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 This is problematic as the safety warnings around the product are very strong and it is likely to be withdrawn from the market in the near future.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 trialled techniques to cultivate strips of varying widths using different types of machinery suitable for working on steep ground. The treated strips were then planted with trees and their subsequent growth monitored.
The project which ran on two sites over two and a half years, included 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 were also carried out for comparison such as a strimming, and manual bashing.
Four tree species saplings will be planted:
- Sessile oak
- Downy birch
- Sitka spruce
The majority of these saplings were 45-60 cm in length which is the industry standard.
The success of each technique was monitored through survival and growth rate of the trees. 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.
- Notwithstanding the reduced amount of experimental data due to Covid, this study suggests that cultivation of bracken is not likely to be a promising option. A slope above 50% is probably the safe limit for working. Most bracken covered land is likely to be too steep for most equipment. The cultivated strips would have to be wide in order to avoid the impact of vigorous growth of adjacent bracken. Complete ploughing or rotavation of the site would be the most practicable option and this could lead to erosion and other environmental problems.
- Planting into dense bracken (using canes and spirals) in the absence of cutting or trampling leads has a poor rate of establishment. Significant cumulative losses continue for at least three years after planting.
- The most resilient of the four species chosen was rowan. Birch was also fairly resilient. These species should be favoured when planting difficult bracken sites.
- The pushing over and smothering effect of the bracken appears to be a greater problem than the direct shading. Trees planted into bracken without any support are not likely to survive. Any support that helps prevents the tree from being pushed over should help reduce mortality. Any canes used should be extra sturdy.
- The impact of the timing of strimming and/ or trampling, on tree survival is a topic of research that would be worth looking at in the future.