British woodlands need diversity from around the world Such a process of colonisation involves a degree of chance in terms of which species and genes get established, and a loss of diversity. Perhaps there were tree species in central Asia with genes more suited to living in Britain, but they were too far away to have a chance of getting here. Ever since the industrial revolution native British trees have been unable to sustainably meet our economic needs. We almost ran out of timber in the First World War, and afterwards we were forced to look abroad for tree species that would grow better. Commercial forestry in Britain now depends almost entirely on Sitka spruce, introduced from the west coast of North America. A massive one third of all British woodland is Sitka spruce plantation. Until now, we have been less willing to look abroad for species or genes to improve our broadleaved woodlands. This needs to change now that global diseases are threatening their very existence. We need to use new technologies, developed for medical genetics, to start a global search for trees that have genes to resist incoming pathogens. Take, for example, the UK's second most common broadleaved tree: birch. This could soon face devastation because it is susceptible to an American beetle called the bronze birch borer, which has not yet reached Britain. We might be able to insure ourselves against this beetle's arrival by planting American birch species, as these have co-evolved with the bronze birch borer and show resistance to it. Or we might hybridise American and British birches to produce a tree that is both well adapted to the British climate, and prepared for a future pathogen attack. Similarly with ash; preliminary data from a German study suggest that American ash species show resistance to ash dieback. So it might make sense to plant American ash in Britain as a replacement for British ash species. But on the other hand, American ash species are susceptible to the emerald ash borer, a beetle from Asia. For resistance to this pest, we need to look to Asian ash trees, which have co-evolved with it. A robust ash tree may therefore need to be a hybrid with genes from both Asia and America. The idea of hybridising tree species is not a new one: in fact, a great deal of natural tree evolution occurs by hybridisation. We can give evolution a helping hand by bringing different tree species together from around the world. Of course, this will to some extent change the appearance of our woodlands, but we can minimise this by careful selection of trees. To do this in a controlled and informed way we need to understand more about the genomes of closely related species of trees, and how they have evolved. There are over 30 species of both birch and ash around the world. We need to do research on their genetic make-up, so that we can harness their genetic resources to preserve our woodlands. We are particularly well placed to do this in the UK because here, over many decades and centuries, silviculturalists have built up comprehensive collections of trees from around the globe. These provide us with superb materials on which to carry out surveys of genetic diversity. For the first time we also now have DNA sequencing technology that allows us to do this in a cost-effective manner. We can use this technology to understand the genetic diversity of foreign relatives of British tree species. Then we can use these data, together with field trials, to select and breed trees that will be resilient to future pathogen threats. Such a research programme will take time, but by proactively working on this now we may be able to preserve British woodlands for future generations. Woodlands with foreign genetic diversity will be better than no woodlands at all. Richard Buggs is Senior Lecturer in Evolution and Ecology at Queen Mary University of London. He recently sequenced a birch tree genome