Posts Tagged ‘invasive species’

Photo by Phillip Sauerbeck

Photo by Phillip Sauerbeck

A version of this post was published February 20, 2014 on The Sieve.

I suspect no other relationship is more complex and fraught than that between humans and trees. I’ve been wanting for a long time to write something about it, but every time I try, I get overwhelmed. Where to begin?

For us humans, it indeed goes back to the beginning: Adam and Eve learned of their own humanity from a tree. Or if you prefer more scientific stories, our ancestors took a crucial step in speciating from other apes by descending from the trees. Since then we haven’t gone far from the tree, so to speak. We have eaten from trees, climbed trees, lived in trees, worshiped trees, studied trees, planted trees, hugged trees, and saved trees. We have also, at various times, cut trees down for fuel, for lumber, to make paper, to make weapons, to clear farmland, to create subdivisions, because they threatened our infrastructure, because we didn’t like where they were growing, and for no reason whatsoever.

After hundreds of thousands of years of shared history, have we and trees come to understand each other better? Three stories I have come across recently suggest the answer is, it’s still complicated.

A local tale

The first story is from my own neighborhood of Mount Rainier, a small city just northeast of Washington, DC in the watershed of a minor river called the Anacostia. The Anacostia River, which flows into the better-known Potomac, was once a commercially significant waterway. But as generations of people felled surrounding trees, bare soil eroded, silting up the water and reducing the clear-flowing river to a shallow muddy creek. One of the many things trees do for us—one of their ecosystem services, to use the fashionable term—is hold our land in place.

The Anacostia may be a trickle of its past self, but it can still flood. And as people have understood for a long time, when you deforest a watershed, rain washes more quickly into waterways, and flooding gets worse. Cities along the river responded to the elevated flood risk by building a levee.

Now, ostensibly to protect the levee, several hundred trees near a channelized tributary of the Anacostia going through Mount Rainier are about to be cut down. The byzantine reasoning is as follows: After Hurricane Katrina in 2005, the Army Corps of Engineers strengthened its requirements for levee certification, which is supposed to guarantee that a levee will stand up to a 100-year flood. People who want to insure property behind a levee have to buy flood insurance unless their levee is certified, so the Army Corps’ rules affect a lot of people. The new rules, in addition to raising the minimum levee height, require that trees growing within 15 feet of a levee be removed, because the Corps believes tree roots could compromise the levee’s integrity. This means that more than 200 trees in Mount Rainier, from small, scraggly things no one is likely to miss to 100-or-more-year-old sweetgums and tulip poplars, are destined for the chainsaw.

A view from the levee. The trees on the far side of the concrete wall are on the chopping block.

A view from the levee. The trees on the far side of the concrete wall are on the chopping block.

(Quick pause for disclosure: I am on Mount Rainier’s Tree Commission, which is advising the city on the levee issue. But all opinions expressed here are mine, and all the facts I’m reporting were presented at a public city council meeting. For a further perspective on the urban forest, check out council member Jesse Christopherson’s blog post.)

In the grand scheme, the number of sizable trees we stand to lose is small. And the county has agreed to give the city two new trees for every one that is cut down, so Mount Rainier could emerge in a few decades with more tree cover than it had before. But I’m struck by the perverse logic of the situation: People cause a problem (increased flooding) by cutting down trees and building in floodplains, and then pursue a technological solution that leads to cutting down more trees.

And for what? The new levee may protect against the current 100-year flood, but what about the 100-year flood 50 years from now, when climate change has loaded the dice in favor of stronger storms? We can’t keep building levees higher forever. A better strategy would be to reduce the peak flows that levees have to deal with, which would mean increasing tree cover and, perhaps, giving the river back some of its historic floodplain.

It’s unrealistic to hope tree cover will make a full comeback in an area as densely settled as the DC suburbs. But there’s plenty of room for improvement over the current 25 percent, the number reported by the Anacostia Watershed Society. The nearby city of Takoma Park, which has long protected its trees, stands out in satellite images for its dense foliage compared to neighboring areas. Other municipalities, including Mount Rainier, are now taking steps in that direction with laws that protect large trees on public and private land. Trees are our allies, and the loss of a healthy tree anywhere in the watershed makes all of us more vulnerable.

Tree-friendly Takoma Park, MD from the sky. From google maps.

