RFS Chief Executive Simon Lloyd argues that to counter the threat of tree diseases and pests we should plant a much greater variety of tree species, both native and non-native, in our woods. This will make them more resilient and sustainable.
“What tree species should I plant in my wood?” is a question at the forefront of the minds of all woodland owners and managers. There has been an exponential increase in the number and virulence of tree pests and diseases in the last twenty years. The climate is getting warmer and more extreme, which is not only facilitating the spread of pests and diseases, but will put increasing stress on many tree species, exposing them to a greater risk of infection. An alarming number of our staple tree species are under threat including ash, oak, beech, horse chestnut, larch and pine. We desperately need a fresh approach.
We have been planting a narrow palette of tree species in the UK. Just five species (oak, ash, beech, sycamore and birch) make up 77% of broadleaved woodland in England. 80% of our conifer woodlands are comprised of only four species. Evidence and experience tell us these species are well suited to prevailing climate and site-specific conditions and produce quality wood products which are in demand. It also means that when a disease threatens any one of them, the impact on the landscape, woodland ecology and productive capacity of our woods is disproportionately widespread. Ash dieback has highlighted this vulnerability very clearly.
We have been overly fixated on native broadleaved species, which narrows the choice markedly. There are only 32 tree species which qualify as native. Of these only a handful are productive trees capable of producing quality timber with a ready market. Still, we may have overlooked some native species which could play a part in replacing ash. Wild service (Sorbus torminalis) and small leaved lime (Tilia cordata) are good examples. Britain has one of the best climates for growing trees and over 600 species grow in our woods, parks and gardens.
The threats to trees
A solution to the problem of rapid environmental change and increased risk of pests and diseases is to diversify the range of species, and broaden the genetic diversity within species. This requires research and forethought by the woodland owner and will depend on management objectives and attitudes to risk. It is a big decision. Get is wrong and you live with the mistake for decades.
Making informed decisions
Fortunately, there is an increasing range of information available to help make informed decisions about the relative merits of alternative tree species. Principle among these is the Ecological Site Classification (ESC) decision support system (www.forestry.gov.uk/esc). ESC matches key site characteristics with the ecological requirements of tree species and, crucially, includes climate change projections for 2050 and 2080, well within the productive life of trees we are planting today.
The instinct for experimentation is alive and well and many enterprising landowners are busy cultivating everything from eucalyptus to tulip trees. The RFS visits many of these woods each year to learn from their examples.
It is pointless to grow many species of broadleaves without a sustained effort to control grey squirrels. Some woodland owners are turning away from broadleaves to avoid this risk. Some broadleaved species are less vulnerable to attack by grey squirrels than others. Ash is notable in this respect. Others include wild cherry (Prunus avium) and common alder (Alnus glutinosa) which can be expected to play a bigger role in our woods in future.
We should not be prescriptive about choice of species. Provide the woodland owner with the best information and let them make an informed decision consistent with their objectives.
Where will all this lead? We should expect to see a much greater variety of tree species, both native and non-native in our landscape. In many ways this will enhance, not detract from the beauty of our woods. More importantly, our woods will be more resilient both ecologically and financially to shocks from pests, disease and climate change. We should embrace this change.