Non-native species are those that have been introduced into areas that have not historically been part of their native range, through either accidental or deliberate human activity.
Some non-natives can expand rapidly, eliminating native species in the process, and completely altering or degrading the quality and diversity of the ecosystems into which they are introduced. In fact, after habitat loss, non-native species are the second most serious threat to global biodiversity.
Controlling non-native species once they become widespread is often very expensive. Government figures have show that non-native species cost the Scottish economy around £264 million a year, with a 2010 report estimating a cost of £1.7 billion per year to the UK economy.
One of the major factors which contributed to the degradation of the Caledonian Forest in the 20th century was the widespread under-planting of ancient pinewoods and oakwoods with commercial timber crops of non-native trees, such as Sitka spruce, lodgepole pine and hybrid larch. Today, there is still an ongoing problem with the original crop setting seed and regenerating in areas of native woodland.
The integrity and healthy functioning of the Caledonian Forest depends on the control and removal of these non-native trees, so volunteers regularly carry out this work on our Conservation Days and Rewilding Weeks at Dundreggan. Although some volunteers are surprised when they first learn that we are cutting down trees in addition to planting them, they soon find great satisfaction in clearing areas of invasive species to provide space for native trees and regeneration of natural habitats.
Many landowners with whom we work use contractors with chainsaws and specialist machinery to clear dense or large areas of non-native forest. In order to keep our work as safe as possible and to enable volunteers to enjoy being in the forest without using noisy tools, we usually remove trees by hand and tend to focus mainly on places where the trees are spread out over a large area. Volunteers work in pairs and are equipped with personal protective equipment such as hi-visibility vests, goggles, gloves and hard hats. We use a variety of techniques:
Simple removal of small trees
If a tree is less than about a finger’s width in diameter, it can usually be easily pulled up by hand. For those that are slightly bigger (up to about 4 cm), we use loppers, and for those that are bigger still (up to about 10 cm) we use a handsaw.
Felling of large trees
If a tree is larger (between 10 and 20 cm in diameter), the safest way to cut it down is to use a “gob” or “wedge” cut. This technique enables a tree to be felled in a particular direction by effectively creating a hinge. We remove any side shoots that remain on the stump, to ensure that it doesn’t grow back, and then brash the trunk. This involves cutting off the branches to get the trunk as close to the ground as possible, enabling it to rot down quicker. This also makes the site safer and easier to cross in future years. Brash and trunks can be moved into drainage ditches, which helps to fill them up, and also means the wood doesn’t smother as much ground flora.
Ring-barking
Another technique that we use is ring- barking, also known as girdling. This is the removal of a strip of bark from around the entire circumference of the trunk of a tree. It results in the death of wood tissue above the damage and, eventually, after a number of months, or even years, kills the whole tree. We use this quick and relatively easy technique on trees that are too large for us to fell by hand, for example in the Dalreichart plantation on at Dundreggan. Removing lodgepole pine and Sitka spruce to help return this site to native forest is an ongoing project.
A very important benefit of ring-barking is that it creates standing dead wood. Dead wood supports a large and complex food chain and is a very important aspect of a forest – in fact, in a healthy forest, approximately 30% of the trees will be dead.
Due to the extent of non-native trees across the Scottish Highlands, removing them will remain a very important aspect of our work in the years ahead. Returning to a site a few years after it has been cleared and seeing native trees such as Scots pine, birch and rowan pushing through the soil is incredibly satisfying, and we look forward to seeing more and more young native forests starting to take root as a result of this work in the coming years.
