Urban Jungle
The changing natural world at our doorsteps | Illustration and text by Patterson Clark
WEEKS:
March 20, 2012
Callery pear Pretty tree going rogue
Pretty tree going rogue
At the margins of the city, commuters can't help but notice tight groves of white-blooming trees in sunny waste areas, old fields and roadsides.
A first glance, the trees might appear to be cherries, which bloom at about the same time and have similar five-petaled flowers. However, these are more likely to be thorny and aggressive escapees of Callery pear, the seeds of which were probably flown in by starlings engorged with the tree's pea-sized fruits.
The Chinese native hasn't always been so invasive. In 1962, the USDA made commercially available a thornless variety of Callery pear, naming it Bradford pear. The tree, hailed for its bright-red autumn foliage and profuse spring blossoms, was widely planted as an ornamental in the United States. It was unable to fertilize itself and therefore couldn't yield fruit. But as the trees matured, they began splitting apart in windy weather and heavy snowfall.
That disappointment inspired horticulturists to develop new varieties of Callery pear that were able to grow up and weather storms. However, this injection of genetic diversity allowed cultivars of Callery pear to cross-breed and fertilize one another, yielding fruits that would transform a once tame and thornless tree into a problem weed that stabs anyone who gets too close.
SOURCES: USDA, Biological Invasions, BioScience, Virginia Tech
March 27, 2012
American robin song It takes a night owl to beat the early bird
It takes a night owl to beat the early bird
People rising very early in the morning may hear the pre-dawn singing of American robins. Urban robins crank up their loopy warbling music even earlier than their rural counterparts.
Although city noise might play a role in that — an early start helps urban birds avoid the din of rush hour — it's more likely that they are responding to light pollution.
In 2003, biologist Mark W. Miller studied robin song starting times in Arlington's Lyon Park neighborhood and compared them with data collected from the same location in 1929.
In '29 the first robin songs began about 45 minues before sunrise, but 74 years later, when Lyon Park nights were awash in electrical light, robins tended to break their silence more than an hour earlier, often before any hint of dawn.
Miller's study also included robins near the White House, in an area with even higher levels of artificial light. Those robins began singing about 3 hours earlier than the '29 Lyon Park robins.
Miller said that the starting times for White House robins might be even earlier, because sometimes they were already singing when he arrived to monitor them.
First morning song of American robins in Lyon Park
DURING BREEDING SEASON
*The half-hour before sunrise or after sunset; car headlights are generally not required.
SOURCES: Mark. W. Miller, University of Alaska at Fairbanks; "Apparent Effects of Light Pollution on Singing Behavior of American Robins," the Condor; Biology Letters, the Royal Society
April 3, 2012
For Eastern hemlocks: Bad news and good news
This year's mild winter wasn't much help to the East Coast's oldest evergreen trees, the Eastern hemlock, which are beleaguered by the hemlock woolly adelgid. The notorious insect pest from Japan, which latches onto the tree's needles and relentlessly sucks its sap, began spreading from the Richmond area in the 1950s and has since killed thousands of acres of hemlocks from southern Maine to Georgia. In particular, it has devastated hemlocks in the southern Appalachians.
Brad Onken of the U.S. Forest Service expects a significant increase in and spread of the insect because of the warmer-than-normal season. However, several consecutive years of good moisture conditions have strengthened hemlocks, he says, "so the immediate impact on tree health will hopefully be minimized."
Overwintering adelgids cover themselves in a white, woolly blanket of wax into which each can lay hundreds of eggs. The bugs are exclusively female.
Did unseasonably warm weather trick the insect into a vulnerable stage at which it could be whacked by last week's freezing temperatures? No, says Onken. Only near-zero temperatures or sustained freezing temperatures seem to knock back the adelgid.
Eggs hatch into crawlers, which settle either on the same hemlock or travel to new hemlocks by sailing off on breezes or hitching rides on the legs of birds — a good reason to place bird feeders far from hemlocks.
Urban hemlocks can be treated with insecticides, but that's not always practical for forest trees, so biologists have been using biological control agents. Among the most promising are a black lady beetle from Japan — the size of the Q on a quarter — which appears to be aiding a recovery of hemlocks in Connecticut, and a beetle from the Pacific Northwest that is increasing in numbers and "is likely to be a major player in the long-term solution," says Onken.
SOURCES: Carole Cheah, Connecticut Agricultural Experiment Station; Fifth Symposium on Hemlock Woolly Adelgid in the Eastern United States
This year's mild winter wasn't much help to the East Coast's oldest evergreen trees, the Eastern hemlock, which are beleaguered by the hemlock woolly adelgid. The notorious insect pest from Japan, which latches onto the tree's needles and relentlessly sucks its sap, began spreading from the Richmond area in the 1950s and has since killed thousands of acres of hemlocks from southern Maine to Georgia. In particular, it has devastated hemlocks in the southern Appalachians.
