Diversity is best defence

We have grown accustomed to the globalisation of trade and are having to adjust to the realities of global climate change and the chaotic weather patterns we are likely to suffer. From time to time in the past we have, through innocence or carelessness, introduced pests and diseases which have affected our crops and our livestock and we have dealt with them with varying degrees of success.

However, the threat we now face in our woodlands and forests is likely to be the most difficult to overcome. Trees already under stress from climate change face an unprecedented salvo of pests and pathogens. These are endemic in closely related species on other continents, but deadly when they reach a population of trees with no natural resistance. The global trade in live plants and plant products has accelerated dramatically in recent decades. Inevitably, some will carry pests and diseases with them. The process may be unstoppable and we may have to accept devastating loss of some species until an equilibrium is established with the new pathogen.

Perhaps this is what we are beginning to see 40 years after Dutch elm disease among the wych elms in Wales, but from a human perspective the process of adaptation in trees is painfully slow. We need to do everything we can to prevent the introduction of new diseases and to deal with the spread of infections but we also need to think ahead, to build as much resilience into our woodlands and forests as we can.

For me, diversity is the best defence: diversity of species, genetic diversity within the different species and diversity of age structure. The trend in British forestry has been to reduce species diversity, to focus on closely-related clonal material and to grow it in evenaged crops. In so doing, we provide ideal conditions for pathogens to prosper and spread. Crops of larch and pine are already suffering badly in Wales and we are watching the spread of ash dieback with great concern, but the consequences of something similar happening in our spruce forests or oak woodlands would be a disaster on a far greater scale. Are we prepared for this? I think not!