Tree-friendly Takoma Park, MD from the sky. From google maps.

The global view

Flooding is a local problem, and the number of trees in the Anacostia watershed will probably always be of concern mostly to the people living in the watershed. But in another important sense, we are all united in our dependence on every tree everywhere. That is because trees store carbon in their tissues. This fact that was perhaps almost incidental until people began putting way too much fossilized carbon into the atmosphere; now, our collective future could depend on it. The world’s forests have become crucial reservoirs of carbon and sponges for some—though far from all—the carbon spewing from our cars and factories.

A new map made by a team of researchers at the University of Maryland gives us a global view of how well this global carbon sponge is working. Unfortunately, the picture, which combines over 650,000 satellite images, is troubling. Major areas of tree loss show up angry red both in tropical South America and the boreal forests of Alaska, Canada, and Russia. Things are more mixed in the U.S. South and Indonesia, and a few pockets of tree gain are sprinkled here and there. But the big picture, which the authors reported in Science, is that the world lost around 1.5 million square kilometers of forest between 2000 and 2012. As Peter Ellis, a forest carbon scientist at the Nature Conservancy (and incidentally a Mount Rainier neighbor) observes on his blog, “we are losing forests a lot faster than they can grow themselves.”

Credit: NASA Goddard, based on data from Hansen et al., 2013.

But there are reasons for hope. Forests in much of the U.S. have staged a major comeback in the past century, and are making small but significant dents in atmospheric carbon dioxide levels. And a recent study published in Nature showed that even old trees keep growing, soaking up more and more carbon the larger they get. Forests can buy us some time to deal with climate change before the worst impacts hit—but only if we leave trees standing.

The specter haunting this whole discussion is the possibility that if temperatures get warm enough, forests could start to release more carbon than they pack away. This could happen through increased respiration (in addition to consuming carbon, trees breathe out carbon dioxide just like we do) as well as increases in decomposition rates and forest fires. Forests as carbon sources would be an unmitigated global disaster, dramatically amplifying global warming and potentially making parts of the world simply hellish. Should this happen, the authors of another recent Nature article recommend that we harvest our forests and turn them into buildings and other structures that won’t decay and release their carbon. If we get to the point where we are cutting forests to save the climate, I don’t think I want to be around for it.

I find it wonderful that trees, which I love anyway, also store carbon, and have the potential to blunt the full impacts of our carbon pollution (though this borrowed time is worth nothing if we don’t use it to reduce that pollution as fast as possible). But I worry about the implications of viewing trees as big sticks of solid carbon—what one might call the widgetization of nature. The Nature authors manage to write a whole article about trees without mentioning a single actual species. That kind of abstract view makes it easy to imagine harvesting those trees if they become carbon sources, regardless of the other benefits they may confer to people or other living things. It’s a view that may see the forest, but misses the trees.

Losing a loved one

As anyone who has been to a forest knows, there is no such thing as a generic tree. There are only white oaks, sugar maples, pitch pines, and so on. And each tree is the basis of a unique food web, many of which contain organisms not yet known to science. Trees of different species are not interchangeable with each other, nor with other things we might discover that do an equally good job of holding carbon. We have only barely begun to learn how the system works. To paraphrase Aldo Leopold, this is not the time to be throwing pieces of the machine away because we think we don’t need them.

This brings me to my third story, in which a particular tree native to my part of the world is disappearing, not because anyone wants it to, but nevertheless for an entirely human-caused reason. As my friends and family know, I have become fairly obsessed with this tree, the eastern hemlock, or Tsuga canadensis. I point it out on hikes and turn branches over to look for the fluffy white egg sacs of the hemlock woolly adelgid. I have written about this invasive insect, which is destroying nearly all the hemlocks in the eastern U.S. The adelgid was apparently introduced to the U.S. in or around 1951, by accident, on a shipment of Japanese hemlocks to a Richmond, VA nursery (the insect is native to Asia and hemlock species there tolerate it just fine). That such a catastrophe could originate from so trivial an incident is part of what makes the whole thing so spooky.