Brad Onken of the U.S. Forest Service expects a significant increase in and spread of the insect because of the warmer-than-normal season. However, several consecutive years of good moisture conditions have strengthened hemlocks, he says, "so the immediate impact on tree health will hopefully be minimized."
Overwintering adelgids cover themselves in a white, woolly blanket of wax into which each can lay hundreds of eggs. The bugs are exclusively female.
Did unseasonably warm weather trick the insect into a vulnerable stage at which it could be whacked by last week's freezing temperatures? No, says Onken. Only near-zero temperatures or sustained freezing temperatures seem to knock back the adelgid.
Eggs hatch into crawlers, which settle either on the same hemlock or travel to new hemlocks by sailing off on breezes or hitching rides on the legs of birds — a good reason to place bird feeders far from hemlocks.
Urban hemlocks can be treated with insecticides, but that's not always practical for forest trees, so biologists have been using biological control agents. Among the most promising are a black lady beetle from Japan — the size of the Q on a quarter — which appears to be aiding a recovery of hemlocks in Connecticut, and a beetle from the Pacific Northwest that is increasing in numbers and "is likely to be a major player in the long-term solution," says Onken.
SOURCES: Carole Cheah, Connecticut Agricultural Experiment Station; Fifth Symposium on Hemlock Woolly Adelgid in the Eastern United States
April 10, 2012
Explosive bittercress fruits
Most people associate early spring with flowers and tender vegetation, not ripening fruits. But for hairy bittercress, which began flowering in early February, it's seed-slinging time!
Anyone pulling bittercress from the garden this time of year has experienced the half-foot-tall weed's assault of seeds. Botanists use the term "ballistic seed dispersal" to describe the explosive nature of ripened bittercress fruits.
The pods, or siliques, consist of a central membrane flanked by two outer strips, or carpels, with about 30 seeds in between.
As cells on the outer surface of each carpel dry out, they shrivel, creating tension on the exterior face of the carpel. Meanwhile, the inner surface of the carpel is kept moist and plump by adjacent mucilage.
When cells at the bottom end of the carpel finally give way, the carpel snaps into an upward curl, propelling seeds as far as 16 feet — an effective strategy for spreading into new areas.
Bittercress reaps another benefit from ballistic seed dispersal: An insect climbing onto a ripe silique, with the intention of making a meal out of it, may suddenly find itself catapulted several feet away, far from its would-be meal.
April 17, 2012
White oak pollen: job done
They are clogging your gutters, carpeting your decks and landing in your salad at that outdoor cafe. White oak catkins are being shed by their trees after concluding their pollen release for the season, much to the relief of people tortured by oak allergies.
Warm spells followed by cool temperatures and low humidity created ideal conditions a couple of weeks ago for the three-inch-long male flowers to release their golden-green dust, a process that each flower accomplishes in two or three days.
Breezes deposit only a wee fraction of the airborne pollen onto the freshly emergent and inconspicuous female flowers that now appear on the stems of new oak shoots.
A pollen grain fortunate enough to land on a female flower's pink stigmas will sprout a pollen tube that grows toward the ovary to fertilize an egg, which could develop into a ripe acorn that will drop in early September.
When open air space among the oak branches is no longer needed for fertilization, young oak leaves rapidly expand, and the spent catkins float to the ground to gather in drifts, turning rusty brown as they decay.
Before July, next year's catkins will begin forming inside buds on the oak's twigs. By autumn, the packed catkins will be mature, poised to expand in the spring of 2013 to discharge yet another round of blond powder.
SOURCES: Forest Science, U.S. Forest Service, "The Ecology and Silviculture of Oaks," staff reports
White oak catkins (male flower stalks) with details of a male flower and its open pollen sacks, lower left, and a female flower, lower right.
April 24. 2012
Inchworm on a thread
Suspended by a barely visible silk filament, a small green caterpillar dangles underneath a box elder tree.
It's a geometer moth larva, otherwise known as an inchworm, named after its distinctive looping and lunging gait. The caterpillar appears to measure its path in units of its own body length.
But a climb back up a silk line happens at a much slower, steadier pace, as the inchworm uses only its front three pairs of legs to hoist itself back up to the leaf from where it leapt.
Inchworms don't bungee jump for kicks; they reserve that option for escaping predatory insects. Inchworms have lousy eyesight but are quite adept at detecting vibrations, especially those made by the wasps and predatory stink bugs that hunt them.
