19 March 2021

Growing Quality Timber in Agroforestry Systems

Kevin O'Connell, Teagasc Forestry Development Officer, explains that there are many definitions of agroforestry. In Ireland we generally practice a form of agroforestry call silvopasturalism – growing trees with grazing systems. Or simply: Trees are grown at a wide spacing with grazing in-between

Silvopasturalism

There are many definitions of agroforestry. In Ireland we generally practice a form of agroforestry call silvopasturalism – growing trees with grazing systems. This, in its simplest terms, is 3 dimensional farming: trees are grown at wide spacing to produce quality hardwood timber over a relatively short rotation and grazing is allowed to continue between the individually protected trees.

The benefits

Silvopasturalism in one form or another has been practiced for millennia, but has only made a revival in Ireland in the last number of years. This can be attributed not only to the generous grants and premia made available by the Department of Agriculture, Food and Marine (DAFM) Forestry Division; but also by farmers becoming aware of the benefits it can bring. Benefits like quality timber production, fruit/nut production, continued grazing, animal health and welfare, silage/hay production, biodiversity, pollutant abatement, improved water quality, flooding and erosion mitigation, carbon sequestration to name but a few.

Growing quality broadleaf trees for timber production under this system, will mean taking different factors into consideration and making necessary decisions. Normally trees do not grow as individuals in the open. This is a man-made landscape, a hedgerow or parkland. Trees evolved and adapted to grow together in communities in a woodland setting, where different trees and plants interact above and below the ground. Under silvopasture, trees have to be “trained” to grow in a different environment.

Under silvopasture, trees have to be “trained” to grow in a different environment.

Site factors

The path to growing quality broadleaves takes place before a tree is even planted. The owner wishes must be taken into consideration. What products do they want and what tree species will provide them? The site factors, mainly soil type is very important. In general the better the soil, the greater the choice of trees that can be selected. Good free draining agricultural soils should be preferred as they will encourage crown development and rapid girth growth. Heavy soils will reduce species selection and they can lead to trees blowing over as they grow due to poor root spread and penetration. The regional, local and indeed micro – climate can have a major influence on the planting site. Climatic factors, including temperature, frost, precipitation and wind and how the desired tree interacts with them should be considered. Other site factors such as topography, elevation, exposure and aspect can also play a major role in the potential growth of the trees.

Having examined the site factors, the silvicultural characteristics and requirements of potential candidate trees species should be investigated. Silviculture is to forestry as agriculture is to farming. It takes into consideration the tree’s growth habits, how they naturally grow straight (apical dominance), its branch structure and formation, is it light demanding or shade tolerant. Light demanding trees will adapt better to open grown conditions than shade tolerant trees. The potential density of the trees crown is important, it should not be too heavy so as to kill off grass growth underneath. The silvicultural requirements are basically what the trees need to grow to fulfil their potential; namely a good free draining soil, with sufficient nutrients under ideal site factors of adequate rain, protection from frost and exposure and on a southerly aspect to take full advantage of sunlight.

Planting & Care

Unlike in creation of a woodland/forest, where in general 3300 trees are planted to the hectare (ha). In a silvopasture system the stocking can vary from 400 tree/ha to1000 trees/ha. In both systems, when the trees are mature and producing sawlog there will probably be about 150 trees left per ha. Because the number of trees are much less in a silvopasture system they must be handled and managed more carefully from the very beginning. The following should be considered:

Select the best quality transplants with a good radially root distribution, a straight stem with a suitable root/shoot ratio and between 90cm and 120cm in height.
In general trees will be delivered as bare root stock and it is very important that the roots of the young trees are not exposed to drying winds and are covered at all time. Plant from the bag!
Mark out with some builder’s lime where the trees should be planted and although time consuming and more costly, the trees should be pit planted. Pit planting is digging a hole, placing the tree in it and covering the roots with the soil. The sod should be inverted at the surface. Trees should be planted to the same depth as they were growing in the nursery. A mark called the root collar can usually be seen, and indicates soil level.
The trees must be planted in an upright position and care must be taken to avoid them being forced out of the vertical. This is especially true when protecting them with the shelter.
The tree shelter should be held in place by two fencing stakes. Ideally one strong split stake, driven in immediately after planting with a stake driver and another lighter stake driven in by hand when putting on the shelter. The shelter will need to be secured with wire to the stakes as the ties provided are not adequate to keep it in place. A flat sided stake prevents the shelter from turning when sheep brush against them.
There is no need to use herbicides to control grass growth around the young trees, the sheep will keep the grass down. Grass may grow within the shelter so some hand weeding may be necessary from time to time.
Formative shaping generally refers to the removal of branches and forks when the trees are young, as the trees get older and taller the removal of branches is referred to as pruning.
Formative shaping should start a year or so after the tops of the trees appear above the shelters. The objective of shaping is to improve the quality of the timber by preventing the formation of knots.
The branch collar is the swelling where the branch grows out of the stem and it contains tissue that will heal the wound after the branch has been cut. When removing a fork or branch it is important not to damage the branch collar and all cuts should be made just immediately outside it. Care should be taken not to leave pegs.
When the trees begin to outgrow and split the shelters, new plastic mesh can be put around each tree. Branches that were inaccessible because of the shelters can be removed at this stage.
The crown of the tree is its power house that controls the production of timber. Over pruning or under pruning will affect the amount of timber laid down. Trees like Wild Cherry, Sycamore, Red Oak, and Birch that show apical dominance should be pruned once every 2 years by removing the bottom whorl of branches. Trees like Oak, Walnut and Spanish chestnut can be pruned every 3 years depending on their growth rates with the emphasis on encouraging the trees to grow straight and removing the bottom whorl. Never prune in spring or autumn when the sap is rising or falling as it can result in excessive “bleeding”. Oak should be pruned in winter and the other species mainly in summer. Do not remove more than 40% of the crown.
The aim should be to produce at least 6 metres of knot free timber. Once this 6 metres has been reached, the crown should be let grow to achieve the target diameter.

Remember not to neglect the grassland management. Removal of trees in thinning operations at the correct time will encourage the growth of grass.

Find out more from the Teagasc Forestry Development Department