Exotic Invasives in Charlottesville
This afternoon, unless a hard rain is falling at 1 p.m., I’ll be standing in a body shop parking lot, in the middle of crumpled fenders, stoved-in hoods, cobwebbed windshields, dented side panels, and shattered headlights, waiting for whoever shows up for a ninety-minute nature hike along the Rivanna Trail that circles Charlottesville.
The hike, which will be focused on invasives, is the third leg of a series of walks promoted by the city’s Parks and Recreation Department, covering different stretches of the trail each weekend during April and May, and I’ve volunteered to be a guide. I’m not an expert on invasives, but I’ll do my best to share what I know. Today’s hike, as we’ll see, will furnish case histories on how some of them have found their way into our lives.
A lot of damage is being done to trees and other native plants around us here in Charlottesville—Eastern Hemlocks being killed by the hemlock woolly adelgid (an invasive insect from eastern Asia) and ash trees threatened by the emerald ash borer (likewise an Asian species). We also have garlic mustard (native to Europe and western and central Asia) threatening sizable patches of herbaceous plants along forest trails, and stilt grass (Asian) has shown the shade tolerance to move into forests and disrupt life on the forest floor. Autumn olive (Asian) and privet (Asian) are spreading throughout the area. Various vines, including Japanese honeysuckle (obviously Asian), Oriental bittersweet (ditto), mile-a-minute vine (eastern Asia), porcelainberry (temperate Asian), kudzu (Asian), and English ivy seem to be everywhere, overrunning patches of woods, smothering trees, shrubs, and choking out other plants. All of these are easily noticeable, starting with the area where we will access the trail. Although invasives will be easy to spot, their impact will be harder to gauge.
For this outing, I won’t spend time on all the various invasive insects, plants, birds, earthworms, and other creature we will encounter. That would be a book in itself. But I do want to focus in a bit of detail on a few representative species, to show something of their history and the tools of their success. Because this is a blog on forests, and because they’re what I know best, those will be trees.
It’s hard to imagine a location more susceptible to invaders than this stretch of the Rivanna. The Free Bridge area where we’re starting is a meeting point of so many vectors for invasives--pedestrians, cars and trucks, wind currents rising and draining each day, and a river flowing below. The parking lot where we’re meeting is at the intersection of Rt. 250, the old road to Richmond before I64 was built and now the main bypass for Charlottesville, and High Street, one of the main streets out of the downtown area. The lot sits at the base of Free Bridge, which carries seven lanes of Rt. 250 across the Rivanna River, and is at the bottom of a natural basin, surrounded by hills. A popular paved section of the Rivanna Trail runs along its edge, busy with hikers, dog walkers, runners, and bike riders.
The Rivanna has been an important transportation route since the colonial era. Thomas Jefferson’s father Peter Jefferson built a mill at Shadwell in 1757, and Thomas Jefferson himself promoted navigation on the river, organizing the clearing of rocks and obstructions from its channel. The original Free Bridge was built in 1801, and before that the site was a low-water ford. Heavy human traffic has been using this same route for more than two hundred years, first with seeds stuck to trousers or the mud of boots.
Many of those travelers, in addition, were deliberately carrying plants and seeds, which they would use to ornament their yards and the city’s streets. Seeds also arrived in horses’ hooves, and in their manes and tails. They rode in on the spokes of horse-drawn carts and wagons. More recently, seeds have been blown here by wind currents and carried by the draft of cars and trucks, or on the vehicles themselves.
The road crosses above a river that has transported more human traffic, as well as floated water-borne seeds downstream, along with plants and pieces of plants that have been plucked from the ground by raging water, then dropped on the flood plain where they’ve taken root--a sizable grove of bamboo we’ll see along the trail, a number of willows, a scattering of Mimosa The Rivanna was a busy trade route for more than a century, with mills dotting its shores and flatboats hauling goods all the way to the James River and Richmond.
It’s not just the volume of traffic by land and water which has historically funneled through this area that makes it so susceptible to invaders. It has also been the site of frequent severe disturbance--everything from clearing to trail building to paving to road construction to bridge construction to shopping mall construction. If invasives have a heaven, it could resemble this spot. To the knowledgeable eye, it’s a mess. At every level, invasives are dominant, natives sparse.
