Dwarf mistletoe

a good record for a “forest pest”

Dwarf mistletoe has appeared in these essays several times, but never as the main attraction, so to say. Most of the biological research has focused on the anatomical and physiological details of the intimate relationship between this parasitic plant and its conifer hosts. I’ll avoid most of that here and highlight aspects of its ecology.

Photo by Bob Armstrong

Dwarf mistletoe is generally hated by foresters because it makes ‘witches brooms’ of distorted branches on the host tree. It has been called the worst disease afflicting conifer trees, an insidious pest, and no doubt a variety of unprintable epithets. But if you look beyond the question of board-feet to be harvested and consider the bigger picture, the biology of the species offers some interesting things. (Besides, light infections of this mistletoe do little or no harm to a tree’s board-foot values, although heavy infestations can do so.)

There are reported to be over forty species of dwarf mistletoe, all in North America, Eurasia, and northern Africa. With that many species, this way of life has clearly proved to be fairly successful. In general, each species has one principal kind of host tree, although additional hosts are sometimes used. For example, our local species is known to favor hemlocks, but it is also found occasionally on shore pines and perhaps spruces. It is typically found at lower elevations, below about five hundred feet.

All the dwarf mistletoes are dependent on their host trees for most of their nutrients and water. When a dwarf mistletoe seed lands on a twig and germinates, it takes several years to reach maturity. Male and female flowers are on separate plants, and there is some controversy about whether pollination is effected mostly by insects or by wind. Once pollination occurs and seeds develop on a female plant, the mature seed needs to be dispersed to somewhere, if it is lucky, it can grow.

The basic dispersal mechanism is called ballistic dispersal: the seed is shot off the stem explosively, travelling at a rate of around sixty miles an hour (not bad for a plant!). Each seed is covered with sticky material, which gums the seed to its landing site. If the seed lands on a needle instead of a twig, rain may wash it down to the twig, where it can eventually grow.

The maximum distance a seed travels by the ballistic mechanism is about fifty feet, and most seeds go much shorter distances than that. It is therefore difficult for the mistletoes to spread much beyond the neighbors of a host tree, unless they get help. Help cometh from birds and mammals. The sticky seeds stick to fur and feathers when an animal passes by a seed stuck on a twig or held on the parent plant. The animals can carry seeds for hundreds of yards before the seed gets wiped off on another host tree. Many different species are known to carry the seeds, including crossbills, juncos, woodpeckers, chickadees, nuthatches, warblers, jays and squirrels.

Far from being just a forest pest, dwarf mistletoes can contribute to the diversity and abundance of the animal community. For example, a study in Colorado found that both the abundance and the diversity of birds increased as the level of dwarf mistletoe (on ponderosa pine) increased. The variety of animals that use dwarf mistletoe is remarkably broad. In fact, of all the parasites in the world, dwarf mistletoe stand out as having the greatest positive direct impact on a diversity of other organisms. Leaving aside the fungi and microorganisms that take advantage of the mistletoe (though they are part of the ecological system too), consider just the animals that make use of dwarf mistletoe.

Lots of insects and spiders take refuge in the shaggy, tangled masses of distorted branches and mistletoe shoots. There is even a hairstreak butterfly whose larva specializes on the shoots of hemlock dwarf mistletoe in Washington and British Columbia. I haven’t found many studies detailing the abundance and diversity of invertebrates that shelter in the tangles of dwarf mistletoe, but it is safe to assume that the numbers are large. These small creatures are food for many small birds, including warblers, wrens, and chickadees, which often feed among the mistletoe shoots. Grouse, thrushes, and others may eat the seeds as well.

The shoots of dwarf mistletoe and the infected twigs are a popular food for red squirrels and porcupines; even deer will eat the shoots off of fallen branches. The squirrels actually prefer infected twigs of lodgepole (shore) pine, and in some regions, porcupines eat a lot of mistletoe shoots. Twigs infected by our hemlock dwarf mistletoe get gnawed by rodents, probably squirrels.

The number of animals that use witches brooms is impressive. Red squirrels, pine marten, and flying squirrels use it for nests, and porcupines may use them for shelter from cold wind. Lots of song birds have been reported to nest in the brooms, including marbled murrelets, red crossbills, western tanagers, robins, hermit thrushes, and pine siskins. Grouse use the tangles as roosts, and ravens, goshawks and sharp-shinned hawks, and some owls will nest there.

That is a very good record for a plant that is often vilified as a pest!

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