Kiss Me Under the Semi-Parasitic Aerial Shrub

A closeup of mistletoe leaves. Photo: Tara Hunt, Flickr CC.
A closeup of mistletoe leaves. Photo: Tara Hunt, Flickr CC.

Mistletoe is the horror of many a person at the annual Christmas Party. Mistletoe may be associated with unwanted advances, but EcoNews readers should also know that this weird shrub is fascinating and important.

Mistletoe is a hemi-parasitic plant, meaning that it draws some, but not all, of its nutritional requirements from its host plant. It attaches to the host plant through its haustorium—the root-like structure that penetrates into the host’s vascular tissue—to slurp up water and sugar. Infections can be so bad that they can kill the host tree, either by drawing too much from the host plant or outcompeting the foliage of the host, practically replacing all of the growth. In most circumstances, however, mistletoe adds complexity and diversity
to our forests.

White mistletoe berries. Photo: Farrukh, Flickr CC.
White mistletoe berries. Photo: Farrukh, Flickr CC.

There are over 1,300 species of mistletoe worldwide. California is home to many native species, including the oak mistletoe, American mistletoe, western dwarf mistletoe, Douglas fir dwarf mistletoe, and fir dwarf mistletoe. Some of their names suggest their preferred host, others are more generalist, like the American mistletoe that can infect ash, alder, oak, willow and more. Despite being a numerous and varied species, the physical form of mistletoe is generally similar: evergreen leaves and white fruit. California is now home to some invasive mistletoe as well, including the European mistletoe.

Given that mistletoe co-evolved with the wildlife of California, it comes as no surprise that mistletoe plays an important role in forest ecosystems. Mistletoe brooms provide an excellent structure for nesting birds. These include northern spotted owls and marbled murrelets, which appear to have a particular fondness for the dense foliage—so much so that 90 percent of owl nests in Southwest Oregon are reported to
be in mistletoe.

Although mistletoe is typically poisonous to humans, the white berries provide food for birds, deer, and other mammals. And just because it is poisonous doesn’t mean there aren’t other uses. Mistletoe has a long history as a folk medicine, treating everything from infertility to arthritis, and there is ongoing research into whether the plant may contain anti-cancer properties that could
be isolated.

Birds spread the growth of mistletoe. The fruit of the mistletoe is covered with a sticky substance called viscin. Depending on the species of bird and mistletoe, the seed may either be regurgitated or defecated. The sticky viscin will cause the seed to attach to the branch where it will wait until it germinates and the haustorium wiggles its way into the bark of the tree. Mistletoe is slow growing, as the haustorium pulls nutrients from the tree until, after around five years, the first leaves emerge.

But not everyone likes mistletoe. Despite its natural role in forests, the Forest Service routinely uses mistletoe infection as a justification for logging—including in old-growth and late-seral forests—despite the important nesting platform that dwarf mistletoe provides for owls. Timber companies hate mistletoe because it can stunt the growth of trees grown for timber. If caught early enough, or if someone diligently removes the new growth, it is possible to remove mistletoe from an infected tree. Otherwise, the only way to remove mistletoe is to remove the
infected branch.

It is not clear how mistletoe came to be associated with Christmas. The usual mistletoe tradition holds that a man can kiss whatever woman stands under the mistletoe, and a refusal by the woman would bring bad luck. The first written record is from famed American author Washington Irving, who wrote in 1820, “the mistletoe, with its white berries, hung up, to the imminent peril of all the pretty housemaids.”