Monday, April 20, 2020

Welcome!


     This blog is an exploration of what forests mean to the lives we want to be living--what threats forests face, what changes they are undergoing, and what the outcome of those changes seems likely to be.  My hope is that it will be a place where news about forests in this region, your ideas and articles on forest topics, and resources for learning about trees and forests in this area can all come together in one place. 

     Among other things, the blog contains several presentations I've developed on tree identification and on the history of naturalists in America; selections from an unfinished book on forests; and various notes on trees and forests.  They're all listed on the right hand side of the blog, under "Pages," and can be accessed simply by clicking on the title of the topic you're interested in.  Photos in the materials are my own unless otherwise indicated.

      Also on the right are links to various tree-and- forest-related websites you might want to make use of.

      Your thoughts are not only welcome but strongly encouraged!  You can either post in the comment section or e-mail me directly at taorivertony@gmail.com.

Tony Russell 


Secluded Farm; October 31, 2011

Friday, January 10, 2020

Forests as Biotic Wind Pumps


An intriguing theroy has been kicked around in the scientific world for more than a decade.  It’s particularly timely right now as the world watches videos of Australia aflame.

Back in 2006 and 2007, two Russian scientists named Anastassia Makarieva and Victor Gorshkov proposed that forests have a central role in driving weather and climate for the entire planet.  The theory, while controversial, explains phenomena that otherwise remain a mystery, such as how continental interiors like the Amazon and Congo can be as moist as coastal regions, and why Australia went from being a heavily forested continent to being heavily desert.

They theorize that vast forests generate winds that pump moisture across continents and around the globe.  Here’s how:  Gas occupies less space as it converts from a gas to a liquid.  This is true when water vapor condenses to form droplets and clouds.  Because the moisture takes up less space than the gas it formed from, local air pressure drops.  

The next key is that evaporation is greater over forests than over oceans, so coastal forests have lower air pressure than the nearby sea.  Thus they draw in moist air from the ocean, and the inrush of air generates winds that carry moisture toward the interior. As this process repeats itself in successive stages, moisture is cycled towards the continent's heart.

The total volume of water involved in this process would be almost unimaginable. The researchers suggest that, at least at the time they were writing, rainforests were sending more water into the atmosphere than evaporated from the surface of the world’s oceans. The Amazon rainforest, they said, was releasing 5.28 trillion gallons of moisture every day via evapotranspiration.

The implications of this theory are enormous.  If it proves valid, it will transform our vision of forests and climate change, since even relatively minor deforestation could cause the heart of a continent to dry out.


Smoke from Australia's bushfires, as photographed by the Japan Meteorological Agency's
Himawari 8 satellite, January 4, 2020
* * *


"The Biotic Pump: Condensation, atmospheric dynamics and climate" by Anastassia Makarieva and Victor Gorshkov in the International Journal of Water, vol. 5, no. 4, pp. 365–385 (January 2010); doi:10.1504/IJW.2010.038729.  Open access. Full text available in PDF format at https://www.researchgate.net/publication/228528788_The_Biotic_Pump_Condensation_atmospheric_dynamics_and_climate

"Biotic pump of atmospheric moisture as driver of the hydrological cycle on land" by Anastassia Makarieva and Victor Gorshkov in Hydrology and  Earth System Sciences, vol. 11, issue 2, pp. 1013–1033 (November 13, 2007); doi:10.5194/hess-11-1013-2007.  Open access.  Available online at https://www.hydrol-earth-syst-sci.net/11/1013/2007/  and downloadable in PDF format.


Tuesday, February 26, 2019

A Walk along the River


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.

Monday, March 18, 2013

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.

Friday, February 22, 2013

Conservation of Tree Species by Botanic Gardens



Thanks to Ruth Douglas for forwarding the following article.  It seems especially timely, given recent discussions on the importance of trees and forests.  I've taken the liberty of breaking the text into more paragraphs to make it more reader-friendly.

                                                                    Tony  


Protected remnant native dryland forest between Mauna Kea and Mauna Loa, Hawai'i.

The Overstory #252
Conservation of tree species by botanic gardens
by Sara Oldfield and Adrian C. Newton
February 19, 2013



Importance of tree species


Trees are of exceptionally high ecological, socioeconomic and cultural importance. As the principal biomass component of forest ecosystems, they provide habitat for at least half of Earth’s terrestrial biodiversity (Millennium Ecosystem Assessment, 2005), supporting 80% of amphibian, 75% of bird and 68% of mammal species (Vié et al., 2009). Forest ecosystems play a major role in the Earth’s biogeochemical processes, and contain about 50% of the world’s terrestrial carbon stocks (FAO, 2010; Millennium Ecosystem Assessment, 2005), highlighting their importance for moderating human induced climate change.

