing or slowly moving water for long stretches, and with mineral or muck soil rather than deep peat. Classic examples in the southeastern United States are forests of bald cypress and water tupelo, where the ground can be under water for much of the year. In Europe, wet woods dominated by alder are a similar type. Tropical regions have freshwater swamp forests along rivers and lakes, and tidal freshwater swamps lie just inland of the saltier mangroves.
Swamp forests are often rich in wildlife. Standing water offers breeding sites for amphibians and fish, and the mix of trees, snags and water draws herons, wood ducks, owls and many other birds.
Bottomland hardwood forests
Bottomland forests grow on river floodplains and are flooded seasonally rather than all year. Where the land is slightly higher, the soil may be dry for much of the summer. Typical trees in North America include several oaks, green ash, sweetgum, elms and sycamore. Their wide variety is linked to small differences in ground height: a few feet can separate wetter and drier zones, each with different species.
Bottomland forests are closely related to other river-edge woodlands, and they rely on seasonal flooding for sediment, nutrients and seed dispersal. They are also productive timber land in some regions, as the soils can be fertile.
Bog forests and peatland forests
In a bog, the ground is made of peat, partly decayed plant matter that builds up where waterlogging and cool or acidic conditions slow decomposition. Trees in boreal and cool temperate bogs are typically slow-growing and often stunted. Black spruce and tamarack (larch) are characteristic in northern North America, while pines and birches grow on some European peatlands. The ground is typically covered by sphagnum moss, which holds water and keeps the surface acidic.
In the tropics, peat swamp forests develop in a similar way, but with tall trees growing on deep peat, particularly in parts of Southeast Asia. These forests are especially significant for their carbon stores.
Peatland forests hold a large amount of carbon in the peat, and their value for soil carbon storage is the subject of much attention. Draining them can cause the peat to oxidize and subside. In the far north, bogs are also linked to permafrost thaw, which can alter how water moves across the landscape and cause trees to die or ground to collapse.
A distinction worth noting is between bogs, which are fed mainly by rain and are acidic and nutrient-poor, and fens, which receive groundwater and tend to be less acidic and richer in nutrients. Forested fens exist as well and often hold different trees.
How trees survive wet roots
Roots need oxygen to work. In flooded soil, oxygen disappears quickly, so wetland trees have evolved ways of coping:
- Air channels. Many species have spongy tissue called aerenchyma that carries oxygen down from the trunk to the roots.
- Shallow, wide roots. Roots stay near the surface where oxygen is better, which also makes trees more prone to being blown over.
- Buttressed or swollen trunk bases. Bald cypress and tupelo have flared bases that spread weight and support the tree in soft ground.
- New roots on the trunk. Some trees sprout adventitious roots above the waterline when flooded.
- Knees. Bald cypress often produces woody projections called knees rising from the roots. Their function is not fully settled, so claims that they simply act as snorkels should be taken carefully.
- Timing. Some species tolerate flooding during dormancy much better than in the growing season.
- Floating or water-dispersed seeds. This helps them reach new sites.
Tolerance varies a lot. Some trees can survive weeks of flooding, while others can survive years. This variation in tolerance is one of the main reasons why species change as you move along even a small gradient of wetness.
Why they matter
Forested wetlands store floodwater and release it slowly, filter sediment and nutrients, supply habitat for a wide range of wildlife, and in some cases store large volumes of carbon. They also provide products such as timber, game, berries and peat in various parts of the world, while many cultures treat them as places of spiritual or cultural meaning.
Threats
Because they are wet, many forested wetlands were historically seen as wasteland and drained for farming, forestry or building. Other pressures include changes in river flow from dams, logging, invasive plants and shifts in climate that alter the water balance. Because they develop slowly, damaged wetlands often take a long time to recover, and a drained peatland cannot easily be returned to its former state. Protection of the water regime is generally the key to protecting these forests.