Tree Planting and Succession: Guest Blog by John Biglin

Following on from last week’s blog on meadows, John Biglin of Anglian Parks and Wildlife has sent me this piece about succession and tree planting.

Sometimes not planting trees and even cutting them down can be good for wildlife

You might ask, how can this be?  Surely all trees and tree planting is great for wildlife.  Well nothing in nature conservation and habitat management is ever that simple.  There are always conflicting land uses and a hierarchy of valuable habitats.

To start looking at this issue we need to understand a bit about what ecologists call succession.  This is the inexorable march of most habitats towards woodland in the UK.  We can illustrate this with an example from two different habitats.

A freshly dug pond is a good start.  Not so good for wildlife yet but it will be.  In time, plants and animals will colonise the pond.  As they die and sink to the bottom they start to build up mud.  As more plants grow, especially at the edges, they drop more leaves and other plant material into the pond and so the mud gets deeper and the water gets shallower.  Add in a few decades of growth and the mud is so deep there is little of the open water left.  At this stage, trees like willow and alder will start to colonise.  These produce large amounts of bulky organic matter, which accelerates this silting up process.  Eventually the pond becomes a muddy patch, probably dominated by willow scrub.  Add in a few more decades and the ground is dry enough for birch and other trees to colonise.  Closely followed by oak and other large trees.  This quickly builds up the leaf litter on the ground and soon you have woodland.

Using another example.  Say we have a piece of land, which has recently been cultivated.  A patch of agricultural set aside would be a good example of this.  The land will have been ploughed, harrowed, treated with pesticides and fertilisers and fairly poor for wildlife.  However, those same successional pressures operate here.  In time, the land will be colonised by weeds.  These ruderal species (technical term for fast growing and seeding weeds) thrive on disturbed land.  There could be a good growth of nettles and thistles too, which love disturbed land.  Again, add in some time and grassland species will start to invade, creating a meadow.  This will be closely followed by more competitive grasses and small scrub trees, such as hawthorn and blackthorn and yet again what was once a wildlife sterile piece of land becomes oak woodland.

Oak woodland is what ecologists call climax vegetation.  This is the habitat that almost all pieces of land will become, if left untouched, in the UK.  Habitats are all part of a continuum from open land or water through to climax oak woodland.  Succession is a natural process and is part of our natural environment.

Back to cutting down trees.  If you look really closely at woodland and see beyond the trees, you will notice that many very dense woodlands are dark and dingy places with no ground flora and little variety.  This is due to the heavy shade that the trees create.  Most plants need light to allow them to grow.  On the edges of the woodland and where there are rides, you will find grassland and this will have a wide range of plant species, and the insects that live on them.  Insects just love warm sunspots to fly around in, for food and breeding.  Birds do too.  Without these areas, the woodland will not be as diverse and as good for wildlife.  In some ways, the best woodland for wildlife is made of lots of rides and clearings, with few trees.  In the past forests were much more open, with fields and clearings.  Not what we mainly see today with trees and woods squeezed into the patches between fields.

This is not to say that there is not life in dense woodland but it may be thriving unseen in the tree canopy.  One of our most stunning butterflies, the purple emperor, lives mainly in the tree canopy feeding on aphid honeydew.  Of course, dead wood is brilliant for a whole range of specialist dead-wood feeding beetles so a bit of a plea here not to be too tidy.

Cutting down trees and reducing the shade they cast is essential in conserving some types of woodland for wildlife.  This is the principle of coppicing.  A vitally important habitat management technique for coppice woodland.  Coppicing involves cutting down the trees to just above ground level.  This allows light to reach the woodland floor with an amazing bloom of woodland flowers in the following spring and in the few years after coppicing.  The stumps, or stools as they are known, of the cut trees do not die but grow up again.  Some tree seeds germinate too.  As the stools cast their dense shade again the woodland flowers decline, until the next time the woodland is coppiced.

When you consider the losses of heathland and old meadows in the UK in the twentieth century the figures are alarming (more than 90% lost since the end of the second world war).  We have a small proportion of what was once here.  Many of our lowland heaths have gone and chalk and other grasslands have disappeared under scrub and woodland.  A change in agricultural practice is probably the main reason for the loss of these internationally important habitats.

People and organisations managing land for wildlife have to take all these factors into account.  For a long time now, considerable effort has gone into cutting down trees to restore heathland and grassland.  This has involved millions of hours of volunteer effort working for a wide variety of wildlife groups.  After many years of this concerted effort grassland and heathland has returned to many places.  Once again, heaths can support good populations of some of our rarest birds and other animals, not forgetting the plants.  Who could not be amazed by the sound of a nightjar flying at dusk, the beady golden eye of an adder sunning itself on a patch of warm grass, or the secret light of the glow-worm at night?  They all need heathland to survive.

So, it can be good for wildlife to cut down trees to create rides and to restore very rare or threatened and fragile habitats such as heathland or grassland.  Where I live if it were not for the cutting down of trees on heathlands we would have lost the Dartford warbler and many other species so dependent on this habitat.

Moving on to tree planting.  If we plant trees on a heathland or a meadow, we are moving succession forward in an unnatural way.  The trees will eventually shade out the grassland and heathland plants and this rare and threatened habitat will again be lost.

In the drive to create woodland in the UK following two world wars, many hectares of conifers were planted on marginal land.  This is precisely the land that is often good for wildlife.  Little patches of remnant heathland often survive in the woodland tracks used to remove the cut trees after felling.  This wholesale planting of conifers has had a similar result as lack of management.  The heathland disappears or just hangs on by a thread.  Opportunities do exist when felling plantations to convert at least part of the areas back to wildlife rich habitat.

So all I would say is please remember that when you are planting trees you may be destroying a much rarer habitat or at least replacing it with a more common and less threatened habitat.  Look at the land you are proposing to plant trees on and assess whether it might be better for wildlife to create a species rich meadow or even just leave natural succession to take place and create woodland (always best adjacent to an existing woodland).  Even if you do decide to plant trees use native species and don’t just plant them in dense blocks.  Provide the rides, which are good for wildlife and of course access for people.  Think about the edges and make little clearings for birds to catch insects in.  Be creative in woodland planting.  Diversity is the key.  Get advice if you need it.  There are many organisations out there that will help and for free.

Think before you plant trees and consider the wildlife value of the tree planting, and woodland you are creating, against what the potential or actual value the habitat may already provide for wildlife.  A landscape improvement by tree planting may be good for visual amenity but bad for wildlife.

Did I mention archaeology…….but that is another story!

Time for a stroll in a woodland glade I think………