Trees Around Us ~ Tree of Heaven (Ailanthus altissima)

Tree of Heaven is the dandelion of the tree world.  It arrives unbidden and unwanted; it thrives with zero care; and it’s the devil to get rid it of once it gets its foot in the door.  It thrives in the face of pollution, poor soil, drought, salt, heat, neglect, or hand-to-hand combat.  And it looses hordes of seeds to be scattered by the wind.

“Tree of Heaven” is its common name, but it’s often referred to simply by the genus part of its scientific name, Ailanthus, and they’re used interchangeably.  Turned off by the tree’s odors (the male flowers are often compared to cat urine, and the leaves and twigs to rancid peanut butter); the litter from its numerous seeds and easily-broken twigs; the hazards of its often-hollow trunk; the tendency of its shallow but wide-ranging roots to damage sewers, sidewalks, and foundations; and the difficulty of ridding it from their yard, frustrated homeowners turn the common name around: they call it “the tree from hell.”  The  picture below catches some of that whiff of brimstone.

Tree of Heaven in Winter
February, 2009

The photo, unretouched, shows the top of a large Ailanthus, bathed in the lurid light of a winter dawn, and seemingly writhing in flames.  This tree is a typical “volunteer” that sprouted in a fencerow behind Burnley-Moran School, at the top of Grace Street hill.  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.

Ailanthus sprouts
April 1, 2008

This second photo is of a group of Ailanthus sprouts.  You can glimpse them across the guardrail at the base of Free Bridge, on the right as you are preparing to exit the city, and they illustrate why Ailanthus is so tough to get rid of.  All of the sprouts in the group (the photo shows only a handful of a large number) have shot up from the roots of a single tree, after someone cut it down.  Ailanthus fights back with a vengeance; simply hacking away at it is a lost cause.  It will sprout from its stump, and any roots or fragments left behind will almost certainly regrow and create a clonal clump.

A friend looked at the photo of these sprouts and gave a small shudder.  “They’re spooky,” she said.  And maybe she was feeling something of its menace.  Tree of Heaven is allelopathic, meaning it produces a toxin that poisons the ground around it to eliminate its rivals.  Seedlings are baby assassins.  They develop 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. Live fast, die young; its average lifespan is only 30 to 70 years.

Tree of Heaven could be considered the prototypical invasive.  It took the classic, well-traveled invasive’s route from east Asia to America; was introduced in 1784; and was sold by nurseries from about 1840 on.  (As a side note, I was interested to learn that the first Tree of Heaven in North America,  the 1784 planting, was in Bartram Botanical Garden in Philadelphia, a quiet, somewhat neglected place that I liked to retreat to when I was in graduate school in that city, many years ago.)  Like many other troublesome invasives, such as Oriental bittersweet, Autumn Olive, Siberian Elm, and multiflora rose, it didn’t sneak into the county.  It was invited and promoted for its virtues.  

In fact, early on, from the standpoint of a developer or urban tree planter, the species must have seemed like the fulfillment of a wish list--a veritable dream tree.  It has few insect enemies or diseases; is tolerant of smoke, salt, and dust; establishes easily; and grows fast. It makes itself at home almost anywhere, up to and including a crack in the sidewalk.  So it was widely planted in U.S. cities in the 19th century.  Yet nowadays it’s hard to imagine someone deliberately planting it.  I say that, but a few minutes on the Internet show nurseries still offering Ailanthus for sale, with the claim that it "will add that special touch to make your lawn look great"!

That shift in our perception of the species--from adaptive and attractive to repugnant and invasive--should cause us to meditate deeply on our normal "good tree/bad tree" categories.  And when we consider that not only the import, but the disturbances and harsh urban environments that favor Tree of Heaven are the work of human hands, we might hold the tree less responsible for its behavior, and introduce our own responsibility into the equation.

