Basket

Our latest Directors’ blog is a guest article by Peter Cunningham, an aquatic biologist who has lived and worked in Wester Ross since 2001. In this article, he looks at the role of riparian woodlands (and alder in particular!) in sustaining wild fisheries, supporting riverbanks and reducing wildfires.

I’m in lockdown in East Lothian (January 2021). There is a burn nearby which I have revisited over 40 years to look for warblers and redstarts in the springtime, to watch little trout darting into hiding places within rooty labyrinths in the summertime, and to look for animal tracks in the winter. Once I found the tracks of two otters by the confluence pool. Last week, I found the footprints of roe deer, rabbit, hare, fox and a badger in the snow; and heard a dipper singing from a stone beneath a riparian alder tree.

Childhood adventures progressed to cycle expeditions through the Moorfoot Hills where lapwing, curlew, redshank, snipe and oystercatchers can still be heard in May and June, to fish for trout and salmon in the upper Tweed and Yarrow waters.

Over many years, Dr Ron Campbell and colleagues at the Tweed Foundation progressed a series of projects in the Tweed headwaters and tributaries to remove obstructions to upstream fish passage, and, in collaboration with many landowners, to fence off sections of river bank from livestock and plant them with alder, willow and other tree species to create more diverse habitats for fish and other wildlife, and ‘dappled shade’ upon the water.

Stream with mix of alder trees and open areas, providing dappled shade, River Carron tributary, c. 2005. The stream-bed was unstable with much freshly deposited sediment due to upstream forestry operations.

The first projects were over 25 years ago. Since then, hundreds of miles of stream habitat have been reopened to wild salmon or improved. By around 2010, Tweed anglers were rewarded with some of the highest rod catches of salmon on record. Alas, over the past ten years the Atlantic salmon has been in decline across much of its range, primarily because of problems at sea, some of which are associated with climate change. Riparian alder trees are wonderful things. Their roots have air-conducting tissues which enable them to grow beneath the surface of the water. Submerged roots trap woody debris, leaves and other organic matter and provide habitat for aquatic invertebrates and places for fish to hide. Alder leaves, twigs and fallen cones nourish the insect nymphs and larvae which in turn provide food for little fish. Alder trees have provided shelter and nourishment for wild trout and salmon for millions of years.

Recent research by Dr Faye Jackson and colleagues at Marine Scotland Science has further highlighted how riparian trees can help to moderate water temperatures by providing shade. Within the next 30 years, peak summer water temperatures are projected to rise too high for salmon to survive in some parts of Scotland where streams are wide, shallow and unshaded.

The need to protect and restore riparian woodland has been widely recognised and many new riparian woodland projects have been undertaken by fisheries managers. Please see ‘SFCC Riparian Woodlands Meeting (2020)’ for some on-line examples from around Scotland.

For most of the past 20 years I have lived and worked as a fisheries biologist in Wester Ross. My work provided an opportunity to explore many river systems and to learn more about how riparian trees can affect fish and other wildlife.

Fat, healthy juvenile salmon from a stream with a well vegetated riparian corridor in Wester Ross.

Much of Wester Ross, including most of the ‘Great Wilderness’, is currently treeless and is managed for red deer. The occurrence of deep peat demonstrates that some areas have had few trees for thousands of years. However, fragments of (once-extensive) networks of ancient woodland can still be found on hillsides and especially along the sides of some of the streams. Saly, though, in some areas woodland cover, including that of riparian woodland, continues to decline as a result of grazing and browsing pressure, and as a result of wildfires.

In contrast to the rivers Tweed and Tyne in the southeast of Scotland, most of the rivers in the northwest of Scotland drain catchment areas underlain by ancient rocks which are hard and resistant to weathering and provide a meagre supply of the life-limiting nutrient, phosphorus.  Wester Ross has little arable land and low densities of people so there is very little input of phosphorus from anthropogenic sources. Instead, stream and riparian corridor fertility is largely dependent upon the more natural ecosystem. This means that natural processes are particularly important. Alder trees are able to obtain nitrogen from the air via nodules containing the bacterium Frankiaalni which form on the near-surface roots. To obtain phosphorus, alder trees depend upon mycorrhizal fungi. In the autumn, the birch and pine woods of Wester Ross are full of diverse toadstools and mushrooms, including those of mycorrhizal Amanita spp. (e.g. fly agaric), Boletus spp., Russula spp., and chantarelle which associate with several tree species but not with alder. Instead, the mycorrhizal fungi of alder trees are mostly host-specific and have also evolved to grow in ground which may be waterlogged for many weeks of the year (Tedersoo et al, 2009).