Yet despite all the problem species I’ve mentioned, we’re fortunate here--at least comparatively so. We haven’t been besieged by the uber-successful and ultra-destructive species that dominate the news and shape the public perception of invasives. Sea lampreys eliminating fisheries in the Great Lakes. The mongoose devastating native bird populations in Hawaii. Water hyacinth choking waterways throughout the southeastern U.S. with dense mats of floating plants. Nutria destroying hundreds of miles of wetlands along the Louisiana coast. Melaleuca, forming monocultures so thick and tightly interwoven in the Everglades area that they eliminate all competition. Kudzu, smothering so much of the southern U.S. that it has been dubbed “the vine that ate the South.” Tens of thousands of Burmese pythons, some as much as sixteen feet long, decimating mammal numbers in the Florida Everglades.
When it comes to invasive trees, they are, by and large, opportunists, moving into disturbed areas. When foresters talk about disturbances, they generally mean one thing: the forest’s closed canopy has been suddenly ripped open, allowing sunlight to fall all the way to the forest floor. The new opening might have been created by a lightning strike, or a blow-down, or a fire, or a hurricane, or some other natural phenomenon, but in the grand scheme of things, the enormous, unrelenting disturbance is human movement and the work of our hands and machines. Invasives trail in the wake of our new roads and highways as cheerfully as if we had built them with their convenience in mind.
Which makes a natural lead-in to the first invasive we’ll encounter on our walk, because on this end of the bridge, growing right up against the guardrail, we’ll pass a small grove of “Trees of Heaven.” Its scientific name is Ailanthus altissima, and it is often referred to simply by the genus part of its scientific name, Ailanthus. Frustrated homeowners, turned off by the tree’s odor (frequently compared to cat urine or rancid peanut butter), the litter from its numerous seeds and easily-broken twigs, the hazards of its often-hollow trunk, and the difficulty of ridding it from their property, convert the common name from “tree of heaven” to “the tree from hell.”
The grove is made up of young shoots that are uniformly eight to ten feet tall. With a little prowling around, we’ll find a stump, about six inches diameter, where someone cut the parent tree down, and deduce that the young shoots have all sprouted from the first tree’s’ roots. The parent tree here was a typical “volunteer.” Tree of Heaven is an opportunist that appears in any neglected or disturbed area—vacant lots, alley edges, property lines. Or just follow a bulldozer. You’ll find Ailanthus springing up along new roads, railroad embankments, right of ways, interstate highways, housing developments, and areas that have been logged.
The shoots we’ll be looking at illustrate why Ailanthus is so tough to get rid of. When it is cut, Ailanthus fights back with a vengeance. Simply hacking away at it is a lost cause. Any roots or fragments left behind will almost certainly regrow.
A friend looked at a photo I took of these sprouts and gave a small shudder. “They’re spooky,” she said. And her sense of their menace may have something to it. Tree of Heaven is allelopathic, meaning that its decaying leaves release a chemical which poisons the ground around it to eliminate its rivals. Seedlings are baby assassins. They can grow a taproot within three months after they germinate, and the young trees grow so rapidly they shade out native trees that try to compete. (Ailanthus may be the fastest growing tree in the United States.)
Tree of Heaven is the dandelion of the tree world. It now arrives unbidden and unwanted; it thrives with zero care; and it’s the devil to get rid it of once it has a toehold. It prospers in the face of pollution, poor soil, drought, salt, heat, neglect, or hand-to-hand combat. And like a dandelion, it looses hordes of seeds to be scattered by the wind.
These small Trees of Heaven are a good starting point to talk about invasive species, because Ailanthus could be considered the prototypical invasive. It took the classical route from east Asia to America, introduced by an adventurous gardener in 1748, and was sold commercially from the mid-1800s on. Now it’s hard to imagine someone deliberately planting it in the U.S. It’s not picky about soil, has few insect enemies or diseases, matures early. and seeds prodigiously. Ailanthus is dioecious, meaning that male and female flowers grow on separate trees; a single female produces as many as 350,000 seeds, with a high germination rate. The twisted seeds are centered in a papery sheath, and are about the size of an ash seed. They hang on the tree in large clumps that persist all winter, releasing slowly over a long time and spiraling off hither and thither with each breeze.
With its long compound leaves, Tree of Heaven can be confused with Black Walnut (and like Black Walnut, is one of the last trees to emerge from dormancy in the spring) as well as sumac. If you’re in doubt, breaking off a twig and taking a sniff will erase it; the smells of both walnut and Ailanthus are unmistakable.
Contemplating a future in which native forests are further decimated by disease, insects, and climate change, and Ailanthus makes ever greater inroads, another friend--Tim Tigner, a woodworker and retired forest health specialist--tried to make the best of it. “You know,” he said, “the wood isn’t really that bad to work with, if you don’t mind the color.”
