Riparian woodlands, as they are known, are those on the banks of natural bodies of water and particularly rivers.
Forests and fields, beaches and sea, woods and rivers. The ancient Celts saw the places where two realms meet as being particularly magical. This principle holds true in ecological terms, with waterside forests being rich and valuable habitats – a home to organisms of woodland and water. Riparian woodlands, as they are known (from the Latin ripa – bank), are those on the banks of natural bodies of water and particularly rivers.
Rivers can be seen as the lifeblood of the forest ecosystems, and their health is crucial to the health of the forest as a whole.
Benefits to river and landscape
Riparian woodlands are beneficial to the river, and the wider landscape, in several different ways.
The physical presence of trees, such as the deep-rooted alder (Alnus glutinosa) on the river’s edge helps to prevent the banks from eroding away. Riparian zones are important sources and storage sites for nutrients and energy. Trees will naturally alter the chemical balance of the water by taking up minerals from the soil and releasing them into the water.
Trees on the river’s edge have a huge and beneficial impact on the biological health of the river. Invertebrates falling into the water from leaves and branches form up to 90% of the diet for a number of fish, such as the brown trout (Salmo trutta). The undercuts and deep pools created along a tree-lined river bank give shelter and shade to salmonid fish. Shade is important as it keeps the growth of water weeds in balance, and regulates the temperature of the water.
Riparian zones act as corridors and enhance connectivity, creating links within and between forest patches. They provide routes along which animals can disperse, as well as certain plant seeds which may be carried by mammals, birds, or even water.
The role of leaf litter
The actual leaf litter falling into the water can form the foundation of the river’s food chain. Most ecosystems have green plants as their primary producer, trapping the sun’s energy and converting it to food by photosynthesis. Many upland streams are too cold or nutrient-poor to allow plants to grow, and so leaf litter from riparian trees becomes the main source of energy.
Once in the water, leaf litter is decomposed by micro-organisms and invertebrates. First of all, bacteria and filamentous fungi condition the leaves. A series of invertebrates classified as shredders and collectors then break down the leaf litter. These include the larvae of mayflies, stoneflies, water-beetles, midges and caddisflies. These all provide food for predators such as salmonid fish.
A diversity of trees is beneficial, because different leaves decay at different rates. The leaves of bird cherry (Prunus padus), for example, can break down within a few weeks, while oak leaves (Quercus spp.) can take months to decompose. This provides a food source in streams throughout the winter.
Alder is a key riparian tree. It has special bacteria on its root nodules, which enable the tree to fix atmospheric nitrogen into the soil. In this way it contributes to improving soil fertility, and through its leaves which fall into the water, to increasing the nutrient input in upland streams, where nutrients are often in short supply.
Dead wood
Dead wood which falls into a stream enhances the biological value of the aquatic habitat. Woody debris slows the flow of water in the stream, and this can help to form still pools, which some fish need for spawning. Dead wood provides shelter from predatory birds and mammals, and it also increases the number of territories for fish – more fish will occupy an area if they are visually isolated from one another. Pieces of wood will also trap organic matter, thereby increasing the amount of food at the bottom of the food chain.
