This traditional holiday decoration becomes very attached to plants nearby.
Window On Nature
By Lowell & Kaye Christie, F47246
In the fall, as the leaves drop from deciduous trees, most of the color disappears as well. But in many areas of the United States, patches of green remain high in the branches. They are clumps of mistletoe, hidden by foliage much of the year but exposed in early winter. These plants survive only because of their close association with their hosts. You see, mistletoe is a parasite.
Perhaps that’s being too harsh. Broadleaf mistletoe has green leaves and carries out photosynthesis to produce its own food. But it can live only when attached to another plant that serves as its source for water, along with other nutrients dissolved in that water. That makes it a hemiparasite, which is at least one step up from being a total freeloader.
More than 1,300 species of mistletoe have been identified worldwide, and although some live on only one type of plant, many are less particular. But they all grow on the branches of trees or shrubs and can’t survive on their own. Instead of having a root system that penetrates the soil, when mistletoe seeds germinate, a modified root called a haustorium penetrates the bark of a host tree or shrub and forms a connection “” kind of like an umbilical cord “” with the water-conducting tissues in the tree.
The haustorium gradually extends up and down inside the branch as the mistletoe slowly grows, and it may take several years before the plant matures and produces flowers and seeds. Eventually the ball-shaped hunk of greenery may weigh as much as 90 pounds, sometimes breaking branches, which will kill both the falling branch and the attached mistletoe.
Although mistletoe doesn’t improve the health of its host, some birds, butterflies, and other insects depend on it. Three butterfly species “” the great purple hairstreak, the thicket hairstreak, and the Johnson’s hairstreak “” need the mistletoe for survival. The great purple hairstreak feeds and lays its eggs on the broadleaf mistletoe, while the caterpillars of the other two dine on the dwarf mistletoe.
The dwarf mistletoe is smaller than the broadleaf variety and has tiny, scale-like leaves. While the broadleaf mistletoe depends mainly upon birds for spreading its seeds, the dwarf seeds explode from the fruit, traveling horizontally into trees as much as 30 to 40 feet away.
Because the dwarf mistletoe conducts less photosynthesis, it takes more nutrients from its host, which may cause more damage to the tree. Sometimes the branch becomes deformed, creating a dense growth pattern called “witches’ broom,” where a mass of shoots grow out of the branch at a single point. This is not good for the tree, but it can be very good for some birds.
In the western United States, witches’ brooms make nest sites for birds such as the northern spotted owl and the marbled murrelet. One study found that 43 percent of all spotted owl nests were associated with witches’ brooms. The same is true of many raptors’ nests, including great gray owls, long-eared owls, goshawks, and sharp-shinned hawks. The thick cover provides protection from predators.
Even mistletoe without witches’ broom attracts birds. A United States Geological Survey (USGS) researcher in northeastern Oregon found that 64 percent of all Cooper’s hawk nests were built in mistletoe. And, of course, smaller birds feast on the mistletoe berries and help the plant extend its range.
The mistletoe produces small, sticky white berries that mature between October and December, when other food sources may be scarce. Many birds eat these berries, but our favorite example is the shiny black desert dweller, the phainopepla. It has a much closer association with mistletoe than most other birds.
When we lived in Arizona, visitors often would ask us where to find specific birds. One was the phainopepla. It’s a member of the silky flycatcher family “” the only one whose range extends into the United States. Our method of locating this particular species was to search in the desert washes for clumps of mistletoe. If there were berries on the plants, there was an excellent chance of finding a phainopepla nearby.
The male is black (the female, gray) with an obvious crest, a long tail, and ruby-red eyes. In flight it shows white wing patches. Its chief food is the desert mistletoe; in addition to eating the berries, it often will build a nest in the plant as well. When the berries are available, the phainopepla will eat more than 1,000 of them in a day. The bird rarely drinks water, even though it loses about 95 percent of its body mass in water on warm days. It gets its water from the mistletoe berries, which derive their water from the host plant.
Because the berries are sticky, some adhere to the bird’s bill or feathers, so the seeds are transferred either by bird droppings or when the bird later preens its feathers or wipes its bill on a branch. The sticky substance on the berries keeps them in contact with the new host plant until they can penetrate the bark. Here’s a case where a bird helps cultivate a future food source while eating this season’s crop.
Although many people, particularly commercial forest managers, consider mistletoe a pest, from a naturalist’s point of view, it’s just another part of the ecosystem. In fact, the presence of the plant often leads to increased diversity. In one study it was found that a high abundance of dwarf mistletoe in a forest means that a larger number and variety of birds live there. Another study documented three times the density of cavity nesters in a forest with abundant mistletoe.
Robert Bennetts, a research scientist with the USGS, points out that while mistletoe does have an adverse effect on host plants, when some of the trees die they leave open spaces and snags that benefit different populations of plants and animals.
Mistletoe “” friend or foe? Like many things in nature, it may depend upon your point of view.