Virginia’s Department of Forestry has explored just that possibility, in a project designed to see if the spread of Tree of Heaven could be controlled by using its wood in a number of ways--for charcoal, for pulp, for cellulose, for lumber, or for cabinetry and furniture. Several attractive pieces of furniture created from Ailanthus by area woodworkers grace the hallway and rooms of the Department’s headquarters in Charlottesville as samples of what can be done. One of the attractions of the project is the prospect that a market could be developed which would actually help fund control measures--that is, the Ailanthus could actually pay for the efforts to combat itself.
Ailanthus is the ideal candidate for a project of this type, since it is the most abundant non-native invasive tree in Virginia (and many other states), grows quickly, becomes a relatively large tree, and is present in high enough volumes that it might make commercial applications practical. Charlie Becker, who heads up the project, was quick to caution me that they don’t want it to be too successful. They don’t want to create an incentive to grow more Ailanthus.
A few steps away from the Ailanthus sprouts is a young Siberian Elm, which has rooted among the rocks laid at the base of the bridge. This seemingly inhospitable site feels like home for the Siberian Elm, whose drought resistance, indifference to cold, tolerance of poor soils, and general ability to weather hard times and tough places makes it a natural in places like this. In fact, there are long stretches in the gritty strip running along the railroad track cutting through town where these elms are almost the only tree or shrub you’ll see. Their heavy crops of wind-borne seeds, carried along by the slipstream of passing trains, has made a long, linear, elm thicket that parallels the track.
The Siberian Elm is sunlight-needy, so we are far more likely to see it and become familiar with it here in the city. It doesn’t manage well in the shade, and can’t compete in a forest environment. Elsewhere in the city--along Locust Avenue, for example--there are some nice mature specimens as yard trees, but the ones we will see today are young and struggling.
It’s likely that the people who show up for our walk will recognize many of these invasive trees, but I will be surprised if many know the Siberian Elm. Even some naturalists are unfamiliar with the tree, though it’s fairly common. Maybe that’s because the tree is so inconspicuous. Its bark isn’t unusual, its simple leaves vaguely resemble those of several other species, and as a wind-pollinated tree it has no showy flowers to catch the eye or fragrance to alert us to its presence.
In fact, Siberian Elm flowers so early that even people waiting eagerly for signs of spring are likely to to miss it. With other elms, it is among the earliest of all trees to flower. When many other trees are still waiting to launch their flowering-breeding-seeding cycle, Siberian Elm has already long completed the process. Looking back in my photo library at pictures I took earlier this year, I found some of the Siberian elms we will be looking at this afternoon. They are in full flower, and the date is February 26.
Ducking under the bridge, we make a brief sortie into a patch of young growth on the north side of the highway, to examine a number of seedlings grown about head high, bristling with vicious inch-long thorns. No one in the group recognizes them, but their leaves and tiny fruit ought to look familiar, because Bradford Pears and related cultivars have been installed in shopping mall parking lots, front yards, commercial developments, median strips, and numerous other sites all over town. (“Bradford” is a named cultivar, but in common usage many later cultivars are generally referred to as “Bradford Pears” as well.) They’re beautiful for a couple of weeks early each spring, when they’re awash in lovely white blooms, and their leaves turn lovely shades of red, bronze,orange, gold, and even shades of purple in the fall.
These specimens growing in the clearing on the other side of the bridge aren’t your familiar pear cultivar, though. Birds feed on the myriad of small round pears that grow out of the profuse Bradford blossoms, then pass along the pear seeds in their droppings wherever seems convenient. The first Bradford cultivar was sterile, but had a tendency to split because of the angle at which its limbs set. Hybrids developed to correct that fault were fertile, and produce viable seeds when wandering bees cross-pollinate them with the Bradford. When we humans supply a hospitable sunny opening, the seeds sprout into the creatures we see here—not Bradford Pears, but their heavily-armed parent stock, Callery Pear, whose original homeland (as you might predict) is China.
Because the trees are so widely planted, so fruitful, so resistant to disease, and so broadly spread by starlings, robins, and other birds, the species has been tagged an aggressive invader. The tough little trees we examine, brandishing those cruel-looking thorns, certainly look the part.
Glancing across the highway to the other side of 250, we’ll see more Ailanthus at the base of the bridge, mingled with Paulownia tomentosa, or Princess Tree. The branches of the Princess Tree are still thick with last fall’s seed pods, split open, that have hung on over the winter. I may be drummed out of the naturalists’ guild, but I like Paulownia.
