Is the introduction of novel exotic forest tree species a rational response to rapid environmental change? – A British perspective

Both plantation forests and native woodlands are currently facing challenges in the form of rapid climatic change and unprecedented increases in damage by exotic pests and diseases. To combat these problems it has been proposed that a range of novel exotic tree species (non-native species that have not yet undergone thorough operational testing or previously been grown at a forestry scale) should be grown as part of an adaptive management strategy, and that non-native (including novel) species should be introduced into native woodland. Justifications for this policy are that novel exotic species are required to maintain forest productivity under climate change, to create a more diverse, and by implication more resilient forest, and to substitute for native species threatened by introduced pests and pathogens. Here we examine these arguments in the context of British forestry, where there is a long history of utilising non-native species. On the basis of this documented experience we conclude that in the commercial sector of British forestry, where production is the main objective, there are strong arguments for undertaking a programme of rigorous testing and domestication of a very limited number of the most promising novel exotics which, in addition to good timber and growth, also have attributes that will allow the development of more naturalistic silvicultural systems and a move away from current clear-fell regimes. However this must be undertaken within a comprehensive risk assessment framework, where candidate species are rigorously screened both for any biosecurity threats, and their potential for causing ecological damage if they become invasive outside their initial planting sites. Widespread planting of candidate species should only be recommended after the completion of full species and provenance trials, and when reliable sources of appropriately adapted seed have been established. Conversely where conservation of biodiversity is an objective we find no support for introduction of any non-native species. This is based on the greater ecological and economic risk they pose compared to the use of native species. Use of non-natives is likely to lead to an increase rather than a decrease in pest and disease problems, and to hinder rather than support the retention of threatened native tree species and their associated biodiversity.