Testing inchworm sensitivity, entomologists Ignacio Castellanos and Pedro Barbosa used a machine to imitate various oscillations caused by wind, rain, birds, herbivores, wasps and stink bugs. Bird vibrations caused the caterpillars to remain motionless and try to blend in with the plant. But predatory insect vibrations inspired inchworms to anchor a silk line and bail, using long lines to avoid wasps, shorter lines to escape stink bugs.
Some parasitic wasps, however, are aware of the maneuver. They can locate the silk line and either reel up a desperate caterpillar or slide down to inject eggs inside it; the wasp larvae then slowly consume their host.
Sources: Pedro Barbosa, Department of Entomology, University of Maryland; Animal Behavior; Annals of the Entomological Society of America; Invertebrate Survival Journal; Ecological Entomology
May 1, 2012
The bluer a robin egg, the better
Not much point in looking around for a nearby nest when you find an American robin eggshell on the sidewalk.
Soon after a chick hatches, the female robin grabs the eggshell and flies off to drop it far from the nest. Leaving the baby behind for a few moments is worth the risk, since the bright white insides of the eggshell can attract predators.
But before the egg hatches, blue-green pigments on the outside surface of the egg might help with camouflage. Pigments might also strengthen the egg and help protect it from solar radiation.
A robin coats her eggs with the same turquoise-hued compound found in our bile and bruises, biliverdin, an important antioxidant. Female robins with higher concentrations of biliverdin in their tissue lay darker, more vividly colored eggs, which apparently sends a strong signal to males.
"Males seem to use egg color to gauge the quality of their mate and the eggs she lays, putting more effort into rearing babies when they are more likely to survive and prosper," says Robert Montgomerie of Queen's University in Canada.
With Philina English, Montgomerie determined that when eggs are more colorful, male robins will invest as much as twice the amount of energy helping feed nestlings.
SOURCES:Behavioral Ecology and Sociobiology; Bird Coloration: Function and Evolution; Stanford University
May 8, 2012
Bulbous buttercup: A toxic invasive plant with a hint of promise
In May, many open lots and meadows are flecked with the sun-tracking, shiny yellow flowers of bulbous buttercup, a poisonous herb that thrives in well-drained, clay soils.
Mowing the buttercup before it sets seed won't do much to eradicate the invasive plant. Its ground-hugging leaves will continue to nourish an underground bulblike stem, or corm.
By month's end the plants will begin to wither as the corm enters dormancy for most of the summer. By autumn, the corm will have sprouted a lateral bud that will grow into a rosette of leaves, which will feed the growth of a new corm by next spring.
Bulbous buttercup is native to Europe. In France the corms were sometimes dried, cooked and eaten as a starch supplement during times of famine. The fresh plant, however, is quite toxic. Some historical accounts describe European beggars rubbing it onto their skin to raise blisters and evoke sympathy from passersby.
But even a noxious invasive weed can sound a promising note: Modern investigations of the plant's bitter toxin, protoanemonin, have revealed that the compound possesses antimicrobial properties that can inhibit growth of methicillin-resistant Staphylococcus aureus, or MRSA.
Sources:British Ecological Society, Phytotherapy Research, American Weeds and Useful Plants, Purdue University
In May, many open lots and meadows are flecked with the sun-tracking, shiny yellow flowers of bulbous buttercup, a poisonous herb that thrives in well-drained, clay soils.
Mowing the buttercup before it sets seed won't do much to eradicate the invasive plant. Its ground-hugging leaves will continue to nourish an underground bulblike stem, or corm.
By month's end the plants will begin to wither as the corm enters dormancy for most of the summer. By autumn, the corm will have sprouted a lateral bud that will grow into a rosette of leaves, which will feed the growth of a new corm by next spring.
Bulbous buttercup is native to Europe. In France the corms were sometimes dried, cooked and eaten as a starch supplement during times of famine. The fresh plant, however, is quite toxic. Some historical accounts describe European beggars rubbing it onto their skin to raise blisters and evoke sympathy from passersby.
But even a noxious invasive weed can sound a promising note: Modern investigations of the plant's bitter toxin, protoanemonin, have revealed that the compound possesses antimicrobial properties that can inhibit growth of methicillin-resistant Staphylococcus aureus, or MRSA.
Sources:British Ecological Society, Phytotherapy Research, American Weeds and Useful Plants, Purdue University
May 15, 2012
Silver maple: a better source for ethanol
As a spring storm advances, winds plow through the branches of a silver maple, rolling over the leaves to reveal their silvery-white undersides.
The flash of silver, for which the tree was named, could also be signaling its potential wealth as a resource for biofuels made from cellulose, the fibrous carbohydrate molecule found in plants.