Hemlocks lining a stream in western Maryland

Hemlock bark was once used to tan leather, but otherwise the tree has never been of much value economically. The wood splinters easily, and except up north, the tree lives mostly in river valleys where few people go. In terms of ecosystem services, the trees’ perpetual shade keeps trout streams cool, and its evergreen canopy captures rain and snow all year round. These are not trivial benefits, but they are not the kinds of impacts likely to inspire a national conservation movement. So it is hard to imagine cash-strapped governments spending a bunch of money to save a tree whose loss will cost few, if any, jobs. Indeed, as Richard Preston reported in 2007 in the New Yorker, governments pretty much haven’t.

But viewed another way, the costs if the hemlock disappears will be immense—literally incalculable. For what is the value of a species? A species lost is lost forever, so one could argue its value is infinite.

To be clear, the eastern hemlock itself is unlikely to go extinct, at least in the near future. Insecticides can protect individual trees if applied regularly, and winters in the upper Midwest and Canada, where many hemlocks live, are too cold for the adelgid to survive (though global warming could eventually remove that protection). But far more than the tree is at stake. Entomologist Louise Rieske-Kinney and her students at the University of Kentucky have studied the organisms living in hemlock-shaded valleys. The scientists have found that certain aquatic flies eat hemlock needles that fall into the streams. Certain spiders eat those flies, and fish eat those spiders. Removing the hemlock is like pulling the bottom block from a toy tower—the rest of the blocks come crashing down too.

Changes in streams may just be the beginning. Preston described in his article climbing (now-dead) hemlocks in North Carolina and seeing a whole world living just in the trees’ canopies. “There were small hummocks of aerial moss, spiderwebs, insects associated with hemlock habitat,” he wrote in 2007. “There were mites living in patches of moss and soil on the tree, many of which probably had never been classified by biologists. The hemlock forest consists in large part of an aerial region that remains a mystery, even as it is being swept into oblivion.”

A hemlock graveyard in Shenandoah National Park

A hemlock graveyard in Shenandoah National Park

Since 2007, scientists have shed some light on this mystery. Talbot Trotter, an ecologist with the U.S. Forest Service in Connecticut, told me he and his students have discovered hundreds of insects, mites, and spiders that seem to live on hemlock branches and nowhere else. The researchers would know more, except there is hardly anyone in the world with the expertise needed to classify the species they are finding.

Meanwhile, forest managers up and down the Appalachians lead small armies of insecticide sprayers into the woods. Their goal is to hold off the adelgid, at least from the largest and most visible trees, while biocontrol researchers try to breed and release an effective adelgid predator. But it is an uphill battle; predatory beetles that have been released often consume all the adelgids in a small area and then disappear. No one to my knowledge has gotten a permanent population established.

The extraordinary effort and care of these scientists and forest managers is the flip-side of the carelessness with which the adelgid was released onto this continent. Hundreds of people are now dedicating careers to understanding the insect and the hemlock, and to slowing—and perhaps eventually reversing—the damage. They’re not doing this because it pays well or because it’s glamorous work, or even because we as a society need the eastern hemlock. They’re doing it because they love this tree.

What mysterious forms live in the canopy?

A tale of survival

It’s not just the hemlocks that need love. The emerald ash borer is destroying the nation’s ashes. Asian longhorned beetles are attacking the great maple forests of the north. A fungal disease spread by a scale insect threatens the beech (another one of my favorites). The mountain pine beetle has brought down millions of pine trees out west and could do even more damage if it makes its way east. Already the American chestnut and American elm have largely succumbed to their own introduced pathogens. Invasive species amped up on climate change are doing what humans with their saws and axes could not—excising whole tree species from the landscape.

And yet, I suspect most trees will come through even this latest round of insults, as they have countless times before. The eastern hemlock was once far more abundant, but pollen studies show that it declined dramatically somewhere between five and six thousand years ago. No one quite knows why, but climate change and disease have been suggested. Still, the eastern hemlock is far from rare, and even in places where the adelgid has ravaged the older trees, new green shoots push their way up. Perhaps thousands of years from now, hemlocks will once again find conditions favorable and spread out over the land.

It is tempting to see trees as passive players in this drama, merely reacting to climate shifts, disease, and now humans and their invasives. But I have come to think of trees as playing the long game. They spread themselves far and wide, bank their seeds for decades or longer, and reproduce both sexually and asexually. For all the clear-cutting and species shlepping we have done on this continent, we have only driven two of its native tree species extinct in the wild, according to USDA plant geneticist Richard Olsen. (And at least one of these, Franklinia alatamaha, is still in wide cultivation). In short, trees know how to survive.

