Management of Silvopastural systems
Pastures:
General:
Silvopastoral agroforestry pastures can be managed in exactly the same way as conventional pastures. Pastures may be fertilised as for conventional pastures, though the current trend for replacement of nitrogen fertilisers with grass/clover pastures might offer some benefits if it reduces the use of machinery and the potential for compaction of the soil which can slow the growth of young trees.
Reseeding:
If pastures are to be renovated or upgraded and soil cultivation is involved, it is better to carry out the work before the trees are planted. Surface seeding may be carried out at a later stage but the trees should be well established. Tree roots occupy the top 10cm or so of the soil for up to five years so the use of machinery should be minimised and any surface cultivation avoided for at least five years.
Medium term:
Experience in a number of trials with sheep across the UK has shown that with sycamore trees planted at 400 per hectare there is no reduction in annual livestock production after 12 years of tree growth.
With very fast growing larch trees on an upland site and ash on a lowland site, both planted at 400 per hectare, reduction of around 10% in annual livestock production due to shading was found after 10 to 11 years of tree growth. Pruning of the larch trees then maintained annual livestock production at around 90% of the original level on the upland site. Despite pruning, the ash trees on the lowland site developed crowns large enough to shade the pasture and reduce annual livestock production further; in this case, if livestock production is to be maintained, the trees will have to be thinned.
Species change:
There was no evidence of any change in the species composition of the pastures under trees of a number of species up to 14 years old, except in the area immediately around each tree where bare ground had been created through the use of herbicide for the first three years. In these herbicide-treated areas, species which had not been in the original seeds mixture tended to appear; the older the pasture, the greater the variety of new species. However these changes in species composition would have very little impact on potential pasture production.
Longer term:
Pruning and thinning of the trees can be used to maintain the level of pasture production. Pruning is also necessary to maintain the quality of the timber to be produced.
Pastures under the canopy of 35-year-old poplar trees, thinned to 156 trees per hectare have shown changes in pasture composition. While perennial ryegrass, white clover, rough-stalked meadow grass and creeping thistle were prevalent in nearby conventional pastures, the pastures under the 35-year-old trees had more shade tolerant species such as creeping bent, Yorkshire fog and annual meadow grass. Feeding values to grazing animals may be reduced with these changes and management, including rates of fertiliser application, may have to be adjusted.
Changes in the seasonal pattern of grass production have been found in pastures growing in more mature silvopastoral systems. Increased production, compared with conventional pastures, has been found in early spring, before the trees flush and in autumn after leaf fall. These increases occur when there is adequate light and temperatures at the extremes of the growing season are higher in the silvopastoral system because of the sheltering effect of the trees. In mid-season with mature trees, pasture production may be lower than in a conventional pasture because of shading. Overall, annual production may be little affected in thinned and pruned silvopastoral systems.
Pasture benefits?
In the east of the country where dry summers can reduce pasture production, the presence of trees in silvopastoral agroforestry systems can increase pasture production compared with a conventional pasture, through reducing soil moisture deficits. This happens because, provided the tree canopy is not large enough to cause significant shading of the pasture, the trees reduce incoming sunlight energy, reduce windspeeds and reduce maximum temperatures, the combination of which reduces evaporative loss of water from the surface of the pasture (evapotranspiration).
Livestock:
General:
Sheep are the best livestock species for silvopastoral agroforestry. The trees require less expensive protection with sheep than with cattle, goats are naturally browsers and often prefer trees and shrubs to pasture species.
There is no need to change the management of sheep in a silvopastoral agroforestry system provided that the tree canopy has not reached a size that will reduce pasture growth. If the tree canopy has been allowed to grow large enough to reduce pasture production, stocking rates of sheep will have to be adjusted accordingly.
Livestock welfare benefits
The trees can actually provide some benefits to the livestock. Shade from the sun is available under the trees in hot summers. Shelter from wind is available at other times of the year. A longer growing season for pastures has been shown under silvopastoral agroforestry, the pastures start to grow a couple of weeks earlier in spring and continue to grow for a couple of weeks later in the autumn compared with conventional pastures. The early "bite" from pastures coupled with the direct benefits of shelter from the trees to the lactating ewes and lambs could lead to a greater milk supply and better lamb growth. The late "bite" from pastures and the direct benefits of shelter from the trees in autumn could lead to ewes putting on better condition prior to mating.
A note of caution:
While over-wintering sheep in a silvopastoral system may appear very attractive because of the shelter benefits, care must be taken as with any pasture to avoid excessive winter poaching and loss of pasture production in the following year.
Trees:
A number of aspects of tree management are dealt with here all of which are important to the success of the trees in the silvopastoral agroforestry system.
The first set of links is about getting going, choosing the right tree species and getting the tree seedlings.
