Kiss Me Under the Hemi-Parasitic Aerial Shrub

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Wednesday, December 18th, 2019

Mistletoe is the horror of many a person at the annual Christmas Party. Mistletoe may be associated with unwanted advances, but for the ecology nerds, we 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. They attach 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 by outcompeting the foliage of the host, practically replacing all of the growth but in most circumstances mistletoe adds complexity and diversity to our forests.

There are over 1,300 species of mistletoe worldwide, and 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 mistletoes 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, including the northern spotted owl and marbled murrelets, which appears to have a particular fondness for the dense foliage–so much so that 90% of owl nests in Southwest Oregon are reported to be in mistletoe.

Although mistletoe is ordinarily poisonous to humans, the white berries provide food for birds, deer, and other mammals. And just because it is poisonous doesn’t mean we can’t find 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 can 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 dward mistletoe provides for owls. And timber companies hate that mistletoe 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.”

 

*this article previously ran in the December/January issue of the EcoNews.