ecology of christmas: mistletoe

I will confess right now to not being a huge fan of the holidays. However, I am very fond of both natural history and kissing, and that has led me to investigate the botanical life history of mistletoe.

Mistletoe is not one species of plant, but hundreds. They are not all related, rather their common traits evolved independently a number of times across the globe. Most mistletoes are not terrestrial plants, but evergreen hemi-parasites. They perform photosynthesis, but send their roots into the living wood of a tree to get water and nutrients.  In most cases, this pilfering doesn’t kill the tree – which would be a bad adaptive move for the mistletoe – but can cause stunted growth and deformities in the host.

Worldwide, 66 families of birds consume mistletoe fruits, which are produced only by female plants. Forty-three bird families have species which nest in mistletoe clumps, and hundreds of insects are associated with the flowers, which have abundant nectar.

The common mistletoe in Europe is Viscum album, one of 30 species of Viscum.  This is the species that was probably the source of the custom of kissing under the mistletoe. In the U.S., there are two primary genera: the true mistletoes, Phoradendron, and the dwarf mistletoes, Arceuthobium. 

Phoradendron species are widespread and their large clumps (above) are found on many hardwood trees and some conifers. The berries (top) are eagerly eaten by birds, although all parts are toxic to humans. The seeds are extraordinarily sticky, so they cling to branches when a bird wipes them off its bill or defecates. The bird probably most closely associated with mistletoe in the U.S. is the Phainopepla (female at right). In winter, these birds defend trees laden with mistletoe, and have a specialized gut that can rapidly process the fruit – they are able to consume over 1000 berries a day.  Phainopeplas frequently nest in mistletoes, as do Cactus Wrens and  Abert's Towhees, among others.  Great Purple Hairstreak larvae feed only on mistletoes in this genus.

The dwarf mistletoes are radically different. Rather than relying entirely on birds for dispersal, dwarf mistletoe berries can eject their seeds, which travel up to 40 feet. Most dwarf mistletoes are leafless, and do little of their own photosynthesis. They grow on conifers in the American west, which respond by producing a lot of twigs and branches at the site of the mistletoe implantation, resulting in a structure called a witches broom. The damage they can cause to valuable timber species makes them unpopular with commercial forestry managers, but dwarf mistletoes, in fact, have considerable wildlife value.  Witches brooms provide nest sites for Red Squirrels, wood rats, goshawks, and Long-eared Owls and other birds. Dwarf mistletoes are the only host plants of the Thicket Hairstreak and Johnson's Hairstreak, as well as a number of other insect species.

Mistletoes are an incredibly fascinating group of plants, with ecological value far exceeding their utility as kissing prompts. You can read more here:

• Mistletoes on hardwoods in the U.S. -- Forest Service page with range maps and characteristics of common Phoradendrons in the U.S.
• Not Just for Kissing: Mistletoe and Birds, Bees, and Other Beasts -- Great article by the USGS.
• More on the mistletoe from the eastern U.S. from Hilton Pond Center (SC), including lots of close-up photos.

Phainopepla photo by Desert Vu. Oak mistletoe photo (Phoradendron leucarpum) by Steve Baskauf of Bioimages.

Comments

[John]

I think there was a story in one of the local newspapers a few years back about harvesting mistletoe by shotgun in Virginia. I am not sure how widespread that is.

[Nuthatch]

Yep, that's the way they are often harvested!

[J Daley]

My friend from Kentucky just told me that they'd harvest mistletoe with shotguns, and do so to prevent it from completely taking over and killing its host tree.

She said, too, that the tradition of kissing under the mistletoe is related to its being percieved as having abortive/contraceptive properties.

Comments about this?

[Nuthatch]

From what I read, the true mistletoes don't often kill a tree, but the dwarf mistletoes can sometimes. I didn't look much into the myth and legend behind mistletoe, although the link I provided gives some background. I came across some info on purported medicinal properties, which vary across the many genera.

[Mike]

This is fascinating, Nuthatch. Thanks.

[John]

I suspect that the tradition of kissing under mistletoe derives from its use in Celtic and Nordic rituals - i.e., pagan ritual watered down for Christian uses. Supposed medicinal properties vary according to species, and abortive/contraceptive uses seem to be associated with the American forms. Pliny the Elder indicates that the European form was used as a fertility drug. But it is difficult to find reliable sources on this on the internet.

[GreenmanTim]

Eastern dwarf mistletoe has invaded Monhegan Island off the coast of Maine and has impacted large sections of the red and white spruce forest there in combination with with a spruce beetle infestation and exposure stress. Balsam fir is less vulnerable and remains a part of the forest, but a wholesale conversion to hardwoods is underway in younger stands where white spruce is particulalry vulnerable.

[Nuthatch]

A hint of the big problem with multiple stressors in ecosystems...Monhegan sounds like a really interesting place for long-term ecological studies, although perhaps not one that will have a happy ending.

[Crafty Green Poet]

This is a really interesting post, thanks.