Managing Woodland

Just as a woodland has its seasons, so also does the management of that woodland.In spring and early summer, bluebells and other woodland flowers are very susceptible to trampling. Birds are nesting on the ground and in trees; they shouldn’t be disturbed. Also, the woodland soils are still wet from the winter rains – any machinery is going to leave ruts in the rides which will bake hard later in the year. More importantly, the woodland soil and its ecosystem will be damaged by heavy machinery.

The woodland soil is a stable ecosystem. Unlike the woodland above, it is not subject to movement. This allows a delicate and complex system of root hairs and mycorrhizal fungi to develop. Any disturbance of this fragile ecosystem will destroy root hairs and fungal threads. This will weaken the trees, slowing their growth and leaving them open to infections. The damage to the soil ecosystem caused by machinery at the wrong time of year can last long after the ruts on the surface have disappeared.

Enjoy the woodland at this time of year by walking the rides and footpaths but leave management work until August.

By August the woodland flowers will have died back, young birds will be skulking in the undergrowth and the woodland canopy will have sucked the moisture out of the ground to leave a hard surface for light machinery to run on. From now until November is the ideal time to get on with managing your woodland in fine weather before it settles into its winter dormancy.

In a monoculture woodland planted for timber production, choosing which trees to thin is quite straightforward. A set of tables will tell you how many cubic metres of timber need to be thinned. The first thinning is generally a line thinning; every third row will be felled and extracted to allow access into the woodland for subsequent selective thinning using bigger machinery. With selective thinning, trees of poor form or growth are removed to allow the straightest and most vigorous trees to expand their crowns.

In a mixed broadleaved woodland with multiple objectives, choosing which trees to thin becomes more complicated. Mark the trees to be thinned with spray paint several months before thinning. This allows time to look again with fresh eyes and reconsider. Divide the woodland into say 5 sections and thin one section each year. After 5 years the first section will probably need thinning again. These are some of the factors that should be considered.

- Promoting good quality timber 50-100 years in the future. This may not seem relevant to some people but by promoting good quality timber trees, especially oak, at this stage you will grow a magnificent woodland and ensure a valuable legacy for future generations. What makes a good quality timber tree? At this stage, form is more important than girth. Broadleaved trees naturally grow branches, but you should be able follow the main stem as it weaves its way high up into the canopy. Trees with heavy branches and no defined leader will make good parkland trees but are not suited to life in a woodland – they should be thinned out.

- Keeping a wide range of species. Inevitably, some of the species of tree planted originally are not going to be suited to the conditions of that site, or the reasons for planting them may have changed. Many of these will die out naturally through competition, others may be removed at the thinning stage.

Other trees may be suited to the site but less competitive than their neighbours. These trees may need a little help by thinning out their immediate neighbours. This is called “halo thinning”.

A wide mix of trees will produce a wide mix of food sources throughout the year, increasing biodiversity.

- Encouraging the understorey. A heavier than usual canopy thinning regime in some parts of the woodland will encourage the development of the understorey. Where there is already a good understorey of hazel, the canopy trees could be thinned even more drastically. The increased light levels will allow the development of a “coppice with standards” regime.  A mosaic of different habitats will maximise biodiversity within the woodland.

In a commercial plantation graphs and tables will tell the forester exactly how many cubic metres of wood are to be removed by thinning to maximise timber production. For a multipurpose broadleaved woodland there are no such rules. A wide range of thinning regimes will produce good results.

If oak trees are lightly thinned, they will grow a tall but narrow stem. If heavily thinned, they will not grow so tall but will have bigger girth. For timber production the thinning regime is less important than buying trees grown from seed of good provenance to produce trees of good form.

For wildlife a heavier thinning that allows more light into the woodland (even if temporarily) is always better. The effects of a good thinning on a young woodland are quite dramatic. The extra light gives a boost to the understory. The canopy trees put on extra girth because not only do they have more light, but there is also less competition below ground for water and nutrients. Trees are healthier and better able to fight off pests and diseases.

Thinning conifer plantations can produce a modest income if the timber is sold for wood pulp or another low value commodity, but this requires the thinned tree to be felled and extracted from the woodland then transported to a factory. Increasing labour costs have led to chainsaws being replaced by harvesting machines which can quickly remove straight rows of conifers, but these machines have no place in a young mixed broadleaved woodland. After centuries of practise, it is a big step to realise that you don’t have to fell and extract trees when thinning. You simply have to change the status of the selected trees from living to dead. Neolithic man discovered that the best way to do this was to hack a ring around the tree with a stone axe. Now the same operation can be done with a small battery powered chainsaw and an application of Glyphosate herbicide to the cut.For the woodland manager, thinning by girdling is cheap, effective and safe. It is an operation that can be done by anyone with the appropriate safety wear. It allows even the smallest, wettest and least accessible young woodlands to be managed effectively.