Trees and forest ecosystems provide a wide range of benefits to people including production of timber, fuelwood and fibre, and ecosystem services such as clean water, flood protection and prevention of soil erosion from watersheds, as well as being of high cultural and spiritual value (Millennium Ecosystem Assessment, 2005; UNEP, 2009). The total value of such services has been estimated at US$4.7 trillion per year (Costanza et al., 1997). Some 1.6 billion people depend directly on trees for their livelihoods (World Bank, 2004), and forest industries contribute around $468 billion annually to the global economy (FAO, 2011). Recent research has confirmed that high plant diversity is needed to maintain provision of many ecosystem services (Isbell et al., 2011).


In situ conservation


It is generally recognized that the most effective way to ensure the long-term survival and evolution of tree species, and the ecological communities of which they are a part, is to maintain viable populations in their native environment (Kramer et al., 2011). This is referred to as in situ conservation. Typically this is achieved through the designation and management of some form of protected area, such as national parks, wilderness areas and nature reserves (Newton, 2007). The extent of the global network of protected areas continues to increase, with nearly 133,000 areas now designated, representing 12% of the Earth’s terrestrial surface (Butchart et al., 2010). Parties to the CBD recently committed themselves to raise this figure to 17% by 2020.

Despite the substantial efforts being made to support the development and management of protected areas, many are currently under threat from human activities such as urban encroachment, infrastructural development, habitat conversion, illegal harvesting and fire (Chape et al., 2005). Additional problems include policy-related issues such as weak government institutions, conflicting policies and resource tenure (Brandon et al., 1998). Because of such problems, and the fact the coverage of protected area networks is not complete, additional conservation approaches are also required.


Ecological restoration and reintroduction


The widespread environmental degradation that has occurred as a result of human activities has led to a growing interest in ecological restoration. This may be defined as the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed (Bullock et al., 2011). Ecological restoration has grown rapidly over the past few decades, both in terms of a scientific discipline and in terms of environmental management practice (Nellemann and Corcoran, 2010). Billions of dollars are now being invested in restoration actions throughout the world (Goldstein et al., 2008), supported by international policy commitments such as the CBD. Many environmental organisations and community groups are actively engaged in ecological restoration projects, but increasingly restoration actions are also being undertaken by other organisations, including governments and large companies.

A number of related terms are widely used. Rehabilitation emphasizes ecosystem recovery, without including the re-establishment of some pre-existing state as a management goal. Reclamation generally refers to the environmental improvement of mined lands, and may incorporate soil stabilization and aesthetic improvement (Newton, 2007). In this case there may be less emphasis on restoring the original biodiversity present at a degraded site, and greater emphasis on restoring productivity. Afforestation and reforestation refer to the establishment of trees on a site, in the former case where no trees existed before, and in the latter case following deforestation (Mansourian, 2005).

It is also helpful to differentiate approaches involving the restoration of entire ecosystems or ecological communities, from those that focus on individual species. For example, conservation actions might focus on restoring populations of an individual tree species that had been depleted by timber harvesting. This could be achieved by artificial establishment of individuals of the tree species concerned, for example by enrichment planting, using planting stock derived from an ex situ population. If a species has been extirpated from its original habitat, it may be a candidate for reintroduction, which aims to re-establish new, self-sustaining populations of a species in the locations where it occurred previously.

In recent years, reintroduction has increasingly been used as a plant conservation tool (Falk et al., 1996). For example, one-fourth of the plant species listed by the U.S. Endangered Species Act include reintroduction as a component of their recovery plan (Kramer et al., 2011). To be successful, reintroductions are dependent on the availability of appropriate material either from other nearby adapted populations or from suitable ex situ populations. Integrated conservation approaches will therefore generally involve an element of reintroduction of an individual species, as described in this manual. However, such reintroduction might form part of a broader effort to restore an entire ecosystem, as also explored further below.


Ex situ conservation


Ex situ conservation can be defined as the conservation and maintenance of samples of living organisms outside their natural habitat, in the form of whole plants, seed, pollen, vegetative propagules, tissue or cell cultures. As many plant species are declining in abundance as a result of human activities, and increasing numbers are becoming threatened with extinction, there is an increasing need for ex situ conservation approaches. Botanic gardens play a major role in the ex situ conservation of plants, but a number of other organisations also maintain ex situ plant collections including academic institutions, non-profit organizations, forest services and other government agencies. Such collections can have value for research, horticulture and education, but here the focus is on their potential value for conservation.
The value of ex situ collections for conservation depends on three main factors (from Kramer et al., 2011):

1) The type of plant material collected (including seeds, explants, and living plants), which varies according to each species’ reproductive biology, seed characteristics, and/or adaptability to ex situ conditions. For species with orthodox seeds (i.e. able to be dried and stored at low temperatures for many years and still remain viable), ex situ collections maintained as seed banks provide the greatest direct conservation value at the lowest cost. For species with recalcitrant seeds (i.e. not able to be dried and stored), tissue culture or cryopreservation collections can also provide high direct conservation value but at a higher cost. Living plant collections can also be of conservation value, depending on how they are collected and maintained.