Its huge panicles of violet foxglove-like flowers are not only beautiful but wonderfully fragrant. Its clusters of seed capsules are ornamental. Under the right growing conditions it produces fine, highly-prized wood that is fire-resistant, water repellent, and will not warp or crack once it’s dried. It’s also a helpful pioneer species, with huge heart-shaped nitrogen-rich leaves that improve soil, and roots that stabilize the soil, preventing erosion.
Paulownia’s virtues have gained it some other admirers—or at least promoters. The first time I came across references to a “Supertree” on the Internet (A beautiful tree, easy to propagate from root cuttings, incredibly fast-growing, and producing marketable pulp and chip within five to seven years! Buy stock from me and get rich!), it turned out to be an ad for Paulownia.
Nowadays “supertrees” seem to abound, some of them patented, and some of them from other realms entirely: decision-analysis software; a $100,000 Peruvian device to scrub pollutants out of the air (claimed to do the work of 1,200 actual trees); and phylogenetic trees intended to show the evolutionary development and relationship not just of trees but all kinds of living creatures, including mammals.
Like Ailanthus, Paulownia tolerates civilization remarkably well. It can succeed in worked-out soils, and traffic and pollution don’t faze it. Also like Ailanthus, it has a few tricks in its survival kit. After fire races through an area, burning out trees, Paulownia’s roots, from safe below ground, send up numerous fast-growing stems, and the tree is back in business. Paulownia’s seed production makes Ailanthus’s 350,00 seem paltry; a single Paulownia can produce an estimated 20 million seeds! Again like Ailanthus, the seedlings grow rapidly, and the tree reaches sexual maturity early, often flowering and producing viable seeds in eight to ten years.
It also has its vulnerabilities. It has to have sunlight and doesn’t do well at all in forest shading. It is not drought-tolerant, and it is prone to root rot. Nor is it nearly as hard on its native neighbors as Ailanthus; I’ve seen it struggling to make its way through on the edge of a parking lot at Ivy Creek. Given those elements, it is hard to believe that it is a threat to forests. It may multiply in sunny and disturbed locations, like the stretch of I64 between Charlottesville and Waynesboro, where you can spot numerous specimens as you whiz along the Interstate. But again, we are talking about a human highway for an invasive, created with something else in mind.
Turning our eyes from the Paulownia, we’ll walk together across the bridge, cars and trucks whizzing by us, making it awkward to try to talk. All we can do is look at the grit, the cigarette butts, beer cans, and broken glass on the street side, or the river on our right. Or, to put it another way, tune into the left hand’s constant bursts of noise, frenetic action, and concrete; or the right hand’s quiet, steady murmur of water; calm; and the narrow band of vestigial green forest on either bank.
Reaching the trail at the east side, we’ll start down from the sidewalk into a mixture of hardwoods that includes several Ailanthus mixed in with the native Sycamores, Black Locusts, Black Cherries, and Box Elders (more about the last three later). Behind us, on the other side of the highway, a large Paulownia dominates the skyline with its picturesque crown. Here, a Mimosa is uphill on our left, next to the filling station/convenience store parking lot, not yet leafed out, with a few of its dried pea-like pods still dangling from its branches.
Mimosa, also called Silk Tree, is an intriguing creature, with exotic fern-like leaves and what appear to be pink-edged pom-poms for flowers. It’s another invasive I have a fondness for, this one for personal reasons: one stood outside the bedroom window of our home in West Africa, the home for my wife and me in the first year of our marriage.
This Charlottesville tree won’t be in bloom until July, when it will be thick with its unique, fragrant blossoms (Dr. Seuss comes to mind again), and it is easy to understand why it was such a popular ornamental when it was first introduced to the U.S. in the mid-eighteenth century. Here near the river is a likely spot for Mimosa trees. Mimosa seed pods have a waxy coating that enables them to float safely downstream, without damage to the seeds inside. They’re comfortable in a variety of soils, so gravelly deposits or sand bars or muddy banks are all workable sites, especially as the opening on the river side gives them ready access to the abundant sunlight they—like the Ailanthus, Paulownia, and Siberian Elm--require.
There is another Mimosa just around the corner from our house here in Charlottesville. It sits on the back end of a lot, and almost certainly started as a volunteer that nobody ever got around to cutting down. Its seeds spread surprisingly far in the neighborhood. We live near the top of a hill, and I can spot seedlings springing up in flower beds, walls, plantings, and gardens for three blocks as I head down toward the river, eastward. Walking our dog this evening, a block to the north, I counted seven in a neighbor’s privet hedge.
