Key to your understanding of the biodiversity crisis: The trees near your home

Wander into the woods in most places in the Eastern United States and you're likely to come across a towering trunk with sandy-colored, diamond-shaped ridges rising to bare forking branches and little holes peppering the bark, signaling where small, green beetles have crawled out and flown away after doing their dirty work. This decaying monument is — or rather, was — an ash tree. Its kind will not be back in your lifetime, perhaps ever.

If you live in the other half of the country, just wait a few years. The emerald ash borer is coming for your trees, too.

Humans are setting in motion a mass extinction of life, only the sixth in Earth's 4.5 billion year history. A recent United Nations report put this in stark numerical terms: As many as 1 million plant and animal species are at risk of annihilation. Such an astronomical figure, while intended to impress, can actually make the threat hard to relate to. Too often we view the global biodiversity crisis as remote or abstract, involving the disappearance of exotic charismatic megafauna like tigers and elephants, or obscure species most of us don't recognize to begin with.

The crisis isn't remote, or abstract. The ash tree demonstrates that real, visible and consequential ecological catastrophes are playing out all around us.

But in 2017, in an announcement that received virtually no coverage in the U.S., the U.K.-based International Union for the Conservation of Nature, which keeps track of threatened species, declared five American ash tree species — white ash, green ash, blue ash, black ash and pumpkin ash — critically endangered, the last step before total extinction. The rarer Carolina ash was also designated endangered.

Six species are in mortal peril, and 10 others are infested, due to one tiny beetle that arrived in the U.S. some 20 years ago, probably on shipping pallets. The ash's 40-million-year run on our continent may have been doomed by this most mundane of events in the era of global trade, noticed by no one.

In truth, the ash itself often goes unnoticed, overlooked in favor of more famous cousins like oak and maple. The great nature writer Donald Culross Peattie noted this in his 1948 classic text on eastern North American trees: "Strong, tall, cleanly, benignant, the Ash tree with self-respecting surety waits, until you have sufficiently admired all the more obvious beauties of the forest, for you to discover at last its unadorned greatness." Or as someone on a local native plants Facebook group put it the other day, rather more plainly: "I didn't realize how common ash trees were until they all started dying."

Green ash is a different beast entirely. In the wild, it rarely grows beyond stream and riverbanks. But in those soggy settings it thrives, holding the ground in place and giving structure to critical riparian ecosystems. Along the Anacostia River near where I live just outside Washington, D.C., banks that were shrouded by green ash are now bare and open to the sky. Invasive shrubs and vines are taking over. Their habitat value is close to nil, and the loss of the rain-catching canopy will make it that much harder for D.C. to meet its clean-water goals.

At least we still have other native trees that could someday replace what is being lost. The black ash-dominated wetlands around the Great Lakes may be transformed beyond recognition. An indigenous basket weaving tradition has grown up there around black ash wood, which peels off in long, pliable strips. I recently saw several of these baskets in the workshop of Alfie Jacques, a woodworker and member of the Onondaga tribe of upstate New York. The craftsmanship and care embodied in each one was a world away from the cheap, disposable commercial ware we're surrounded with; it's said that the best basket makers weave them tight enough to hold water. The black ash basket-making tradition has survived colonialism, genocide and centuries of cultural oppression — but it may not survive the ash borer.

Of all the ashes, though, my favorite is probably the strange and wonderful blue ash. Centuries old blue ashes rise from the horse farms and hollows of central Kentucky, where I grew up; these trees are ancient emissaries of a pre-agricultural past. As they grow, their central trunks often collapse, and growth transfers to secondary limbs, which reach skyward like so many arms. This gives the tree a humanlike form; one thinks of Job pleading with a capricious God. Talk about a tree with personality.

When these trees are gone, dozens of other species will disappear, too. More than 40 insects and other little critters are known to live only on American ash trees, and this is surely only a partial tally. Many birds prefer ashes for feeding and nesting. A 2013 study found that tadpoles grow larger when feeding on ash leaves than on maple, which is replacing ash in many places.

A tree like the ash is the Vito Corleone of the woods. He held the family together. When he lost his grip, everything fell apart. It's counterintuitive in the age of mass extinction, but the total disappearance of a native tree species is an almost unheard-of event. The ash extinctions would be the first in America since Franklinia alatamaha , an extremely rare tree to begin with, vanished from its native habitat around 1800. But the loss of a tree as a significant piece of the forest has become a grimly regular occurrence, thanks to a reckless, cowboy-style global trade system that moves just about anything across an ocean that can be sold for a profit. In the past century, imported diseases have wiped out the mighty American chestnut and American elm as functional forest species, and insects are now doing the same to eastern hemlock and all the ashes. A new and mysterious disease could make American beech the next major forest tree to succumb. Our pines face assorted beetles of varying voraciousness.

But scientists and forest managers increasingly express a fear that the system is being pushed to the brink. Perhaps most worrisome, our oaks and maples face diseases and insects that are already here and could explode without warning: oak wilt and sudden oak death for oaks; Asian long-horned beetle for maples. If either of those widespread and hugely important tree genuses go, we could find ourselves facing the ecological apocalypse that the U.N. authors warn of.

Even as we're losing trees, we're learning more and more about the importance of forests to humans. The U.S. Forest Service estimates that American forests offset nearly 15 percent of our carbon emissions, making them one of the few sizable brakes on an increasingly out-of-control climate change process. Scientists keep finding new evidence that exposure to trees can benefit physical and mental health. Perhaps most strikingly, researchers are discovering that forests' recycling of water can account for half or more of the precipitation in continental interiors, and can influence weather halfway around the globe. In other words, if we lose our forests, we may lose much of our rainfall, which would represent an existential threat to the nation and the world.

Yet our response as a society has been strangely muted. A century into this slow-motion disaster, we lack even the rudiments of a coherent plan. The main strategy that has sheltered nature from capitalism's drive to exploit resources — creating parks or preserves — is useless against diseases and insects, which are oblivious to such designations. Federal inspections and regulations to keep tree pests out of the country have helped somewhat, but shipping companies still fail to systematically treat wood products and packing materials that could harbor dangerous insects or diseases. As you read this, the next tree killer could be steaming toward an American port.

It's often said that a society should be judged by how it treats its most vulnerable. A society could also be judged by how it cares for its trees. On both counts, we are falling far short.