Mistletoe: Friend Or Foe? Phoradendron leucarpum

As folks speed down forested highways during much of the year, both sides of the road become an emerald blur that hides many secrets of the woods. With a little practice, however, motorists can differentiate various tree species based on trunk, by shape of limb, or even on subtly different shades of green, but a leafy canopy still prevents us from seeing what's within the foliage. When autumn comes and deciduous trees drop their leaves, we suddenly find evidence of hidden activity: A perfectly round two-inch hole excavated by industrious Hairy Woodpeckers; a haphazard but functional stick nest thrown together by a courting pair of Red-shouldered Hawks; a dangling, cone-shaped nursery crafted by a colony of paper-making Bald-faced Hornets; or--almost anywhere in the more temperate eastern and central U.S. states--a large, pendulous, still-green cluster of mistletoe (above right). Hilton Pond Center has mistletoe we've wanted to write about for the past several Decembers, but since this unusual plant dwells in very top-most branches of our Pecan trees it's not exactly accessible--especially for descriptive close-up photos. This week, however, a basketball-sized bunch of mistletoe fell to earth--just in time for a pre-holiday discussion of this interesting and unusual plant that, for a tree, could be either friend or foe.

The name "mistletoe" is used world-wide for a diverse assortment of plants, some of which aren't so closely related. What these flora DO have in common is their life-style: They are so-called "stem parasites" that live on branches of trees and shrubs. Mistletoes are all grouped in the Santalales (Sandalwood Order) along with Buffalonut, Bastard Toadflax, and--of course--Sandalwood, but plant taxonomists disagree about family relationships among them. Some botanists put all the North American mistletoes into the Loranthaceae while others place only our western mistletoes in that family, moving so-called mistletoes like the one at Hilton Pond Center into the Viscaceae.

There's even some disagreement about the species name of our "Christmas Mistletoe"; it's definitely a Phoradendron, but you can find it listed as either P. seratinum, P. flavescens, or P. leucarpum. So why do we even bring up this taxonomic bugaboo? Primarily to emphasize that just because plants look alike or have similar lifestyles doesn't necessarily mean they're closely related; we also like to point out occasionally that plant taxonomy is an evolving science subject to change as floral studies continue. For the record, we're siding with taxonomists who say our "Christmas Mistletoe" at Hilton Pond Center is in the Viscaceae and is properly referred to as Phoradendron leucarpum.

We should point out that "Christmas Mistletoe"--i.e., P. leucarpum--is also called "American Mistletoe" because it is found in the New World, and "Oak Mistletoe" since oaks are among those trees upon which it grows. An Old World relative named European Mistletoe, Visca album, resembles P. leucarpum and is prominent in Druid and Nordic mythology. Frigga, the Norse goddess of love, fertility, and marriage, had a son named Balder who was slain by an arrow made of European Mistletoe. Somehow Balder came back to life, after which Frigga consecrated mistletoe and gave a kiss to anyone who passed beneath a cluster of it--hence the modern-day custom of smooching under mistletoe. Druids also saw mistletoe as a symbol of peace, and the Romans may have used the plant in their Saturnalia celebrations of winter solstice. We opt for the patriotic epithet and hereafter refer to the P. leucarpum that grows at Hilton Pond Center as "American Mistletoe.")

American Mistletoe does occur primarily on the limbs of oaks, Pecan, and hickories but has been found on more than 100 kinds of hardwoods. (Western U.S. mistletoe species typically occur on conifers.) At any given locale American Mistletoe is often restricted to just one host species, i.e., it may thrive on your Pecans but not on any of your oaks. Conversely, here at 11-acre Hilton Pond Center we've found Mistletoe on just three towering old trees: A Common Persimmon near the roadside, and a Shagbark Hickory and a Pecan whose branches intermingle in the canopy behind our old farmhouse. A big Southern Red Oak whose limbs touch those of the hickory oddly bears no mistletoe.

Any host tree selectivity may indicate a unique and slightly different genetic make-up for a local mistletoe colony, or it may be a reflection of something entirely different--perhaps unusual microclimate or even unique soil minerals that potential host trees handle differently. (One Australian study showed mistletoes there were more common on roadside trees than in deep woods, primarily because water ran off highways onto roots and made highway trees less susceptible to drying.)

So just what is the relationship between American Mistletoe and the tree on which it grows? Historically, it was thought American Mistletoe was a true parasite that extends its roots into the host tree and sucks up all its nutrients, but we don't see it quite that way. When germination occurs for a well-placed, quarter-inch long, grayish-white American Mistletoe seed (above), it first sends out a hypocotyl (embryonic root) into a crevice in the host tree's bark. The tip of the hypocotyl is called a "haustorium," a specialized structure that attaches to a limb on the host and subsequently penetrates--i.e., grows into--its bark. Once inside, the haustorium invades the host's conductive tissue and begins absorbing water, minerals, and nutrients. Months or even a few years later the young mistletoe plant also sends up an epicotyl--a short stem and the first two "seed leaves" (above left)--all of which are green and capable of photosynthesis. As the seedling ages, it produces leafy stems that, in turn, branch into more leafy stems that together make the American Mistletoe look like a green bush growing on the host tree. In fact, the mistletoe is best characterized as a shrub, even though it has no terrestrial roots.

