If you’ve 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 giant trees in London, are hundreds of years old and are a sight to behold. Urban trees also have a variety of lesser-known benefits, from flood control to improved mental health. It’s no wonder, then, that efforts to expand urban forests are growing around the world. But there’s a problem.
Take Boston, Massachusetts, for example. There, researchers have been scrutinizing local trees for decades. About 40 percent of the seedlings there die within seven years of being planted, too soon to realize the benefits. The same is true elsewhere. In New York, more than a quarter of the trees planted in 2009 died within nine years. Despite tree-planting efforts, between 2009 and 2014, urban tree cover across the United States declined by approximately 36 million trees per year. What is happening?
We know these trees face unique challenges, from excessive warmth from urban heat islands to a lack of nutrients from dog urine. Inadequate and under-resourced management is also a factor. But now new research suggests that all of this may be the root of another problem with urban trees: an unhealthy microbiome. Without the right kind of microbial communities, we may be more susceptible to the stresses we face in cities, and the situation looks even worse for the most isolated people.