The nature of planting sites varies considerably depending on whether it is a new planting site (afforestation) or a site being replanted following clearfell (restocking). Restock sites tend to be more variable due to stumps and branches left on the ground after harvesting, heavy weed growth, the presence of damaging weevils etc., factors that make the successful replanting of these areas more difficult than on new sites.
In addition to matching the tree species to the site conditions, foresters must also select the correct size and quality of planting stock for each site. Larger more robust plants are generally favoured on restock areas as they are able to withstand the more difficult conditions on these sites while smaller plants are used where growing conditions are more favourable.
Plants are transported from the nursery usually in special polyethylene bags or waxed cartons to keep them fresh. They are stored on site under shade to keep them cool until ready to be planted. Care is required in handling plants at all stages of transport and planting as poor survival can be expected from stressed or damaged plants.
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The use of mounding and bare-rooted transplants requires that plants are manually planted. This back breaking work is usually carried out using a narrow spade or mattock. Mechanised planting of containerised stock is possible using a special head on the mounding machine but, as containerised plants only account for a small proportion of the annual planting programme, it is not widely practiced.
There are three basic methods commonly used to manually plant forest trees:
- Slit planting – most common method where the slit is made with the spade or mattock and the transplant is inserted into the ground and the slit closed with the heel of the foot and the plant firmed.
- Angle-notch or T-notch planting – similar to slit planting except that two slits are made, either in the shape of – a right angle or a T, and the soil is then levered up. The plants are inserted into the hole created and again the plant is firmed.
- Pit planting – a common method used on flat or uncultivated ground where a small pit is dug and the plant inserted into the hole and the loose soil back filled and firmed.
Research has shown that poor planting can affect the survival, growth and wind stability of trees. Poorly planted trees, particularly lodgeple pine and Douglas fir, can suffer from windthrow if their root systems are badly distorted during planting. Fast growing broadleaves such as ash can also suffer a similar fate. Careful planting is required to ensure that the roots are placed in the planting hole in a way that will allow the tree to develop a radial root system which provides all round support to the stem. Good wind stability is important in Ireland as high winds are a common feature of our climate. Therefore, attention to planting quality is an important aspect of forest establishment as it can have both serious short and long term effects on the success of a plantation.
Bare-rooted stock is planted when the transplants are dormant i.e. when they are best able to withstand the stresses of the lifting, transportation and transplanting into their new location. Bare-rooted material is normally planted between October and April, although this planting season can be extended using cold stored planting stock.
Containerised plants, because they come complete with soil attached to the roots, can be planted at any time of the year, but best results tend to be during spring to mid summer.
Correct timing of planting can have a critical impact on successful establishment. Planting during very cold (particularly frosty periods) or dry conditions is generally avoided.
The distance that trees grow apart can have a significant impact on their growth and development. Widely spaced trees tend to produce large heavy branches, which in the case of conifers such as Sitka spruce or Douglas fir, results in weak and very knotty timber. Closely spaced trees on the other hand tend to have very fine branches, but with little room to develop, tend to have tall thin stems with little individual tree volume. Similarly with broadleaved species such as ash or oak, open grown trees produce lots of heavy branches (good for firewood) but little straight material for sawn timber.
Tree spacing is therefore important in a commercial forest situation where quality timber production is a priority. Spacing for conifer species is normally 2×2 metres or 2,500 plants per hectare. Broadleaves, however, are planted at closer spacing because of the tendency of some species to fork or produce crooked stems. Planting densities of 6,600 for oak and beech and 3,300 for other broadleaves in various layouts are used. The closer spacing creates more competition resulting in straighter and finer branched trees which, when they reach a height of approx 5 metres, are thinned to give more space to the remaining trees to develop.
In biodiversity or nature conservation areas wide spacing is beneficial as it allows more light onto the forest floor resulting in a greater diversity of ground flora and fauna. Also it provides good habitat for canopy living species which are important part of the forest biodiversity. Wide spacing is also used in some amenity and recreation areas where the full development of the tree crowns is an attractive feature of the landscape.
Planting is normally avoided in areas adjacent to water courses. Buffer zones of 5 metres are left unplanted on either side of streams and rivers to avoid tree shading. The reduction in light, particularly from evergreen conifers, can cause a decline in aquatic insect life which in turn can affect the fish populations. However, some group planting of native broadleaf species in the buffer zone is acceptable to create dappled shade.
Unplanted areas are also left adjacent to houses, (60m), archaeological monuments (15m) and public roads (20m). Relevant guidelines produced by the Forest Service on best practice are:
In established forests, seed fall from mature trees can create naturally regenerated seedlings in areas where there is sufficient light and space for them to grow and develop. Natural regeneration is sometimes used where it occurs in sufficient quantity to restock sites where the parent trees are of acceptable quality. However, the cool moist summers in Ireland are not favourable for seed production and there can often be many years between seed crops; also, heavy weed growth can suppress germinating seedlings. Consequently, this method of regeneration has proven to be unpredictable and unreliable and has therefore not been widely used in this country, particularly in commercial conifer plantations where uniformity of crops is sought.
Some species, particularly native species, produce seed on a regular basis such as ash, birch and alder and increasingly natural regeneration of these species is being encouraged within exiting forests.
In recent years the introduction of continuous cover silvicultural systems has created a greater interest in natural regeneration as this is one of the main methods of restocking stands using these systems. Small openings are made in the forest canopy, or stands are thinned, to provide good conditions for seed production and germination. The resulting seedlings are thinned out over time and eventually replace the parent trees in the stand. By this method of regeneration continuous forest cover is constantly maintained.
Prosilva Ireland is an organisation of foresters and forest owners that is dedicated to promoting close to nature forest management and actively encourages the use of natural regeneration in continuous cover systems.