Life and death and riparian alder trees. Figure based on observations from several streams in Wester Ross and from info via google! Speculations are things where I’m looking for further info.

Alder trees grow well where there is frequent flooding. Flood plains are typically fertile places, where nutrient-rich silt settles out when river levels are very high. Alder trees can grow quickly when there is sufficient phosphorus. Fifteen years ago I planted several species of tree on my croft near Gairloch; the tallest tree is an alder planted on an old midden.

In the 1950s, Donald McVean conducted a series of trials at Beinn Eighe National Nature Reserve and was able to demonstrate that so long as there is sufficient phosphorus, alder trees are even able to grow on peat. Nearby, along the Kinlochewe River, phosphorus-rich discharges from the village have enhanced stream fertility and there are many tall riparian alder trees; juvenile salmon grow relatively quickly feeding on insect larvae which graze on benthic algae or upon decomposing leaf litter. However along streams in other parts of Wester Ross, away from human habitation, there are are dead and dying alder or the trees have long since been lost. Some of these streams still support high densities of salmon fry and parr; however, the juvenile salmon are typically small and thin for their age compared to those where healthy riparian alder trees grow. Projected smolt production (the number of salmon that migrate to sea) per unit wetted area of stream can be low despite seemingly high numbers of hungry little fish. Some of these streams are also inhabited by relict freshwater pearl mussel populations, often with little evidence of recent recruitment. Is there still adequate nourishment to sustain freshwater pearl mussel reproduction?

Governments around the world have focussed much attention and resources on addressing problems associated with eutrophication of freshwaters. Many rivers especially those which flow through urban and arable areas within the United Kingdom are much cleaner than they were 100 years ago.  There has been less attention on oligotrophication, where nutrient deficiency has become a problem and ecosystems have become malnourished.

Ultimately, all life on Earth depends upon the circulation of nutrients including phosphorus. In the past, runs of wild salmon into headwater streams were much larger than they are now, and bears, wolves, people, many birds and other animals would have helped to transfer marine derived phosphorus into riparian ecosystems, helping to sustain the productivity of other forms of life. Fish and animal bones are a rich source of phosphorus; some mycorrhizal fungi are able to extract phosphate from apatite (Plassard and Dell, 2010).

Salmon carcasses are pulled up on to the river bank by otters; their bones are rich in phosphorus. This picture was taken on a small island in the Little Gruinard River.

During the early 2000s, much money and effort went into native woodland restoration schemes in Wester Ross. Some of these schemes have grown well. However, there have been wildfires almost every spring especially through areas with heather. Many young woodlands have been lost. As wildfires are predicted to become more frequent and more intense in future years, there is a need for coordinated planning and management of firebrakes at the landscape scale. This requires a strategic approach, like flood management planning.

Parts of Wester Ross burnt in 2018 (green) and 2019 (red). Extensive areas of young native woodland and riparian woodland were damaged or destroyed by fire. Map from Ben Leyshon, SNH, presented at Wester Ross Biosphere HEF Wildfire Workshop, March 2020.

Could streams with riparian corridors with alder trees and understory green grass (rather than flammable heather) be managed to act firebreaks? The roots of alder trees extend down below the water table from where they can draw up water to maintain higher soil moisture content nearer the surface during dry periods.  Note that gorse which also grows well in riparian areas can be highly flammable, so it is not just a matter of promoting growth of any sort of riparian vegetation. Lightly grazed alder woodlands might be something to aim for?

Many land owners and managers have a growing understanding of how riverbank trees can support deer and livestock by providing shelter and by enriching soils, as well as supporting fish populations. Livestock and deer sometimes seek the shelter of woodlands; and during springtime the green grass which grows beneath alder trees before the trees come into leaf is of high nutritional value. There are already many examples of successful riparian woodland projects in Wester Ross as elsewhere in Scotland.

Wester Ross Fisheries Trust is currently being reformed and will continue to support riparian woodland recovery in collaboration with others. If you are interested in joining a field team when the pandemic is over, please get in touch via email (). And look out for dippers!

References:

Plassard, C. and B. Dell (2010): Phosphorus ecology of mycorrhizal trees

Tedersoo, L. et al. (2009): Revisiting ectomycorrhizal fungi of the genus Alnus: differential host specificity, diversity and determinants of the fungal community

Parrott, J. and McKenzie, N. (2000): Restoring and Managing Riparian Woodlands. Scottish Native Wood, Aberfeldy

Cunningham, P. et al. (2020): Skye and Wester Ross Fisheries Trust Review, September 2020


0 Comments

Leave a Reply

Avatar placeholder

Your email address will not be published. Required fields are marked *