Ethanol made from corn starches is currently cheaper to make than cellulose ethanol. But it yields only 26 percent more energy than is used in its production. Cellulose ethanol yields 80 percent more energy than is used to make it, with net-contribution of greenhouse gases.
Silver maple is an ideal source of cellulose. It can be planted in a wide variety of sites. The tree sprouts profusely after being cut, "is a fast-growing species and is relatively pest-resistant," says Eric Holzmueller, from the Southern Illinois University Department of Forestry. "In order to produce the maximum amount of biomass over time," says Holzmueller, "the species would be planted in a plantation, in rows like corn."
Silver maple trees spaced about 11 feet apart can, after three years, yield 6.3 dry tons of biomass per acre annually, potentially producing more than 500 gallons of ethanol.
Saplings, ground into pellets, can be sent to a biofuels plant, where enzymes from microorganisms break the cellulose into sugars, which are fermented to produce ethanol.
Silver maple might help lead us into clean energy independence, but it is a lousy tree for landscaping. Its roots can penetrate and clog water systems, and although its rapid growth is desirable for making shade, the wood is weak, brittle and prone to rotting, causing many limbs
and twigs to shed from mature trees after heavy snowfall and during windy storms.
SOURCES:Department of Energy, National Plant Diagnostic Network, Oak Ridge National Laboratory, Genera Energy, Microbiology and Molecular Biology Reviews
May 22, 2012
Peirce Mill restoration bears fruit
A 19th-century apple orchard begins the first phase of its return to Washington this month at freshly restored Peirce Mill in Rock Creek Park.
Three winesap apple trees will be in the ground by June, and as many as two dozen will follow in the next year or two. The Peirces grew winesaps in their orchards flanking Rock Creek in the mid-1800s.
They also sold trees from a nearby nursery. An 1857 Peirce catalogue listed about 50 varieties of eating and cider apples, says Steve Dryden, author of "Peirce Mill: Two Hundred Years in the Nation's Capital." That number "may seem large," says Dryden, "but such breadth was normal in the 18th and 19th centuries, when orchardists and the public relished a floral diversity developed over the centuries."
"Almost every farm had an orchard, as hard cider was preferred to water — safer, it was thought — though the easy availability helped produce a very boozy atmosphere in our country."
Dryden assumes that the Peirces made hard cider and brandies from the fruits they harvested. A stone house near the mill was known in Peirce family lore as "the distillery."
The new orchard is a project of the Friends of Peirce Mill. Students from the District's Harriet Tubman Elementary School will help with planting and upkeep of the trees. "We hope they will be involved in the care of the orchard for years to come," says Dryden, program manager for the nonprofit group, which raised $1 million to restore the mill.
"We may do demonstration apple-pressing to make fresh cider at some point," he says.
Meanwhile, after a decade-long restoration, Peirce Mill — the only operational water mill in Washington — is once again grinding corn and wheat. Demonstrations of the milling process are scheduled for May 23 and 31, and June 6.
Owned and managed by the National Park Service, Peirce Mill is open 11 a.m.-4 p.m., Wed.-Sun.
May 29, 2012
Privet The scent of commencement
The scent of commencement
For many students, their first breath of summer freedom is thick with the heavy fragrance of white-blossomed Ligustrum, or privet, which is commonly planted on school campuses. Perfuming many an outdoor graduation ceremony, the scent invites attendees to close their eyes and drift into a languid torpor.
Privet odor has a stimulating effect on moths, which at night will actively seek out the highly visible flowers. During the day, bees and butterflies are also drawn to the nectar-laden blooms in May and June.
Scientists have closely studied the alluring effect that privet perfume has on the small cabbage white, an invasive butterfly from Europe, Asia and North Africa.
More than 30 compounds comprise the plant's floral scent, and five of those chemicals attract the cabbage white. Two of the most powerful attractants are well-known aromatic compounds:
• Phenylacetaldehyde, also found in chocolate, has a honeylike odor. Scores of insects generate the compound on their own to communicate with other insects.
• Phenethyl alcohol has an aroma that is also present in orange blossoms and in the flowers of hyacinths. The compound, used as a fragrance in personal care products, is also deployed by many insects as an attractant.
Although each chemical by itself can attract cabbage whites, the most powerful draw happens when all five compounds are combined, creating a synergistic effect.
The butterfly reveals its interest by uncoiling its proboscis, anticipating a sip of late spring's dizzy nectar.
SOURCES: Journal of Chemical Ecology, Environmental Working Group, pherobase.com
A small cabbage white, Pieris rapae, prepares to land on Japanese privet, Ligustrum japonicum.
