Establishing trees in dense bracken
Bracken is a very aggressive plant that can quickly dominate large areas of land if not controlled. The dense canopy makes it very difficult for existing plants to compete and also inhibits any seed germination. Dense bracken growth has two major negative impacts on tree establishment. One is shading which will reduce the sapling growth rates, and the second is that at the end of the growing season the collapsing fronds tend to flatten and bury the young trees. However, once the trees are established, they will outcompete the bracken which is then unable to survive. The main method currently used to treat bracken prior to planting is by spraying Azulox herbicide, typically by helicopter. This is problematic as the safety warnings around the product are very strong and it is likely to be withdrawn from the market in the near future. There are also situations where its use is not permitted such as on organic land and or near watercourses or water supplies.
Bracken can be controlled by cultivation (and or ploughing). This method is rarely used on farms because of the difficulties of working on steep ground and the costs involved. However, there is specialised machinery that could be used for this purpose. Because trees are planted 2m to 3m apart it should be possible to prepare in strips rather than ploughing the whole slope.
This project will trial techniques to cultivate strips of varying widths using different types of machinery suitable for working on steep ground. The treated strips will then be planted with trees and their subsequent growth monitored.
The project will run on two sites over two and a half years and will include ground preparation using a mini digger cutting shallow benches, a mini digger with cultivator attachment, a crawler tractor with cultivator, and a forestry scarifier and a robocut machine with a cultivator. Alternative techniques of post planting weeding will also be carried out for comparison such as a strimming, and manual bashing.
Four tree species saplings will be planted:
1) Sessile oak
2) Downy birch
4) Sitka spruce
The majority of these saplings will be 45-60 cm in length which is the industry standard. One plot will be planted with larger saplings of 1 metre directly into untreated bracken to establish whether the extra costs of larger plants and stakes would be a more cost effective alternative than the experiments involving mechanical control.
The success of each technique will be monitored through survival and growth rate of the trees. A cost benefit of each technique will also be made and an estimate of the likely costs of each technique when employed on a larger scale. Once the trees become sturdier and grow higher than the bracken, the impact of the bracken will be sharply reduced. It is the first one or two years after planting where bracken competition is critical.
The results will be relevant to both to conventional and organic farms and provided the proposed techniques are effective they are likely to see widespread adoption particularly at a farm scale. For large-scale woodland plantings the comparative costs of mechanical techniques versus helicopter spraying is likely become a more critical factor. However, mechanical ground preparation confers other advantages than bracken control alone and it is by no means a foregone conclusion that mechanical control would not be a more cost-effective option.