In early April, when Ailanthus begins to sprout out, its new foliage can be as colorful and eye-catching as a flower, as you can see in the first photo below.  The second shows the same sprouts that looked so spooky above, this time clothed in the fresh, hopeful green of new growth.  It's worth having many impressions of a tree.

Ailanthus is dioecious, meaning that male and female flowers grow on separate trees.  When a friend commented last year that the Ailanthus were in flower, I realized that he had mistaken the colorful seed clusters on the female trees for flowers.  In our area, the winged seeds--which are called samaras, just as maples' winged seeds are--become a cheerful red as the seeds are maturing.  This is actually a trait that varies with varieties and geography.  The more common version stays a greenish yellow.  Both bleach out as the seeds fully ripen and dry.  

Tree of Heaven's invasive potential is magnified because a single female produces an astounding 350,000 seeds, with a high germination rate, and fruit production begins at an early age.  Stories abound of even one- and two-year-old Ailanthus producing seeds.   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 into the distance with each breeze.

A cluster of Ailanthus seeds

Ailanthus’s huge compound leaves--up to 41 individual leaflets--are the largest of any tree in Virginia, and act as disposable branches.  The long, pinnately compound leaves can be confused with those of Black Walnut (and like Black Walnut, it is one of the last trees to emerge from dormancy in the spring) as well as sumac.  If people are having trouble distinguishing between Ailanthus and Black Walnut, it is likely because we focus so much on using leaf collections to identify trees in school.  The reality is that, leaves aside, the two species could hardly be more different.  Their bark, their twigs and buds and leaf scars, their scents, their flowers, their fruits, their forms--nearly everything about them is distinctly different.

Newer-growth Ailanthus twigs are thick but lightweight and flimsy, with a smooth light chestnut-brown bark, and look nothing like Black Walnut twigs.  If you’re in doubt, breaking off a twig and taking a sniff will erase it; the smell of Ailanthus is unmistakable.  You could also check out the bark; that of Ailanthus resembles the skin of a cantaloupe.  Or look for the one to five rounded lobes at the base of Ailanthus leaves.  Each lobe has a small, dark rounded gland protruding on the underside of the leaf.

Ailanthus bark

Black Walnut bark

Currently Ailanthus ranks as the 42nd most abundant tree in Virginia, already surpassing many natives and moving steadily up the list.  I remember hiking through the woods at Ivy Creek and coming across an oak that had come down in a storm.  Perched in the earth atop its rootball was an Ailanthus seedling, already making its way toward the sun.  That’s important to keep in mind, because Ailanthus needs sunlight for its seeds to germinate and its seedings to thrive; thus it prospers on disturbed sites and in canopy gaps, but can’t get a toehold under a closed canopy.  It often establishes within forests along roads or after logging.  One obvious strategy for fighting the spread of Ailanthus, then, would be to minimize human disturbance of natural woodlands.  

The Virginia Department of Forestry has investigated ways of marketing products made from Ailanthus’s wood as one means of Ailanthus control.  These include things like furniture (some pieces of which can be seen at the Forestry Building on Ray Hunt Drive), charcoal, pulp, and biomass.  It’s a tricky business, trying to create an incentive for people to cut and work with the wood while not creating an incentive to grow more of the trees.  

With its increasing abundance and its many flowers per tree, Ailanthus inevitably ends up being visited by honeybees.  I don't have any firsthand experience with this, but several resources claim that the honey from the foul-smelling male flowers is itself foul-tasting--but that the nasty taste disappears as the honey ages, leaving behind an especially nice-tasting honey.  I'd welcome some input from beekeepers on their experience with Ailanthus!

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.”

          © Tony Russell, 2013

Comments: 

Peter Dutnell wrote: A note on the Ailanthus altissima.  It seems to me, quite a few have died and are dying as a result of the last two or three dry and hot summers; this also predisposes to a fungal affliction.  Now whether this means they die from the top down or bottom up remains to be seem.  I only know of one insect which affects or uses the tree, the Ailanthus webworm, and as the tree has a male and female plant, then pollination has to take place. The question is, with such a large female seed count and few male mature trees, could this effect an invasive control? The web worm Atteva punctella is the pollinator and is common in the Fall and appears to pollinate quite a few plants so that control would be ineffective. 

