Mistletoe: A Merry Parasite

Oak mistletoe, Phoradendron leucarpum, growing on a hawthorn (Crataegus sp.). Photo by Jeffrey S. Pippen

Despite occasionally being a bit of a Grinch, something I love about the holiday season is that we fill our houses with wild flora. Dozens of conifer species are displayed in homes as Christmas trees, wreaths of hemlock and fir hang from doors, and sprigs of American holly brighten up rooms with their glossy green leaves and red berries. And, of course, don’t forget the mistletoe. Most famous in America as a kiss-inducing holiday decoration, mistletoe is an important component of forest ecosystems that is incredibly valuable to many wildlife species and can exert pressure on even the largest trees.

Mistletoes are flowering plants in the Santalaceae family. Yes, Santalaceae. It seems unlikely, but the family’s name has nothing to do with Kris Kringle. It is derived from the sandalwood genus, Santalum, which is also a member. There are around 1,300 mistletoe species worldwide, and 2 rough groups in North America: American mistletoe (the genus Phoradendron) and dwarf mistletoe (the genus Arceuthobium). Both types are found in the Chesapeake Bay Watershed, though American mistletoe occurs more in southerly parts of the watershed and dwarf mistletoe occurs in sub-boreal conifer pockets of northern Pennsylvania and New York.

All mistletoes are hemi-parasitic, meaning they parasitize trees but are not fully dependent on them for all functions. Mistletoes attach to stems of live trees and send root-like structures under the bark to tap the tissues for nutrients and water. The mistletoe’s xylem and phloem (water and nutrient transport, respectively) cells connect with the host plant’s xylem and phloem, forming connections called “sinkers” that the hemi-parasite uses to siphon resources. Mistletoe leaves are green in color and can complete photosynthesis for energy, but would be unable to survive without the tree. It is thought that they evolved in the tropics, where there is stiff competition for soil, and moved into temperate zones as the Pleistocene glaciers melted away.

Cross-section of a stem infected with a Phoradendron mistletoe. Photo by Carolina Biological Supply Co.

Phoradendron means “tree thief” in Greek; American mistletoes parasitize deciduous trees and are obviously visible in the winter when the host’s leaves have fallen. They develop evergreen foliar growth that can develop into quite large, branching, orb-shaped masses. These leafy growths are what are commonly used as decorations. Phoradendron mistletoes will survive as long as their host tree or the host branch does. They produce berries which contain viscin, a sticky substance that helps the fruit stick to new stems. The berries either fall off and colonize stems below or are dispersed by birds. The sticky seeds can adhere to the plumage, feet, or beaks of birds to be scraped off onto new stems while preening, or get eaten to and later deposited on branches in droppings. Bird droppings are the primary way the plants spread; indeed, the word “mistletoe” means “dung on a twig” in Anglo-Saxon.

Arceuthobium means “juniper-living”; dwarf mistletoes parasitize conifers and must have different life histories to cope with the constant shade of their hosts’ needles. They grow beneath the bark for several years before sending out foliar growth, causing infected branches to swell up or grow into dense tangles called “witches’ brooms”, and can even cause crown die-back in severe infections. Their foliar growth is small and does not last long; male stems wither and die after pollinating and female stems persist over the summer to bear fruit in the fall. Dwarf mistletoe berries also contain viscin, but are not spread by birds. Instead, water pressure builds up in the berry cells, while simultaneously the cells at the base of the berry die. Eventually, the pressure builds to a point where the berry explodes, expelling the seed at up to 50 miles per hour, with a range of up to 60 feet! The seed will stick to anything it hits, hopefully a suitable conifer that it will adhere to all winter and begin rooting into in the spring.

Eastern dwarf mistletoe (Arceuthobium pusillum). Arceuthobium mistletoes are not as attractive for ornaments, but are important members of forest ecolosystems. Photo by Joseph O’Brien, USDA Forest Service

A dwarf mistletoe seed being shot out by built-up water pressure. Photo credit: U.S. Forest Service.