Thinning young broadleaved woodlands can produce large quantities of firewood. But, instead of felling and extracting all the trees for firewood, it would be better to select just those trees that make the best firewood and leave the rest of the timber on the woodland floor.Living trees build up a store of carbon and nutrients in their wood. The tree bark, and the chemicals that it produces, protects the tree from insect and fungal attack. As soon as the tree dies its defences are down, allowing insects and fungi to start recycling the stored carbon and nutrients into the woodland ecosystem. If a large proportion of the thinned timber is removed as firewood, or another commodity, the woodland will become impoverished over time.

Ash makes the best firewood because it is easy to split and dry, and it burns well. The trunk grows long and straight with a thin bark, making it easy to saw into firewood lengths. Three years or so after girdling the trees will be dead. Now is the time to fell those trees selected for firewood. The stem will still be quite sound so the tree can be felled conventionally with a chainsaw. Beware of dry branches in the crown “exploding” as they hit the floor. The tree is less likely to be caught up on other trees because it will already have lost its twigs and some smaller branches.The wood has been drying in the open air for 3 years. Logs up to 10cm diameter can be taken directly to the wood burner, even in Winter. Larger material could be split and stored under cover to dry further.  

The woodland edge should have been planted with a totally different species mix from the woodland interior. Its management should also be totally different. The dense cover in years 10-15 will slowly grow and open up at the base. This is particularly true for hazel and dogwood. Sections of the woodland edge should be coppiced every 5-10 years, and the coppiced branches laid over the coppiced stumps to prevent deer from nibbling the new shoots.

Shoots from the coppiced stumps will regrow up to 2 metres in the first summer after coppicing.

Occasional trees could be left to grow to maturity. This will open up the woodland edge and leave tantalising glimpses into the woodland interior, but don’t forget that one of the functions of the woodland edge is to protect the interior from draughty winds.

Another function of the woodland edge is to provide shelter and food for wildlife. This is done by coppicing short sections each year and leaving brambles. Bramble patches in full sunshine will produce plenty of flowers, blackberries and nesting cover.

Generally, rides and glades will have been designed into the planting plan. For the first few years they could be mown every month during the summer to define the edges of the planted wood and allow access on foot.

As the wood grows, the glades will become mini meadows and should be managed accordingly. Only the centre of the rides needs to be mown. This allows the development of a woodland edge flora.At the thicket stage (5 to 10 years after planting) the centre of the wood becomes impenetrable. Now is the time to establish a system of footpaths which allow access within 10 metres to all parts of the wood.

If the wood was planted in a spiral pattern, it is easy to clear paths between the rows of trees, leaving about 20 metres between the cleared rows. In denser planted woodlands or woodlands planted in straight lines, it will be necessary to fell young trees to establish winding footpaths. The footpaths can be maintained by mowing once or twice each summer.

15-20 years after planting any rides up to 6 metres wide will become overshaded and parts of the ride will become bare ground. You may want to widen the rides to keep a green ground cover. Similarly, the woodland will become more open as trees are stem pruned and thinned. It is no longer necessary to mow the footpaths. A new system of footpaths may be formed naturally.

This is a short chapter because pests and diseases need not be a problem in a new woodland that has been planted with an intimate mixture of native broadleaved trees and shrubs.Voles, rabbits, hares and, to a large extent, deer are only a problem in newly planted woodlands. Once a woodland has become established (stem diameters greater than 15cm) guards can be removed, and fences taken down. Animals can come and go as they please; they become part of the woodland ecosystem.Grey squirrels are a different matter. From April to July, they can strip the bark from young trees to get at the sweet sap beneath. The timber value of young oak trees can be lost forever in a few weeks.Fortunately, grey squirrels prefer to strip the bark from maples and hornbeam, only choosing oak trees if nothing better is available. If your wood has been planted with a good proportion of field maple and hornbeam to act as sacrificial trees then, at the very least, you should get adequate warning of high squirrel population and be able to do something about it. If your young wood has large patches of pure oak, then you have a big problem.

Other pests and diseases are found on specific trees only. Native trees have evolved over millennia to cope with them. A pest or disease may kill a few trees of a certain species. Very rarely a pest or disease will kill the entire population of a certain species. But never will a pest or disease destroy a mixed broadleaved woodland. There will always be another tree ready to grow into the space left by a dead neighbour.

For the forester growing a specific crop of trees, pests and diseases are a continuous threat. For the forester growing a woodland ecosystem they are insects, fungi and other organisms that are part of food webs and biodiversity.

A new woodland planted on ex-arable land will have high levels of nutrients in the soil. As the growing trees shade the woodland floor, plants such as nettles and cleavers will dominate. A few years later roosting birds will excrete elder seeds. Elder bushes also thrive on nutrient rich land. The rather depressing site of woodlands with an elder understorey and nettle ground flora is typical of many 20 year old planting schemes.

The solution is not to spray off the nettles with herbicide and plant native wildflower plugs. The solution is to take an ecological approach and alter the environmental conditions of the woodland.Coppicing, pollarding and thinning trees as described in earlier sections will all encourage a healthy canopy and understory. After a few years even nettles will struggle to survive the dense shade at ground level. It is surprisingly easy to hand weed the few remaining nettles in June and July. Wearing long trousers, sleeves and leather gloves, nettles can be easily pulled, bringing a section of root creeping along the surface with them. Pile the nettles into a wheelbarrow and dump them outside the wood.