Tony's response:  Spoken like a true naturalist, Peter.  You have some interesting thoughts in your e-mail, and I wonder if you'd be willing to have our correspondence here posted to the blog, as I'm sure a number of folks would be interested in your follow-up.

I've been following the Verticillium fungal issue with Ailanthus for several years now.  As you probably know, a significant number of dead Ailanthus were spotted by Donald Davis of Penn State University back in 2002 in south-central Pennsylvania, and he subsequently identified Verticillium albo-atrum fungus as the killer.  He immediately recognized its potential as a biological control to help check Ailanthus's progress.  In his experiments, he used a special hatchet to drive the fungus into the tree's sapwood, where it grew into the tree's vascular system and clogged it up.  Since the vascular system runs both up and down, I don't think the direction of the disease's progress is one-way.  However, wilting leaves seem to be the first tell-tale symptom of the tree's distress.  The holdup in using V. albo-atrum as a biological control is the need to be very sure that it doesn't attack other species in the forest, such as oaks, maples, and ashes, if the fungus is introduced to kill Ailanthus.  Studies I've read to date are encouraging in that the killing fungus seems to be highly host-specific to Ailanthus.

Ailanthus webworm (now known as Atteva aurea, formerly known as Atteva punctella) is an ermine moth and an interesting story, since it has jumped species from its original larval host plant, the Paradise Tree, to make wide use of Ailanthus.  I haven't seen anything to indicate that this webworm is a potential check on the spread of Ailanthus, but I'd be glad to hear of anything you've come across that suggests that is a possibility.

Like tent catepillars, Ailanthus webworms create nests of loose webbing around their larvae in the crowns of trees and feeds on the trees' leaves.  It can't survive cold winters, but migrates north from Florida and points south each year.  As I said, I haven't seen anything that indicates Atteva aurea does enough damage to act as an invasive control.  It does serve as a pollinator for Ailanthus, but in most areas probably not the primary one.

Ailanthus webworm moth
Photo by JJ Wilson et al, from Wikimedia Commons

A 2008 master's thesis by Jessica Thompson at Virginia Tech focused on the pollination biology of Ailanthus,  She found that its flowers were easily accessible to a variety of insects and required no specialization on a pollinator's part to access pollen and nectar.  She identified a variety of pollinating visitors, with beetles most numerous and flies next.  The most frequent visitor was a beetle called Chauliognathus marginatus, commonly known as Margined Leatherwing or Soldier Beetle.

One line of thinking in attacking Ailanthus is that the very factor that makes it such an aggressive colonizer--its ability to create clonal colonies from root sprouts--also makes it highly vulnerable.  That is, if you can introduce a fungus or other disease to one of the stems in the colony, it may be spread throughout the entire colony and wipe the whole thing out.

Peter responded:    Further on the Ailanthus.  I had read of the hatchet fungal introduction  somewhere, but the info from Jessica Simpson at VT I found very interesting. The inhibiting chemical that Ailanthus puts out, does it have a name?  And how pervasive is it?

Tony wrote:  Thanks for your willingness to share our conversation, Peter.  The allelopathic chemical is called ailanthone.  The chemistry is over my head, but it's a quassinoid and highly phytotoxic (which simply means poisonous to plants).  Two interesting things about it.  One is that while it's killing plants around the tree, it doesn't have any impact on Ailanthus seedlings, which means the plant has some way of preventing being poisoned by its own kind.  And two, it loses its phytotoxicity within five days in natural soils because of microbial activity.  That makes me think the most phytotoxic period might be when leaves are falling and the ailanthone is gradually leaching out of the leaves into the soil over an extended time.

The USFS Silvics Manual says that in experiments over 35 species of hardwoods and 34 species of conifers were vulnerable to allelopathic action by Ailanthus.