Natural colonisation and regeneration

Natural colonisation is the process by which trees and woodlands establish on sites which were previously unwooded. Natural regeneration is the process by which existing woodlands regenerate. In practice, the distinction is blurred where there are scattered trees in wood pastures, commons, heaths and other areas.

Natural regeneration has several advantages in woodland creation:

• The species will be matched to the site, and local genetic stock will be conserved.
• A natural, uneven distribution will result, with structural diversity.
• There are no costs in purchasing or planting trees, but fencing or other protection may be needed.

Natural regeneration is supported by the Woodland Grant Scheme, both for restocking areas in existing woodlands, and for creating new woodlands by extending from an existing woodland. Where natural regeneration is practical and appropriate, grant aid for planting will not normally be granted. Between 1992-97 about 15% of approved new planting was through natural regeneration, the majority of it in Scotland.

Natural regeneration requires:

• A good source of seed nearby. Trees do not produce seed every year, and seed of most species does not remain viable for many years. Most species germinate in the year following production.
• Sparse ground vegetation. Seeds will not germinate easily in thick grass, bramble or other dense ground vegetation, and seedlings will be suppressed by such growth. For this reason, natural colonisation by trees is often associated with poor stony substrates, sandy soils and other sites where grass and other dense ground vegetation cannot survive.
• Protection from animals and people. Squirrels, mice and voles may eat seed before it has germinated. Rabbits, deer and stock may browse on the newly germinated trees. Heavy use by people will trample and destroy seedling trees.

Some tree species, which are called colonisers, produce seed more frequently and abundantly than other species. The seed is usually light in weight, so it can be dispersed easily by the wind, and the trees tolerate a wide range of soil conditions. Examples are beech, willow, pine and sycamore. Heavy seeds and fruits of other species may be taken by birds or animals, and dispersed some distance, but distribution will only be patchy.

Table 6g below indicates the seed characteristics of native trees (based on Harmer, 1999).

Table 6g: Seed characteristics of native trees

Natural colonisation as a way of creating a new woodland can be very successful in particular locations, such as urban fringe land, abandoned industrial sites and other areas where soils are poor and there are good seed sources nearby. Such woodland is less of a target for vandalism than are newly planted trees.

If a particular site for woodland creation already has evidence of natural regeneration, this can be allowed to develop. Fencing or individual protection of trees may be necessary, depending on the likelihood of damage from animals or people. In vandal prone areas, fencing or individual protection can be counter productive.

Where regeneration is very dense, treeshelters or spiral guards can be used as a way of thinning or ‘respacing’ the seedling trees. Seedling trees can be protected at the desired spacing, for example 2m apart, with the others left to be browsed or trampled. Seedlings of other species can be planted at the same time, to grow up together with the natural regeneration. This can be easier to manage than trying to introduce them at a later stage, when some thinning of the canopy may be necessary.

More usually, colonisation of new sites will tend to spread slowly from the edges, or in the vicinity of a particular tree, so the development of a new woodland will be gradual and patchy. This is not necessarily a problem, as the range of habitats produced will have high wildlife value.

Natural regeneration as a way of restocking existing woodlands is not an easy process. As well as the factors listed above, it requires sufficient space and sunlight in the canopy to permit regeneration and good early growth, but not so much sunlight that weed growth becomes prolific.

Ground preparation

Provided the conditions listed above are fulfilled, germination can be encouraged by preparing the ground in the vicinity of the seeding trees. The area should be cleared of grass and other ground vegetation, by hand weeding or use of herbicide. Ash and beech in particular require weed-free ground for germination, while oak can germinate through a grass sward or other growth. Raking the ground can help provide a good surface for germination.

Viable beech seed is only produced on average about every 10 years, in a year following a long, hot summer which stimulates the production of flower buds. Beech leaf litter is very slow to break down, and if the seeds fall onto deep litter they find it difficult to root through to the mineral layer below. In a beech ‘mast’ year, you can encourage germination by raking the seed-covered ground, to mix the litter and mineral layer and trample the seed into the soil, where it will also be protected from squirrels. On the continent of Europe this mixing and trampling is done by wild boar, and in the past in Britain was probably done by boar and domestic swine. This action can also be imitated by dragging logs across the ground, or by shallow rotavating, but care must be taken not to damage surface roots or compact the ground.

Tree seeds germinate best on soil which is well drained at the surface, with plenty of leaf mould or mulch to suppress other growth. Ground that has become compacted through trampling or machine use and is wet at the surface will not result in good germination.

The soil in newly-cleared areas of woodland often ‘re-wets’, due to the sudden reduction in transpiration when trees are removed, and to disturbance of the soil structure through felling operations. It’s best to get tree cover established again as soon as possible, as otherwise such areas get rapidly overgrown with rushes and tussocky grasses, making regeneration or planting difficult. it may be possible to dig temporary drainage ditches to improve surface conditions and allow regeneration to take place. Cultivation is unlikely to help in soils which are already wet. If natural regeneration is unsuccessful, planting should be carried out the next season, choosing planting positions on any drier mounds or ridges on the site.

Protection and early care

Techniques for protecting naturally regenerated seedlings are the same as for planted seedlings. However, the need is likely to be greater, because regeneration necessarily occurs near or within existing woodland, which will provide cover and shelter for the animals which damage young trees. Natural seedlings appear to be less palatable to deer than planted trees are, possibly because transplants retain higher nitrate content from nursery applications of nitrogen, which make them more inviting to eat.

Deer have become a serious problem in many woodlands, and will affect the success of regeneration, and also the regrowth of coppice, which may be cut as part of a woodland management programme. Electric fencing can be used around woodland regeneration plots and newly-cut coppice coupes, to deter deer. Dead hedges of woven coppice material or brashings can also be successful.

Dead hedges have the advantage of using locally available material, and tend to result in fewer negative comments from visitors than does the presence of electric fencing. It’s possible for a pair of volunteers to erect about 20m (22 yards) of 2m (6ft) high dead hedging in a day, so the technique is a viable one for small regeneration plots.

Erect stakes about 2m (6’) high, using the stoutest coppice stems available. push them into the ground, first using a crowbar to make a hole as necessary. Then roughly weave the cut material between the stakes, treading on it as you go to pack it down tightly. The result should be a dense barrier about 1m (3’) wide and 2m (6’) high, with no gaps visible through it. The dead hedges eventually rot down, benefiting the woodland ecosystem, and avoiding any removal costs.

Treeshelters are normally the best method for protecting seedlings, and provide a useful marker for weeding operations. Weeding must be continued for at least three years, until the leading shoots of the trees are well clear of competing weeds. As a rule of thumb, trees should be at least 1.8m (6’) high and growing well. Bramble can be a problem as it can clamber higher, but will not adversely affect strongly growing trees. Eventually the bramble will be suppressed, but sometimes it may be preferable to clear it, or cut the stems to prevent it spreading, at the stage before the canopy closes.

Sometimes natural regeneration is so successful that a dense sward of seedling trees results. These can be thinned by cutting swathes through with a scythe or brush cutter while the trees are still tiny. Seedlings can also be transplanted within the site. They tend to grow away well as they are of an appropriate strain, and have the right mycorrhizal association in place.

If thick stands of regeneration are left to grow up to sapling stage they must be thinned.