Mimosa is yet another Asian species, native across southern and eastern Asia, ranging from Iran to China and Korea. It has been heavily imported and widely grown as an ornamental in the U.S. since the mid 1700s. It’s easy to see why.
Mimosa’s flowers are not only picturesque and sweet-scented, but bloom over an extended period in the summer. Its form is graceful, and its delicate, unusual leaves, catch people’s eye as well. Plus, it’s easy to grow, is considered a good-to-excellent excellent nitrogen-fixing legume, is resistant to drought and pollution, and is said to have powerful medicinal properties. Mimosa is also highly popular with pollinators, producing abundant nectar for honey bees, hummingbirds, and butterflies over the course of its long blooming season. For an “enemy,” this a tree with an abundance of wonderful qualities!
Mimosa is the last of the exotic invasive species we’ve see on the walk. All of these species seem much more abundant than they are in truth because of their high visibility. They aren’t shade tolerant and can’t establish themselves under canopy trees in woods and forests. They are open grown or edge species, which gives them a high visibility. They’re what we see constantly along the side of the road or in a clearing or disturbed area—the places people are carving out or traveling through.
All of them except Siberian Elm have thin bark that makes them especially vulnerable to fire. Siberian Elm is vulnerable to fire anyway because of its shrubby nature, its tendency to crowd together, and its concentration in dry habitats. All of them except Callery Pear also have seeds that would be destroyed as fire raced through the duff on the forest floor. Our emphasis on fire suppression has certainly favored these trees.
Once we start into the woods, we’ll see English ivy in the shade, not just forming a thick ground cover, but numerous vines climbing the trunks of trees, racing toward the canopy. Chickweed and garlic mustard will be ubiquitous along the trail, crowding out native wildflowers. In sunnier sites, we’ll see huge swaths of woods overgrown with Oriental bittersweet here, Japanese honeysuckle there, often a mixture of both. They are so aggressive that they sometimes cover every square inch of trees and form mats of green linking them.
In places, as we walk, we’ll see trees jackknifed as if by a storm, where the sheer weight of the vines’ biomass has broken the tree tops, or sheared off branches that dangle, suspended by the very vines that toppled them. We won’t see any kudzu here, although there are a number of infestations around town and in the county.
That’s a quick look at the exotic, sometimes called alien or non-native invasive tree species locally—all of which happen to be widespread in the U.S. “Exotic” and “alien” specifically indicate that the plants originated in foreign countries; non-native is more general, often meaning simply that the plant is from somewhere other than the place or environment in which it is now found. To illustrate, Sessile Oak (Quercus petraea), a British species, would be an exotic if it were growing here in Charlottesville; Blue Oak (Quercus douglasii) growing here, far from its home range in California, would be a non-native.
For several reasons, most of our exotic invasives come from Asia, especially from China, Japan, and Korea. One of the major factors is that northern and central European species were devastated by the abrupt arrival of the last Ice Age, which, triggered by a sudden slowdown of the Gulf Stream, converted the climate from a warm temperate zone to Arctic conditions within a period of only six months, with ice and glaciers spreading hundreds of miles southward. Europe, as a consequence, became species-poor, leaving it with few potential invaders, while much of China, Korea, and Japan retained their species.
Another factor is our voluminous trade with China, Taiwan, Japan, and Korea. According to U.S. Census Bureau Foreign Trade figures, the U.S. imported $364.9 billion dollars worth of goods from China in 2010, with sizable annual increases every year since 1985, the first date shown ($3.9 billion in 1985, $4.8 billion in 1986, $6.3 billion in 1987, ..., first topping $100 billion in 2000, passing $200 billion in 2005, and $300 billion in 2007). http://www.census.gov/foreign-trade/balance/c5700.html
No authoritative inventory of the world’s native tree species exists, so assessing and measuring biodiversity is far from an exact science. Still, it’s clear that Europe has the fewest native tree species (Iceland has only three!), with the Amazon Basin of South America having the highest, and East and Southeast Asia having high species richness. Scandinavia has fewer than 50 species per country, and central European nations are in a range from 50 to 200. China, Korea, and Japan have over 1,000. China and the others are also cut off from the U.S. by the Pacific on one side, and much of Eurasia, plus the Atlantic, on the other. Thus, sheer species numbers, a huge volume of trade, and the fact that species developed in isolation from each other make the invasive potential of Asian species much higher than that of European species.
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