The trees in Boston are among the most studied in the world Ian Dagnall/Alamy
IF YOU have ever walked down a tree-lined street on a hot day, you know the value of urban trees. They provide shade and respite from the concrete jungle. Some, like the great trees of London, a few of which are centuries old, are simply magnificent to behold. City trees have a range of less obvious benefits, too, from flood control to improving our mental health. It is no wonder, then, that around the world efforts to expand urban forests are growing. But there is a problem.
Take Boston, Massachusetts, where researchers have scrutinised the local trees for decades. There, around 40 per cent of saplings die within seven years of planting, far too early to realise their benefits. It is the same elsewhere. In New York, more than a quarter of trees planted in 2009 were dead within nine years. Despite planting drives, urban tree cover across the US fell by about 36 million trees per year between 2009 and 2014. What is going on?
We know these trees face unique challenges, from excess warmth due to the urban heat island effect to a surfeit of nutrients from dog urine. Poor, under-resourced management is also a factor. But now, emerging research suggests that all these may be the roots of another problem for urban trees: unhealthy microbiomes. Without the right kind of microbial communities, they may be more susceptible to the stresses they face in cities, and things look even worse for the most isolated …
