A winter walk through an oak-hickory forest is a study in earth tones.
Though gold, red and purple only a few weeks ago, the forest has faded. The forest’s brown leaves now carpet the ground and crunch underfoot.
While a few conifers continue to display vibrant green foliage throughout the year, their canopies sit high overhead, thereby diminishing their visual contribution.
But observant hikers will, however, notice scattered tufts of green vegetation growing throughout the understory. Against the otherwise-brown backdrop of the forest’s lower levels, these splashes of green are conspicuous and out of place.
These leaves appear as though they’ve sprouted directly from the tree’s branches, but they represent an entirely different organism.
You are looking at mistletoe – an evergreen parasite that makes its living mooching off trees in hardwood forests.
Parasitism is a common life strategy. In fact, some scientists estimate that parasitic life forms outnumber non-parasitic forms by a margin of 4:1. Some evolutionary lineages – such as the 5,000 or so living tapeworms (Cestodes) – are completely comprised of parasitic species.
But parasitism is very rare in the plant kingdom.
According to the Warnell School of Forestry and Natural Resources, only about 1 percent of all angiosperms – flowering plants such as grasses, oak trees and mistletoe – are parasitic.
But mistletoe isn’t just any old parasite, it is a type that botanists refer to as an obligate, hemi-parasite. This means that while it must parasitize a host, it also supports itself to some degree.
Mistletoe sucks water and sugars right out of the tree, but it also produces chlorophyll. This allows it to manufacture some of its own food via photosynthesis. Some mistletoe species don’t produce very much chlorophyll, but they all pull a portion of their own weight.
“Mistletoe” is the colloquial or common name associated with roughly 1,300 different species. Each one is a member of the Order Santalales.
Orders – like all other higher taxonomic units – vary in size, and they don’t represent equally long paths from the most recent common ancestor. But, to give you an idea of how diverse some orders are, you are in the same order with this.
Note that while all mistletoes are Santalales, not all Santalales are mistletoes. Plenty of non-mistletoe Santalales grow as shrubs or trees. This means that a parasitic lifestyle has likely evolved several times within this order.
All of this is to say that while all mistletoe species are related, they aren’t particularly close relatives.
Given this, and the fact that mistletoe species exhibit quite a bit of diversity, it is probably more helpful to think of the term “mistletoe” as a lifestyle rather than a group of closely related organisms.
There is plenty of debate regarding the classification and nomenclature applied to the many of the mistletoe species growing in the U.S., as well as the rest of the world. Botanists and the books their peers author frequently contradict each other.
The dominant species found in my home state of Georgia provides a great example of the confusion surrounding these plants.
Most authorities agree that it is a member of the genus Phoradendron, but there is widespread disagreement about the species to which it should be assigned.
The Warnell School of Forestry and Natural Resources considers the species American mistletoe (Phoradendron serotinum serotinum) but the US Department of Agriculture considers it oak mistletoe (Phoradendron leucarpum).
Regardless of the name botanists eventually agree upon for the dominant species native to Georgia, it is quite common throughout the southeastern and eastern portions of the U.S., ranging from Texas to Ohio and Pennsylvania.
But the evolutionary details of the group and academic arguments about their identifications matter little to the average nature lover. Unless mistletoe flips your geek switch and you dive down a rabbit hole of field guides and range maps, you can pretty much consider mistletoe to be mistletoe.
All mistletoe species connect to their hosts via stalk-like structures termed haustoria (singular: haustorium). The haustorium not only anchors the plant to the tree, but it also serves as a conduit for those delicious resources it is stealing.
Different species exhibit slightly different foliage, but the clumps growing on the pear tree outside my window are all covered in inch-long green leaves. They’re thick and bear a waxy cuticle, an adaptation that helps to slow water loss. This is part of the reason that mistletoe remains green during the winter. Most broadleaved trees that retain their leaves year-round – including holies and southern magnolias, among others — use the same trick.
Most mistletoe clumps are about the size of a basketball, but older specimens occasionally form round clumps approaching 3 feet in diameter – about the size of a beach ball.
Mistletoe is a dioecious species, meaning that individual plants have either male or female flowers, unlike most plants, which bear flowers of both sexes. Whether male or female, these flowers are pale green, inconspicuous, and bloom during the late summer or early fall. A few months later, they give way to clumps of small, white berries, called drupes.
Like many other winter fruits, mistletoe drupes attract plenty of avian attention. The tiny mistletoe seeds lurking inside are essentially hitchhikers, waiting to ride the bird-intestine expressway into different parts of the forest.
Once excreted, the seeds often manage to get a foothold in the crook or a branch or a rough spot on the trunk thanks to their sticky coating. Shortly thereafter, the seed begins a six-step process allowing them to penetrate the branch, set up shop and produce shoots and leaves.
Mistletoe is a keystone species, meaning that it impacts the surrounding habitat more than you’d normally expect, given its relatively low abundance.
Other important keystone species include:
But mistletoe is one of the few plants to earn the keystone label.
The plant’s influence is obviously felt by the trees it inhabits, but it also plays an important role in the lives of many other species too. For example, a variety of nectar-feeding insects sustain themselves, in part, by visiting mistletoe (they also inadvertently help pollinate the plants in the process).
But birds are the biggest beneficiaries of mistletoe. Many rely on the berries as food, while others use the clumps of dense leaves for nesting.
Scientists have even collected data empirically demonstrating the value mistletoe provides birds.
In 2011, researchers David M. Watson and Mathew Herring removed most of the mistletoe from an experimental plot. The following year, they found that a staggering 35 percent of the resident bird population had disappeared.
I can’t find any similar studies that have been conducted on mistletoe species found in the U.S., but it certainly wouldn’t be surprising to find that American species are similarly important to bird populations.
Homeowners and property managers are rarely in the value mistletoe provides to birds; they’re typically more concerned with the threat it presents to their trees.
Mistletoe’s theft certainly comes at a cost, but the price paid varies from one tree to the next. Some trees continue to thrive and grow while sporting a weighty collection of mistletoe plants, while others struggle to survive once colonized.
A mistletoe plant or two growing on a 25-year-old, otherwise-healthy oak isn’t likely to bring the tree crashing down next week. Chances are, the tree will continue to grow and thrive, if, perhaps, slightly less vigorously than before.
On the other hand, significant mistletoe infestations can certainly cause trees to decline. And although they probably don’t directly kill many trees, mistletoe plants seem to show up in a lot of the crime scene photos.
Like most other parasites, mistletoe presents the greatest threat to trees that are already dealing with other stressors. Trees suffering from water deficiencies, insect infestations and other health problems are much more likely to be troubled by mistletoe than healthy individuals.
Removing mistletoe – including the entire haustorium, which penetrates deep into the branch – can help to improve the health of an infected tree, although the degree to which this benefits the tree is unknown and likely minimal. Regardless of whether removing the parasite will tangibly improve the health of the tree, neglecting to remove all portions of the haustorium will lead to regeneration, and the infestation will continue.
Accordingly, it’s rarely very feasible – or for that matter, advisable – to remove mistletoe effectively. Nevertheless, it is worth having a professional (and by that, I mean an ISA-certified arborist) inspect the tree and recommend a sound course of action.
Just be sure to leave removal to the professionals. Mistletoe typically grows above heights at which amateurs should dabble.
Like all the most interesting components of the natural world, mistletoe is complicated and characterized by contradiction.
It is taxing to the trees on which it grows, but it is an important component of many ecosystems.
You could say that mistletoe is bad for the trees, but good for the forest.