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 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.
There are many species of animals and plants which thrive in riparian woodland. For example, most species of bat in Scotland use riparian woodlands for foraging. Daubenton’s bat (Myotis daubentoni) in particular roosts in riparian trees. It feeds over water, swooping down to scoop invertebrates (and, very occasionally, small fish!) from the surface.
The European otter (Lutra lutra) needs a healthy aquatic ecosystem, with plenty of fish. As already noted, trees along the water’s edge help to increase the number of fish, with obvious benefits for the otter, which often builds its holt (den) beneath the overhanging roots of riverside trees.
The European beaver (Castor fiber) is now extinct in Britain, but this keystone species of riverine habitats formerly played a central role in the functioning of our riparian ecosystems. The dams beavers create provide habitats for other creatures which require pools to live in. By trapping organic matter, the pools make more food (ie invertebrates) available to fish, and store water during dry periods. Beavers diversify the habitat by felling shrubs and trees such as aspen (Populus tremula). This natural coppicing encourages the growth of new shoots, and also stimulates vegetation succession. Contrary to some common misconceptions, the dams made by the European beaver do not hinder fish migration.
The proposed reintroduction of the beaver has still not been approved , so we are unable to catch up with the other 13 countries in Europe who, since 1920, have recognised the importance of beavers and reintroduced them to their rivers. In the meantime, there is no doubt that the extinction of beavers in Scotland through hunting and habitat destruction has been a great loss to our riverine ecosystems.
The goldeneye (Bucephala clangula) is a water-dwelling bird which nests in holes in old trees, and so is another obvious beneficiary of riparian woodlands. The spectacular kingfisher (Alcedo atthis) may be seen perching on a branch over the river, searching for its prey, and herons (Ardea cinerea) also depend on fish, and often nest in riverside trees. Dippers (Cinclus cinclus) hunt for invertebrates on the beds of rivers, and are a sign of a healthy river.
The freshwater pearl mussel (Margaritifera margaritifera) was once fairly common throughout the UK. It has suffered huge losses as a result of over-harvesting for its pearls, as well as pollution from aquaculture and agricultural run-off. It has a fascinating life-cycle, as the larvae attach themselves (harmlessly) to the gills of fish and eventually fall off to settle on the river bed. Individuals can reach an astonishing 100 years old. They may be found half-buried in the river bed, filtering organic matter and thereby cleaning the water and gravel beds in fish spawning areas. It is possible that their decline has contributed to the decline of salmonids. The River Moriston, along the southern edge of Trees for Life’s Core area for forest restoration, has been designated a candidate Special Area of Conservation (SAC) under the EU Habitats Directive, on account of its important population of these molluscs – Scotland contains half of the remaining pearl mussel colonies in the world.
In many parts of the Highlands, the riverside woodlands have been removed, with detrimental effects for the entire riparian ecosystem.
As with all of our native woodlands, riparian zones have suffered greatly due to human impacts over the millennia. Imprudent heather burning, clearance for agriculture, pollution, overgrazing and large-scale conifer afforestation have all played their role. However, as riparian woodland is such a variable habitat, our impact upon it has also varied.
Our remaining riparian woodland includes some of the most pristine and least disturbed native woodland in Scotland. Some of the less accessible upland gorges are a refuge for the more sensitive Atlantic mosses, ferns and lichens. In contrast, floodplain forests are among the most threatened habitats in Europe, as the rich alluvial soil has been favoured for agriculture.
The knock-on effects have been wide ranging, with one of the most noticeable being that numbers of salmon (Salmo salar) and sea-trout have dropped dramatically in the last 50 years. This is due to a variety of factors, including the degradation of the aquatic environment.
Humans need wooded rivers, even if we don't always realise it. Increased flooding is partly linked to the loss of riparian forests. Floodplain woodlands act like a sponge, absorbing and then slowly releasing floodwater, moderating its impact on the landscape. In this way they can reduce the cost of flood protection measures. Equally they protect our water by slowing and absorbing some of the chemical run-off from agricultural land.
A number of organisations, including Trees for Life, are working to help protect and restore this part of our natural heritage. In Achnashellach Forest, owned by Forest Commission Scotland (FCS), our volunteers have been restoring the alder carr, a type of wet woodland which was previously drained for forestry. We are now working alongside FCS and blocking up the drainage ditches to raise the water level once again.
Situated to the south of the Athnamulloch bothy in Glen Affric, the Gleann na Ciche exclosure is particularly important because it protects the riparian woodlands which extend from the upper reaches of the watershed down to the floodplain. In Glen Affric there are areas where we have erected stock fences to protect areas of eared willow (Salix aurita) and alder alongside the Affric River. Birds such as the willow warbler (Phylloscopus trochilus) can already be heard returning to these areas, and it seems likely that water voles (Arvicola terrestris) in the vicinity may be benefiting from the increased amount of vegetation.
Furthermore, we have been increasing the amount of aspen on the shores of Loch Beinn a Mheadhoin. Part of our aim is to increase the glen's long-term potential as suitable habitat for the beaver. Any successful beaver reintroduction would need to ensure that there was enough aspen to support beavers and other aspen-dependent organisms. Our work along Allt na Muic in recent years has essentially involved restoring a riparian corridor between Glens Affric and Moriston.
On a wider scale, it is now FCS policy to take care of riparian zones, avoiding planting with non-native conifers up to the edges of rivers, and instead maintaining a buffer zone of native trees. Forest Habitat Networks, a strategy recently put in place by Scottish Natural Heritage, will include wooded riparian zones as a means of linking forests across the landscape.
It is clear that riparian woodlands are a vital part of the forest ecosystem. Water has been likened to the lifeblood of the forest – as with a human body, where this flow is impaired, the whole system suffers. It follows that it is crucial to ensure that the 'blood vessels' are cared for to ensure the healthy functioning of the entire forest.