Yes, it’s not the trees I worry about. It’s the overconfident, impulsive, short-lived primates.

Writer’s note: this post was amended to reflect the fact that the Anacostia levee was not built in response to 1972’s Hurricane Agnes (it was actually constructed in the 1950s).

Kentucky’s Red River Gorge is a giant sandstone layer cake hollowed out by water and covered in life. On its ridges and slopes grow the mixed hardwoods of the eastern U.S.—oaks, maples, tulip poplars—along with hardy pines, mountain laurels, and an occasional bigleaf magnolia. But in the Gorge’s stream-carved ravines, plants from a colder time still find a home. Descend into one of these valleys on a hot summer day and you may find yourself suddenly wrapped in a cave’s chilly breath and a hemlock grove’s dusky shade.

Hemlocks by a stream in the Red River Gorge

A hemlock-lined stream in Red River Gorge

Since before I can remember, I have been hiking the Gorge. As a child, a hike was pure bounding adventure, and it was only thanks to the adults that we eventually settled down in some beautiful spot to eat our sandwiches. As a teenager with friends, a hike was freedom, a chance to expand from our city selves. We would lace our boots and set out full of spirit and chatter, eager to rack up miles, scramble up boulder piles, run full speed down slopes in the rain yelling. We gazed up at the huge cliffs and arches and allowed ourselves to be awed.

But of the millions of trees we passed on these hikes, I don’t think we even stopped to look at one. We were too restless, too hungry for challenge and too in thrall to the grandeur of rock. If we had paused for a moment in a quiet valley, we might have seen the hemlocks full of life. We might have enjoyed their light green fringes of new growth, and felt their downy needles.

In Kentucky, the eastern hemlock is one of those species left over from a colder time. As the last ice age ended and glaciers receded poleward, trees and other plants moved across the landscape, tracking their ideal climates. The hemlock, perhaps a slower migrater than most trees, eventually found its way to the understories of cool Appalachian river valleys. As it grew, its thick needles, which fan out in dense, overlapping branches, cast a shade so deep that few other species could grow in it. Thus the hemlock created its own preferred conditions, and perpetuated itself in its new home.

As the hemlock grew on the banks of streams, its perpetual shade also cooled the flowing water. Trout, dace, and salamanders thrived. The trees dropped needles into the streams, and stoneflies arrived to eat them. Spiders arrived to eat the stoneflies. Over time, a community of life grew up around the hemlock. Ecologists call it a keystone species.

Hemlock needles in a stream

Hemlock needles

Today, I offer a lament for the Red River Gorge hemlocks, which shaded my childhood hikes without my knowing it. They shade less now because their lower branches are mostly bare. In a few years they may not shade at all. A tiny insect is extracting these trees one by one from the forest.

The hemlock’s struggle is, on one level, simply biology. But it is biology made tragedy by our careless shuttling of species around the globe. The eastern hemlock has cousins in Asia and the West Coast, separated by eons of evolution. In that time, a sucking insect called the hemlock woolly adelgid evolved to live on the Asian hemlocks’ sap. Various predatory beetles evolved to eat the adelgid, and the ecosystem maintained a kind of dynamic balance.

So it went for countless millennia; so it still goes for the hemlocks of China and Japan. But in 1951, a shipment of ornamental Japanese hemlock carrying adelgids arrived at a nursery in Richmond, Virginia. Soon thereafter, an insect and a tree that had never met were united, and by an unfortunate quirk of biology, the eastern hemlock proved both edible and defenseless. The adelgids pierce the sap conduits at the base of the hemlock’s needles, and the tree starves to death.

The adelgid's woolly egg sacs

The adelgid’s woolly egg sacs

The adelgid also proved to be an almost ideal invader. A female adult can, without even mating, lay up to 300 eggs at a time, and the insect has two generations in a year. The tiny nymphs don’t fly, but they are easily given wing by wind or birds. Nevertheless, the adelgid did not immediately sweep across the eastern forest. For several decades it spread slowly, perhaps as it evolved ways to better exploit the vast resources in its new and predatorless environment. In the 1980s, a hurricane that swept adelgids from Virginia up to Long Island and Connecticut seems to have triggered a faster expansion. Since then the bug has been spreading north, south, and west at up to 15 miles per year, devastating hemlock stands from Georgia to Maine.