The establishment is the phase of planting the trees and getting them growing out of the tree shelters or net guards, this phase can last from one to five years depending on the growth rate of the trees. The next phase, the post-establishment phase, involves more work with the trees to ensure a good timber crop at the end of the day and acceptable levels of livestock production.
Timber trees:
The selection of the timber tree species to plant is critical to the success of a silvopastoral system. They must, in the first place, be able to grow on your site and they must meet your requirements in terms of, for example, timber value and benefits to the environment. Several tree species are suitable for use in silvopastoral agroforestry systems, though not all of them have been tested as agroforestry trees.
Timber tree species:
Potential timber value is important for the financial return when the trees are finally felled. Different tree species produce timber with different qualities resulting in them being suited for different uses. For example, timber which can be used for furniture will have a higher value than timber which can be used as a building material. Tree management, especially pruning, will greatly influence the final quality.
Apical dominance is a desirable characteristic. Trees with strong apical dominance tend to grow with a single stem and a minimum of forking. Forking requires additional pruning to maintain good form.
Light branching is a desirable characteristic. Trees with light branches will have small knots in the timber they produce which generally means better quality timber. Light branches are much easier to prune. Trees with heavy branches will be more difficult to prune and may require to be pruned sooner.
Native trees can often be slower growing but they are considered to contribute more to the environment than the faster growing, introduced species.
Non-timber tree species:
Tree species not normally planted for timber may be used for environmental or landscape effects. These trees would usually be native species such as birch or rowan.
Site suitability:
It is very important to plant the correct species on your site. If a site is unsuitable for a particular tree species it will not grow well in terms of either growth rate or growth form or both, indeed in some cases the tree may not even survive. Tree species must be carefully selected on the basis of the climate and soil quality of the individual site. Advice can be sought from professional forestry advisors, for example through the Institute of Chartered Foresters.
Planting stock quality
The choice of planting stock is important, planting a relatively small number of trees at wide spacing means that the highest possible percentage of trees must establish and grow well, so planting stock quality must be good. It is also important to get planting stock without too much variability so that all of the trees establish and grow at similar rates.
Planting for broadleaves (other than poplar) and conifers will be rooted, usually one- or two-year old transplants. For silvopastoral agroforestry systems in which tree shelters or tree guards are being used, the plants will probably be bare rooted and can be in the height range 20 cm to 30 cm. Container (pot or cell) grown plants are often taller and invariably more expensive than bare-rooted plants but the extra expense might be justified when small numbers of trees are being planted. If fenced enclosures are to be used the bare-rooted plants could be taller, for example, in the range 30 cm to 90 cm.
Poplars, unlike the other broadleaves, can produce roots from young dormant shoots when they are inserted into the ground. Most of the commercially available poplars can be propagated in this way from one-year-old wood. Most nurseries will provide unrooted planting material in preference to rooted plants. The unrooted sets or cuttings which are supplied by the nursery are planted by sticking them into the soil having previously cultivated the planting area.
Planting density and pattern (landscaping):
The recommended planting density is 400 trees per hectare for all species.
Planting at 400 trees per hectare strikes a reasonable balance between the developing canopy size of the trees and the impact of the canopy on pasture production. You can expect up to ten years before pasture production will be reduced at which stage pruning can open up the canopy and prevent further loss of pasture production. On the other hand, with 400 trees per hectare, there are enough trees to spread the effects of the animals, which will use them for shade and shelter, so that no individual tree suffers from severe soil compaction which can reduce tree vigour.
There are many ways in which the planting pattern can be varied to create an interesting landscape.
· Mixed tree species can be planted in the same field, creating a mixture of texture and colour.
· Trees can be planted in the same field in different years, creating a varied age and size structure.
· Trees can be planted in a rigid square pattern with every tree 5 metres from its neighbours. This arrangement makes the use of farm machinery simple but often doesn't "fit" into the landscape.
· Contour planting, in which the trees follow the natural curves of the land, can be a more pleasing pattern but may make driving the tractor and fertiliser spreader more challenging.
· Trees can be planted more densely close to existing woodland, connecting with the woodland and fanning out to lower densities across the field creating the effect of the woodland "spilling" into the grazed field.
· The lengths of the rows of trees, whether straight or curved can be varied so that the planted area has curved rather than straight edges and planting right up to fencelines (especially straight fencelines) should be avoided.
· Planting close to existing mature hedgerow trees should be avoided so that open space is left around them. In any case, the growth of new transplants could be reduced beside mature trees.
These choices offer a great deal of flexibility and the opportunity to create interesting landscapes which follow the land form and which are tied into landscape features.
Within the general pattern, regular, contour or variable density, individual locations which are frequented by the animal should be avoided. Don't plant trees around watering or feeding points or too near gates or on sheep lairs. These areas will suffer from soil compaction, the trees are more likely to receive the attention of the animals and as a consequence, the trees will not perform well on them