Nutrient levels in the soil can be reduced by increasing the carbon levels. In the same way that nets of barley straw can reduce algal blooms in ponds, so can dead wood (which is mostly carbon) reduce excess nutrient levels in the woodland soil. (The dead wood encourages microbial growth which requires the uptake of nutrients).

The dead wood produced by coppicing, pollarding and thinning should be left in the wood as stacks of logs or heaps of branchwood. More dead wood could be brought into the wood from outside to soak up even more nutrients, so long as it has not been treated with preservatives.

The best source of high carbon material is large diameter logs. As the logs slowly decompose, they will host a rich and changing assemblage of insects and fungi that will soak up the excess nutrients in the soil. After a few years ground conditions will be ideal for tree seedlings and for introducing a woodland flora.

Patches of dense shade and reduced nutrient levels are the ideal sites for introducing a woodland flora. Bluebells and other bulbs that grow and flower in the Spring before the trees come into leaf are best suited to these sites.

Never dig up plants to put in a woodland. It is illegal, doomed to failure and looks ridiculous. Seed can be collected from wild bluebells in mid-July. Rake over a patch of bare shady ground to make a shallow tilth. Sow the seed in the Autumn and rake it in, then wait for rain to germinate the seed.

Many woodland flowers rarely produce seed so other methods are needed. Creeping shoots of yellow archangel can be half buried in the woodland soil. Roots will form at the leaf nodes. After a few years this rare indicator of ancient semi-natural woods will spread through your new woodland, even outcompeting nettles if the environmental conditions are right.

If you are lucky, you will get a natural looking swathe of young plants that can then be managed by altering levels of shade. You will never replicate the flora of an ancient woodland but, by working with nature, the woodland flora will reflect the environmental conditions of your wood. Be prepared to be surprised and delighted by other plants appearing in your wood without any help.

Sooner or later all trees die. It is therefore essential that trees can regenerate and so perpetuate the woodland ecosystem. How a wood regenerates depends on the interactions of many different factors that can only be understood by observing woodlands at first hand over many years. It is not something that can be learnt just from books or websites. This is only a general introduction.

Most trees regenerate from seed. When a young broadleaved woodland has developed a dense canopy, the ground vegetation will disappear, and tree seedlings will appear on the bare ground. Photo of seedling. The tree seed has enough food reserves to enable the seedling to establish in the dense shade. But even if the seedling avoids being eaten over the winter, it will not survive the second Summer because there is not enough light to allow photosynthesis.

If the woodland is managed by thinning the canopy trees over the Winter, then there may be enough light for the seedlings to survive a second summer. But the extra light following the thinning will allow a flush of competitive weeds which take advantage of the high nutrient levels in a young woodland and smother the tree seedlings.

It seems logical to open up the canopy to allow more light to reach the woodland floor and encourage regeneration. But opening up the canopy in a young woodland will only encourage a ground flora of nettles and brambles. This produces even more shade on the woodland floor than before, so any germinating tree seeds are unable to survive even the first Summer.

Despite all this, certain species of tree have the ability to survive for several years before being shaded out by older, more established trees. It is only if a catastrophic event destroys most of the tree cover that young trees can break through and regenerate the woodland.

In hot dry climates the catastrophic event is often wildfire. Wildfire kills all young trees and leaves a bare, nutrient-rich woodland floor. This allows tree seeds that can withstand extreme heat to germinate and grow fast in the abundant light. Herbivores will be attracted to this lush growth, so it will be the trees that can tolerate intense grazing and competition which will form the next generation of trees in the woodland.

In central and northern Europe, the catastrophic event is more usually violent gales such as occurred in southern England in October 1987. In the Summer following windblow the uprooted trees have no leaves allowing young trees to grow up through the dead branches. The young trees have plenty of light and the dead branches protect the young trees from herbivores. Competition between species, and individuals of the same species, will decide which trees form the next generation.

Clear felling can leave similar conditions to windblow, provided that low value trees, branches and roots are left on site. Clearance of all woody material for pulp or biomass will leave inhospitable conditions for natural regeneration and an impoverished woodland soil.

Some trees such as aspen, cherry and wild service send up suckers from the ground if the parent tree dies. The established root system allows the sucker to grow fast and push through competing ground vegetation.

A fallen lime tree will start to root wherever the stem touches the ground. It will then send up new shoots along the trunk. After several years there will be a straight line of young lime trees.

Oak struggles to regenerate in woodlands, except in the lightest soils. Although new seedlings will sprout readily in grassy areas ready to grow on if mowing and grazing stops. For woodlands in the regenerating phase with plenty of light, it would be a good idea to plant young oak trees of good provenance at about 5 metre spacing protected against deer with 1.2 metre tall guards. This “enrichment planting” will allow young oak trees to be part of the next generation of trees in the woodland.

It is fascinating to watch natural regeneration over several years. Don’t be afraid to get involved by helping some species and removing others. Like all aspects of woodland management, it should be a fifty-fifty partnership with nature.