2) The protocols used for collecting; in general, welldocumented, wild-collected ex situ collections that capture as much genetic variation of the species as possible will have the greatest conservation value. Botanic gardens often maintain collections of living plants represented by one or more specimens per species, and from sources that are of wild or non-wild (cultivated or unknown) origin. As only genetically diverse and representative collections are appropriate to directly support in situ conservation (e.g. through reintroduction), living collections represented by only a few individuals will often be of limited value. However they can nevertheless be of indirect conservation value, for example through research, horticulture and education. It is also important to note that ex situ collection efforts must be conducted carefully to ensure wild populations are not placed at additional risk.

3) The subsequent maintenance of viable germplasm, which plays a critical role in determining the ultimate conservation value of an ex situ collection. Without proper curatorial management, the conservation value of a collection, or the collection itself, can be entirely lost. Collections with the most direct conservation application are genetically diverse and representative of the species, and must be managed to ensure the material is genetically sound and available for conservation activities over the long-term.

Many living collections today do not meet these standards, owing primarily to genetic issues such as having too little genetic diversity, being of unknown provenance, or losing genetic diversity via drift or adaptation to cultivation and hybridization. Management of ex situ collections should also minimize the risk of loss due to random events or natural disasters (such as staff changeover, theft, fire, disease, or other catastrophic loss) by ensuring that collections are maintained at more than one site. Additionally, curatorial oversight of living collections through time is crucial to maintaining associations between collection data (e.g. provenance) and specimens.


Integrated conservation approaches


In recent years, increasing emphasis has been placed on integrated conservation approaches, in which in- and ex situ approaches are combined, often together with reintroduction and ecological restoration. The traditional idea that the role of botanic gardens was to hold cultivated stocks of threatened species during a period of habitat degradation, in what has been described as the “ark paradigm,” is no longer believed to be sufficient (Havens et al., 2006). Rather, for botanic gardens to be effective with respect to conservation, the species banking approach must be integrated with other conservation approaches focusing on habitats and ecosystems (Havens et al., 2006).

The concept of integrated conservation of plant species is described by Falk (1987), who notes the need for multiple conservation approaches to be employed. Given the variety and complexity of threats to biodiversity, a single approach, such as legal protection for a species or the acquisition of land, is unlikely to be successful. According to Falk (1987), integrated conservation is based on the assessment and synthesis of three sets of information:

(i) determination of the biological entity of concern, including definition of the target level of biological organization (such as species, sub-species, variety or race);
(ii) identification of the threats to this entity; and
(iii) consideration of the full range of conservation resources that may be brought to bear on the problem.

Integrated conservation approaches deliberately seek a broad base of information about a conservation problem and employ a wide range of complementary tools to accomplish a given objective (Falk, 1987). Such approaches are typically highly site-specific and based strongly on local context, in contrast to traditional approaches that are more general in scope. Individual approaches can be of value at different scales; for example, seed banks are well suited to conserve genetic diversity within a population, but are incapable of conserving communities or ecosystems. However, they may play a role as part of an overall integrated strategy to address diversity at multiple levels of organization.

Rather than being viewed as separate and distinct, in and ex situ conservation approaches can therefore be viewed as part of a spectrum of compatible, mutually reinforcing methods (Falk, 1987). For example, Falk (1987) provides the example of the successional management of a fire-adapted ecosystem, such as a prairie or savanna, which may involve fencing, site preparation, controlled burns, and reseeding with native species.

Such a management regime may differ from a reintroduction program only in terms of the number of years during which a particular species is absent from the site, or from ecological restoration only in terms of the number of species that are the focus of conservation efforts.

Integrated conservation of tree species therefore includes both in situ and ex situ action, linked by restoration, reintroduction and collection, to support species survival. This process can be supported by research, horticulture and education that can ultimately increase the success of conservation efforts (Figure 1). Botanic gardens and related organizations can play a major role in integrated plant conservation throughout the world, and are uniquely positioned to be able to support such efforts (Havens et al., 2006).