Though we often only think of mistletoe at holiday parties, it plays a significant ecological role in our forests. American mistletoes, while toxic to humans, are important food sources for many forest wildlife species. They bear fruit from late fall through early winter, a time when there isn’t much else for resident wildlife to eat. The berries are an important food for birds and small mammals, and the foliage is eaten by porcupines and larger mammals like deer and elk if they can reach it. Dwarf mistletoes are less important for food but the thick, scruffy, “witches’ brooms” that they create are excellent nesting sites for songbirds, red squirrels, flying squirrels, hawks, and owls. Studies in the western US have shown that a whopping 43% of spotted owls and 64% of Cooper’s hawks build their nests in “witches’ broom” tangles resulting from dwarf mistletoe infections.

“Witches’ brooms” caused by infection of spruce trees by eastern dwarf mistletoe, Arceuthobium pusillum. The growths are not formed by the mistletoe, but are plant reactions to the infection. The spruce on the right seems to be uninfected. Dwarf mistletoe infections can kill their host trees, but the “witches’ brooms” are fantastic cover for wildlife. Photo by Daniel Mosquin.

Oak mistletoe (Phoradendron leucarpum) berries are very important for wintering wildlife. Photo by Mary Keim.

Mistletoe is important for many insects too. Their early-blooming flowers are important nectar sources for bees and other pollinators when not much else is available, and many insects only live on mistletoe foliage. Three butterfly species in the US are mistletoe obligates (meaning their caterpillars can only feed on mistletoes), but only one, the great purple hairstreak (Atlides halesus) is found in the Chesapeake Bay Watershed.

American mistletoes rarely kill their host tree; their life history strategy is to remain with the host and produce berries as long as possible, so they need their host to survive. Recall that dwarf mistletoes, on the other hand, die after ejecting their berries. They do not need their host to remain healthy and can weaken their host substantially, increasing the likelihood of mortality. Dead trees are critically important for cavity-nesting birds and roosting bats, and once they fall over, a host of organisms that will live in or consume the log. Dead trees also mean more light to the forest floor, allowing vigorous herbaceous and shrub growth and enabling seedlings and saplings to grow into the next generation of canopy trees. These small canopy gaps are functionally patches of young forest, where the flush of vegetative growth yields more food and cover for wildlife. Many species require canopy gaps to successfully breed, and they are also important cover and food sources to deer, wild turkeys, ruffed grouse, bats, and other forest wildlife.

Like many of our traditions, appreciation for (and yes, kissing under) mistletoe developed and evolved as societies rose and fell. European mistletoe (of the genus Viscum) closely resembles leafy American mistletoe, so European colonists and subsequent immigrants easily transferred their customs over to North America. Many European cultures, from the ancient Greeks to the Nordic peoples and the Celts, revered oak trees. Viscum mistletoes grow on oaks and stay green throughout the winter, when the trees have lost their leaves (and, it was possibly thought, their “life forces”). This is likely why mistletoes were imbued with such importance, especially over the winter; they held onto life when even the mighty oak could not. They were seen as vessels of strength and vitality and were often used in ceremonies marking the winter solstice, which of course are mirrored today by Christmas and other traditions and celebrations around that time of year.

As Christianity swept through Europe the old traditions became muddled, but mistletoe remained an important winter symbol. It was often hung in doorways to deter demons and witches, and was widely thought of as a universal healer. The custom of kissing beneath it may have come from the belief that it stimulated fertility, or perhaps from an ancient Norse myth that resonated through the centuries. In the story, the beloved son of Frigg, the goddess of love and beauty, was killed by an arrow made from mistletoe. The gods all agreed that the plant should never hurt anyone again, but rather be dedicated to happiness and usefulness. Frigg’s tears became the white berries of mistletoe, and the goddess swore that she would kiss anyone who walked beneath it.

No matter what the origin of the kissing custom is, American mistletoe makes a lovely winter decoration. Rather than an excuse to smooch, it’s a great opportunity to talk to guests about forest ecology and the fascinating biology and cultural history of this strange little parasite. Look up into the empty branches of oaks and sweetgums for a cheery clump of mistletoe this winter. It will likely be too high up to reach, but that’s a good thing; mistletoe is a valuable member of our forests and will do a lot of good up there in the treetops.

To learn more about forest ecology and management, contact Ryan Davis at rdavis@allianceforthebay.org or visit forestsforthebay.org.