Mistletoe is frequently spotted hanging above lovers' heads in terrible holiday specials--but only during one month of the year. That makes it easy to forget that more than 1,300 species hang in forests year-round, parasitizing thousands of tree species around the world. Or, rather, hemiparasitizing, which means the plant is partially self-sufficient: it has its own leaves to collect sunlight to convert into energy, but feeds off of a host tree for water and nutrients.

Semantics aside, mistletoe has been long-considered the epitome of the word "parasite:" a blood-sucking bush that plugs into trunks and branches, strangling and killing trees around the world. This airborne parasitism is so successful that mistletoes are thought to have evolved it five separate times on their own.

But the purely negative opinion is starting to change. Mistletoe may parasitize trees, but it also provides valuable services to forest animals. Its flower nectar and berries are a food source for birds, insects (such as moths, beetles, bees and flies), and mammals. Its greenery is a resource-rich patch used as shelter and, for some birds, a sweet nesting spot.

How does mistletoe get up on a tree trunk in the first place? In short: sticky poop. Mistletoes rely heavily on birds and mammals to distribute their seeds; in fact, 90 species of birds in 10 families are considered mistletoe specialists. The berries are brightly colored and covered in a sticky glue called viscin, which can survive an animal's digestive tract. Thus, even after its been pooped out, the seed can still stick to the nearest branch... or to another animal to ferry it elsewhere.

Once smeared onto a proper spot on a tree trunk or branch, the mistletoe seed germinates and sprouts a penetration peg that mechanically drills down into the tree until it hits a nutrient pipe. The peg is then followed by a haustorium, the root that plugs into the arboreal host to draw off nutrients and water. The next step is growing leaves, flowers and berries, a process that can take 2-6 years in some species.

Once it blossoms, the magic begins, and the mistletoe plant sprouts flowers and nectar to feed insects and birds, such as hummingsbirds. After they are fertilized, these flowers turn into berries... and the process begins all again.

Mistletoe also provides a broader ecosystem service: it provides abundant nutrients to the forest floor when it drops its leaves. Most leaf-losing trees hoard their precious nutrients, and suck their leaves of every last nutrient-rich drop before they let them fly. But, because of their parasitic nature, mistletoe plants aren't too concerned with preserving nutrients--they can always just steal some more from their host! So down go the nutrient-rich leaves, which stimulate the growth of other plants (and the host trees themselves).

This may be a more important ecosystem process than producing fruit, flowers, and nesting shelter. In a recent study, scientists removed the mistletoe from stands of eucalyptus forest in Australia. Three years later, one-fifth of vertebrate species previously living there had left the woodlands--an astounding result. Because many of these species that abandoned ship ate insects on the forest floor, the authors believe this leaf-litter mechanism explains the biodiversity loss more than the removal of mistletoe berries and flowers.

Its those same beautiful berries that attract birds and bees to spread mistletoe seeds that compelled Europeans to decorate their living rooms with it around Christmastime. (Well, that and pagan culture.) So next time you see some hanging at a party, forget the kiss--grab your girl or guy by the arm and explain that you want to get hemiparasitic with them.

Watson D.M. & Herring M. (2012). Mistletoe as a keystone resource: an experimental test, Proceedings of the Royal Society B: Biological Sciences, 279 (1743) 3853-3860. DOI: 10.1098/rspb.2012.0856

Watson D.M. (2001). MISTLETOE - A KEYSTONE RESOURCE IN FORESTS AND WOODLANDS WORLDWIDE, Annual Review of Ecology and Systematics, 32 (1) 219-249. DOI:10.1146/annurev.ecolsys.32.081501.114024 (PDF)

Photo Credits:

(1) White mistletoe berries, Flickr user swamibu CC BY-NC 2.0

(2) Mistletoebird, Flickr user 0ystercatcher, CC BY-NC-SA 2.0

(3) Dwarf mistletoe, Jill Siegrist, CC BY-NC-SA 2.0