Mature clusters of American Mistletoe bloom in late fall or early winter, producing blossoms quite unlike those on most other plants (right). Individual clusters of American Mistletoe apparently can bear either staminate (male) flowers or carpellate (female) flowers--requiring cross-pollination by a neighboring cluster of the other sex--or they may be bisexual and self-pollinating. When pollination does occur, each female flower produces a white, ghost-like berry that some sources say does not mature until the following autumn. Even though mistletoe inflorescence isn't very showy, it's still the state flower of Oklahoma, as noted on a U.S. commemorative stamp that also depicts the Scissor-tailed Flycatcher (left).

Globular berries of American Mistletoe are translucent and usually occur in clusters of a half-dozen or more. In close view the berry--about three-eighths of an inch in diameter--looks almost like an eyeball (below), but the orange spot on the terminal end is simply the remains of the old flower that formed the fruit. Many frugivorous birds are attracted to succulent mistletoe berries, which have exceedingly sticky contents and often adhere to the bird's bill and feathers. As the bird wipes its bill against a twig to remove the sticky pulp, the mistletoe seed inside often lodges in the bark of a potential host tree, allowing establishment of a new American Mistletoe plant.

We can attest first-hand to the stickiness of a mistletoe berry's contents, since it took us nearly ten minutes using our fingernail, a scalpel, a paper towel, and a cloth rag to scrape off enough mucilage to clean the seed for a photo. Incidentally, there's evidence the mistletoe berry contains chemicals that speed a bird's digestive tract, meaning that any seed a bird manages to ingest will pass through it very quickly--possibly in time to be deposited on the host tree branch where the bird is still perched while eating. Much of the pulp around the seed is intact and still sticky after being voided, assuring that it sticks to SOMEthing. And that brings up the meaning of the word "mistletoe," which is derived from Anglo-Saxon words for "dung" and "twig"--an appropriate tie-in since mistletoe often sprouts where a bird has left its calling card on a branch.

It's worth noting that all parts of American Mistletoe are thought to be poisonous to humans; this includes the leathery, two-inch-long, parallel-veined leaf (above)--whose distinctive texture we find quite pleasing--but the tasty-looking berries are especially problematic. Sources vary in describing how toxic mistletoe might really be, from "ingesting just one berry results in instantaneous death" to "eating a few berries will have little effect." (An operator at South Carolina's Poison Control Center advised us by phone the latter was likely true but that eating several berries can cause severe gastrointestinal upset and dilated pupils and could lead to seizures and death. He also noted that European Mistletoe was far more deadly than its American counterpart.) Pets such as dogs may also be susceptible to mistletoe fruit, but--despite its toxicity--American Mistletoe is the host plant for caterpillars of the Great Purple Hairstreak (below), a butterfly whose larvae people probably shouldn't eat, either.

American Mistletoe's scientific epithet has interesting origins. The word leucarpum appropriately means "white berry," while Phoradendron comes from the Greek phor meaning "thief" and dendron meaning "tree"--referring to the belief that mistletoe is robbing something from its host. There's no doubt when an American Mistletoe grows with its haustorium permeating the conductive tissues of a Pecan branch that it "steals" water and minerals and, more than likely, some tree-produced nutrients from its host. It's also apparent the haustorium disrupts the Pecan's normal growth since there's often a sizeable malformed knob around the base of the mistletoe's main stem (below left). But unless the host tree is completely covered with mistletoe clusters we're not sure it's appropriate to call the American Mistletoe a true parasite, and here's our logic.

We need to remember that American Mistletoe doesn't become visible until winter, when the host tree drops its leaves and goes dormant. At that time the mistletoe still has its own leaves, which are deep green. This green indicates chlorophyll, of course, a sure sign mistletoe is carrying out photosynthesis and making its own food in the form of simple sugars. We found it interesting that even the stems of American Mistletoe seem to help with photosynthesis, a supposition borne out when we noticed on older stems the upper portion--i.e., the surface closer to the sun--was much brighter green than the shaded lower portion (below).

Since mistletoe's haustorium is so closely interwoven with the xylem and phloem of a Pecan on which it grows, it stands to reason that--especially in winter--some of a mistletoe's food production might be transferred back to the host tree at the same time the Pecan is providing mistletoe with water and minerals absorbed from the soil. Because American Mistletoe can photosynthesize and is not entirely dependent on its host for food, some authorities call it "semiparasitic," but if it actually provides a modicum of food back to the Pecan, then perhaps the relationship is really one of mutual cooperation.

Like most aspects of nature here at Hilton Pond Center, the association between American Mistletoe and its host tree is not as simple as it seems. The tree shades out its mistletoe companion all summer but provides food, while mistletoe thrives in winter sun and returns the favor when the tree is leafless and unable to carry out photosynthesis. So--for a tree--is mistletoe friend or foe?

We don't know.

POSTSCRIPT #1: Our friend Craig Tufts, chief naturalist for National Wildlife Federation, reports that American Mistletoe grows almost exclusively on Red Maples in his neck of the woods--upstate Virginia near the mistletoe species' northern limit. Craig also reminds us that one other mistletoe is native to eastern North America. It's the Eastern Dwarf Mistletoe, Arceuthobium pusillum, which occurs from Pennsylvania through New England and southern Canada, and west to Minnesota This species--which looks nothing like American Mistletoe--is a primary cause of death in Black Spruce (see photo of male shoots on Black Spruce, at right); it is less common on other spruces, Eastern Larch, and several kinds of pine.

POSTSCRIPT #2: Rachel "Gunner" Golden of the North Carolina Office of Environmental Education tells us the first and only time she fired a shotgun it allowed her to harvest mistletoe from a tall tree, all the better to brighten up the holiday season.

Be sure to scroll down for an account of all birds banded or recaptured during the week, plus other nature notes of interest.

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"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

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