Figure 1. Integrated plant conservation combines in situ (on-site) and ex situ (off-site) conservation approaches to support species survival. In situ conservation protects species in their native habitat, while ex situ conservation ensures plant material is available for research, horticulture, and education activities that ultimately support reintroduction efforts, to prevent species from going extinct. (Adapted from Kramer et al., 2011)

References


Brandon, K., Redford, K.H. and Sanderson, S.E. (eds.) (1998). Parks in Peril. People, Politics and Protected Areas. Island Press, Washington D.C.
Bullock, J.M., Aronson, J., Newton, A.C., Pywell, R.F. and Rey-Benayas, J.M. (2011). Restoration of ecosystem services and biodiversity: conflicts and opportunities. Trends in Ecology and Evolution, 26(10), 541-549.
Butchart, S.H.M., Walpole, M., Collen, B. et al. (2010). Global biodiversity: indicators of recent declines. Science, 328 (5982), 1164-1168.
Chape S., Harrison, J., Spalding, M. and Lysenko, I. (2005). Measuring the extent and effectiveness of protected areas as an indicator for meeting global biodiversity targets. Philosophical Transactions of the Royal Society B, 360, 443 455.
Costanza R., d’Arge, R., de Groot, R., Farber, S., Grasso, M., Hannon, B., Limburg, K., Naeem, S., O’Neill, R.V., Paruelo, J., Raskin, R.G., Sutton, P., and Van den Belt, M.. (1997). The value of the world’s ecosystem services and natural capital. Nature, 387, 253-260.
Falk, D.A. (1987). Integrated conservation strategies for endangered plants. Natural Areas Journal, 7, 118 123.
Falk, D.A., Millar, C.I., and Olwell, P.. (1996). Restoring Diversity: Strategies for Reintroduction of Endangered Plants. Island Press.
FAO. (2010). Global forest resources assessment, 2010 Main report. FAO Forestry Paper 163. Rome, Italy. (Also available at www.fao.org/forestry/fra/fra2010/en/).
FAO (2011). State of the World’s Forests 2011. FAO, Rome.
Goldstein, J.H., Pejchar, L. and Daily, G.C. (2008). Using return-on-investment to guide restoration: a case study from Hawaii. Conservation Letters, 1, 236-243.
Havens, K., Vitt, P., Maunder, M., Guerrant, E.O. and Dixon, K. (2006). Ex situ plant conservation and beyond. BioScience, 56(6), 525-531.
Isbell, F., Calcagno, V., Hector, A., Connolly, J., Harpole, W.S., Reich, P.B., Scherer-Lorenzen, M., Schmid, B., Tilman, D., van Ruijven, J., Weigelt, A., Wilsey, B.J., Zavaleta, E.S. and Loreau, M. (2011). High plant diversity is needed to maintain ecosystem services. Nature, 477, 199-202.
Kramer, A., Hird, A., Shaw, K., Dosman, M. and Mims, R. (2011). Conserving North America’s threatened plants: Progress report on Target 8 of the Global Strategy for Plant Conservation. Botanic Gardens Conservation International US.
Mansourian, S. (2005). Overview of forest restoration strategies and terms. In Forest Restoration in Landscapes. Beyond Planting Trees, eds. S.
Millennium Ecosystem Assessment. (2005). Ecosystems and human well-being: current state and trends. Island Press, Washington D.C.
Nellemann, C. and Corcoran, E. (eds.) (2010). Dead planet, living planet - biodiversity and ecosystem restoration for sustainable development. United Nations Environment Programme, Arendal, Norway. http://www.grida.no/publications/rr/dead-planet/
Newton, A.C. (2007). Forest ecology and conservation: a handbook of techniques. Oxford University Press, Oxford.
UNEP (2009) Vital forest graphics. FAO, UNEP, UNFF. UNEP GRID Arendal, Norway.
Vié, J.-C., Hilton-Taylor, C. and Stuart, S.N. (eds.) (2009). Wildlife in a changing world an analysis of the 2008 IUCN Red List of Threatened Species. Gland, Switzerland: IUCN. 180 pp.
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ORIGINAL SOURCE

This article was excerpted from the original with the kind permission of the publisher and authors from:
Oldfield, S. and Newton, A.C. 2012. Integrated conservation of tree species by botanic gardens: a reference manual. Botanic Gardens Conservation International, Richmond, United Kingdom.http://www.bgci.org/files/Worldwide/News/SeptDec12/tree_species_low.pdf

Botanic Gardens Conservation International (BGCI) is an international organisation that exists to ensure the world-wide conservation of threatened plants, the continued existence of which are intrinsically linked to global issues including poverty, human well-being and climate change. BGCI represents over 700 members - mostly botanic gardens - in 118 countries. We aim to support and empower our members and the wider conservation community so that their knowledge and expertise can be applied to reversing the threat of extinction crisis facing one third of all plants.
Descanso House
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Telephone: +44 (0)20 8332 5953
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Web: http://www.bgci.org/