Why Rivers need trees - reviewing Peter Wohlleben's book

Peter Wohlleben’s  book ‘The hidden life of trees’ (translated and published by Harper Collins 2016) makes fascinating reading as a water specialist. It is based on his experience in European woodlands. I am familiar with the ‘why fish need trees’ booklet but there seems to be much more to trees than provision of woody materiel and slowing the flow through wet woodland habitat. The book is well referenced, so you can check the sources, and forests seem to be more critical to the water cycle than I had thought.

The UK water cycle diagrams show water vapour arising as clouds over the ocean and blowing over land where rain (or snow) is deposited as the altitude rises. However Peter points out that this works only on coastal regions- 400 miles inland the clouds would run out. Not an issue in the UK and central Europe, but elsewhere forests intercept rain (27m of leaf cover per m of canopy) and re-evaporate some immediately, which blows on inland. So in summer trees can use, then evaporate, 8500 cubic yds of water per square mile- allowing rain to travel inland into the amazon basin.  So coastal deforestation will lead to desertification inland, in large continents.

We may know that conifers attract rain since they were planted around reservoirs, but apparently this is due to turpenes being emitted by them as a defence. Turpenes cause moisture to  condense over the trees. This causes rainfall which, with the cooling through reduced sunlight, ensures that conifer habitat is kept cool and moist which suits them (and water collection).

Forests build up humus and deeper soil which, with the tree’s leaves reducing the vigour of heavy rainfall, allows forest floors to absorb rainfall. This provides a slower, cooler and more reliable headwater source and  more stable downstream flows. However like rivers, very few of UK forests are ancient or natural and Peter notes that commercial forests perform very poorly in comparison; in heavy rain, undisturbed forests lose 1-14 Tonnes per square mile of soil whereas forests on cultivated land can lose 2900 tonnes per square mile, ten times what they can create in a year, leading to soil depletion and muddy watercourses.

Trees also provide local climate control directly, beeches for example provide a humid environment, with slower air movement which reduces transpiration and protects the moisture in the forest floor. Mature woodland can reduce extreme summer temperatures.  Evaporation from the leaves cools the air, reducing transpiration and saving water for the trees to use later.  Canopies over streams cool the flow through shade. (See the Woodland Trust’s ‘Keeping Rivers Cool’ project which is important for salmonids in southern England where water temperatures are approaching lethal limits in warm summers.) Trees can also regulate their leaf cover - providing dense shading in summer and after autumn, the bare branches allow better sun penetration and reduce freezing especially under deciduous trees. Which is why parking under trees in winter keeps the windscreen frost free. Conifers are less effective at stabilising temperatures but rarely grow near rivers naturally.

Trees of course then provide dams and obstructions as they fall and die, creating ponds, flood plains and especially support the morphology of our lowland streams. Plantation trees tend to be grown fast and harvested young so that they are taller and weaker whereas in mature forests, trees maybe hundreds of years old and the young trees beneath are dwarfed until one of the elders finally dies. This slow growth makes them resilient to storms and grazing and provides large root structures. So natural woodlands will persist longer and plantation trees may give us debris dams ore quickly.

The book is worth reading for the fascinating information and questions around trees - why alders and willows survive flooding, the oldest and biggest trees etc. However my key take is just how critical healthy, natural forest is in a catchment, moderating water flow, providing channel stability, deadwood and cooling, as well as providing allochthonous food to aquatic fauna.

Fiona Bowles, RRC Board

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