Mostly dead hemlock

A dying hemlock in Red River Gorge

The hemlock woolly adelgid is a relative newcomer to Kentucky; it was first found in the far southeastern part of the state in 2006. Now it occupies about a fifth of the state’s counties, including Powell, Wolfe, and Menifee, where the Red River carved its gorge. On trees where the adelgid has taken up residence, the lower branches go first, but the insect can take several years to reach the higher ones. It may not be too late for Kentucky’s hemlocks—but they need help, and fast.

Scientists have found two ways to protect hemlocks. One is to inject or drench trees in an insecticide like imidacloprid (the same class of chemical implicated in honeybee colony collapse disorder) every few years. This method has the obvious drawback that it introduces a dangerous pesticide into a potentially vulnerable ecosystem. It’s also far too expensive to save even a sizable fraction of the trees in a forest. But it can temporarily preserve trees deemed particularly valuable for aesthetic or ecological reasons.

Most forest managers believe only long-term solution to the hemlock woolly adelgid invasion is to replicate the natural systems that keep the insect in check in its native Asia. That means establishing populations of adelgid predators—a process known as biocontrol. Throughout the east, scientists are breeding, screening, and releasing several different species of predatory beetles in places with heavy adelgid outbreaks. Ironically, getting a desirable exotic species to take hold in a new area can be a real challenge. And in the South, where warm winters allow the adelgid to spread faster and more virulently than further north, trees are often too far gone by the time beetles build up enough population to make a dent in the bug’s numbers. Biocontrol may not be able to stop the adelgid invasion, but it will be a critical piece of any future efforts to restore hemlocks to the eastern U.S.

Kentucky foresters and scientists can at least piggyback on the experiences of states where the adelgid hit earlier. But these places tell a mostly sorry tale. Despite millions of dollars and many smart scientists devoted to saving the mighty hemlocks of North Carolina, the trees there have mostly succumbed. In the valleys of the Great Smoky Mountains, grey skeletons line the riverbanks. In Joyce Kilmer Memorial Forest, a rare remnant of eastern old-growth, giant leafless trunks point skyward as mushrooms pock their decaying bark. Forest managers even took the extraordinary step of felling some of the dead trees with dynamite so they would not endanger hikers.

Two hemlock trunks in Blanton Forest

Two huge hemlock trunks in Blanton Forest

Sadly, a similar story is now playing out in Kentucky. And because it lacks world-famous sites like the Smokies and Joyce Kilmer to focus public attention and donations, the state seems to have far fewer resources to devote to its trees. As far as I can tell, only one Kentucky state employee works full-time on the hemlock. Forest managers are forced to triage, directing limited insecticide and biocontrol budgets to save select populations along streams or in heavily visited areas. And scientists doing adelgid research are stretched thin. Of those scientists, University of Kentucky forest entomologist Lynne Rieske-Kinney probably spends the most time on the insect. She has studied a variety of questions: what stream insects depend on hemlock; why are some hemlocks more resistant to the adelgid than others; what will happen to Kentucky forests if the hemlock disappears? Nevertheless, she told me the adelgid accounts for only a little more than a quarter of her research effort.

It’s impossible to know whether Kentucky, with more money, could be the place where the hemlock makes a stand. But it does seem clear that without more resources and greater public urgency, most of the state’s 70 million-plus hemlocks will soon be lost.

A hemlock graveyard in Virginia's Shenandoah National Park

A hemlock graveyard in Virginia’s Shenandoah National Park

I admit I’m a latecomer to the hemlock’s fan club. I learned of the adelgid less than two years ago, from scientists in Wisconsin who worry that warming winters will allow the adelgid to reach their magnificent forests (for now, cold winters limit the insect’s northern expansion). Since then, I have seen too many hemlock graveyards—in North Carolina, in Georgia, in Virginia and Maryland, where I now live. I have also hiked among the gorgeous, still living hemlocks of Red River Gorge and Blanton Forest in eastern Kentucky. I have felt their cool shade on a hot June day. I don’t want to come back to Kentucky in a few years and find more graveyards.

Will future Kentucky teenagers romp through hemlock-shaded valleys? Or will their Gorge hold only skeletons and a few scraggly survivors of the adelgid invasion? The next few years will likely tell. And I hope that in Kentucky, scientists will be able to give this story a different ending.