All the Trees Will Die, and Then So Will You

May 10, 2017, 10:00 AM

A story from Wired.

The POLYPHAGOUS SHOT HOLE borer, a brown-black beetle from southeast Asia, never gets bigger than a tenth of an inch. It breeds inside trees; pregnant females drill into trunks to create networks of tunnels where they lay their eggs. The beetles also carry a fungus called Fusarium; it infects the tunnels, and when the eggs hatch, the borer larvae eat the fungus.

Unfortunately Fusarium also disrupts the trees’ ability to transport nutrients and water. Holes where the beetle bored into the tree get infected and form oily lesions. Sometimes sugars from the tree’s sap accumulate in a ring around the hole—that’s called a “sugar volcano.” The tree dies, and the wee baby beetles fly off to continue the circle of disgusting life.

This would just be a scary story for arborists and tree-huggers, except: Fusarium dieback is on track to kill 26.8 million trees across Southern California in the next few years, almost 40 percent of the trees from Los Angeles to the Nevada border and south to Mexico. That’s more than just an aesthetic tragedy. It means that thousands of human beings are going to die, too.

I’m not just being a monkeywrenching fearmonger. Dead trees mean dead people, and scientists are finally starting to figure out why. In the 1990s, spurred by a program to plant half a million trees in Chicago, researchers started trying to quantify the value of a tree beyond the fact that one is, like, at least slightly more lovely than a poem. It’s a field of study today called ecosystem services. “I’ve been trying to quantify the impacts of trees on rainfall interception, pollutants in the atmosphere, cooling and energy used by buildings, CO2 stored and emitted,” says Greg McPherson, a research forester with the US Forest Service who conducted the latest study of SoCal’s trees. “But I think those are the tip of the iceberg.”

And at the base? Public health impacts—and differences in illness and death in populations that live near greenery versus those that don’t. It’s only been in the past few years that anyone has been willing to go out on a limb and associate morbidity and mortality numbers with nature. Oh, sure, everyone agrees that trees pull particulate-matter pollution out of city air. Simply by dint of being shady, trees reduce the “urban heat island” effect that drives people to run their AC all the time, a contributor to climate change. And, yes, trees inhale carbon dioxide, another win for the climate.

But fighting disease is a whole other question. What is a “dose” of nature? What’s the response curve? By what mechanism would a walk in the park alleviate, let’s say, heart disease? Is it the park? Or the walk? (Some Japanese researchers think trees literally emit life-giving chemicals, like that weird M. Night Shyamalan movie where trees kill people, but in reverse. No, wait, that’d be people killing trees, which actually happens. The converse, then.)

Whether the mechanism is stress reduction, pollution reduction, or increased physical activity, somehow trees make a difference. The biophysics is less important than the epidemiology. In 2013 another researcher with the US Forest Service named Geoff Donovan took advantage of the fact that another beetle, the emerald ash borer, killed 100 million trees across 15 states in the US. Using statistical models to rule out the impacts of a whole bunch of other potentially confounding factors—race, education, income—Donovan’s team was able to connect illness with places that had ash borer infestations and concomitant loss in tree cover (which you can see in satellite imagery).

His result: Counties with borers had 6.8 additional deaths per year per 100,000 adults from respiratory disease, and 16.7 deaths from cardiovascular disease. Over the arc of the paper, that means 100 million dead trees—roughly 3 percent of tree cover on average—killed 21,193 people. “The implicit thing I’m saying here is that if you either kept the trees or increased the amount, you’d g...