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Brooms, burls and pine tree galls

Posted: March 22, 2013 - 12:00am
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A western hemlock has a large burl on the trunk, because of a mistletoe infection.   Photo courtesy of Paul Hennon.
Photo courtesy of Paul Hennon.
A western hemlock has a large burl on the trunk, because of a mistletoe infection.

On a recent exploration in the muskegs along the Dan Moller trail, two hikers asked about the dark, knobby growths on the branches of the shore pines. The knobs are called galls, although they are not induced by insects or mites; in this case, the causative agent is a fungus known as western gall rust.

The gall rust fungus spreads by means of airborne spores. When a spore lands on a pine needle, it germinates and grows into the twig, where it taps into the tree’s vascular tissue that orchestrates nutrients and water throughout the tree. The gall rust is a parasite that lives off the tree, but seldom kills the branch. It takes over local control of the tree’s growth hormones and causes extra growth near the site of infection. It is apparently fairly long-lived, eventually maturing and making more spores that fly on the breezes to other pines.

A recent survey in Southeast Alaska found that as many as 86 percent of shore pines are infected with this gall rust fungus. Some of the galls are as large as a human cranium, but most are smaller. The majority of galls form on branches, but some afflict the main trunk, where they began when the tree was small and had needles close to the trunk. One third of the pines in the survey had gall rust infections on the trunk.

A quick look around a muskeg shows that there are dead needles “outboard” of the gall, and the end of the branch or the top of the tree has died. Very recent studies have found that it is not the gall itself that usually kills the branch. Instead, it appears that branch death is caused by secondary infections of other organisms. One secondary agent is another fungus (a species of Nectria) that invades the gall and ultimately clogs up the vascular system of the branch, killing it. Other secondary agents are insects (probably beetles or moths whose larvae dig tunnels through the gall and interrupt the flow of nutrients and water to the end of the branch). The survey indicated that about 20 percent of branch deaths could be ascribed to Nectria and about 20 percent to insects (the remainder were unknown). I have to wonder if woodpeckers can find the insect larvae inside these galls and dig them out.

Another common tree parasite in Southeast is hemlock dwarf mistletoe. Dwarf mistletoes have co-evolved with their conifer hosts for millions of years and now about 30 species occur in North America. In our rainforest, the only species is the western hemlock dwarf mistletoe, which infects western hemlocks all over Southeast. However, hemlocks north and west of Cross Sound (up the coast toward Anchorage) are not afflicted, perhaps because some climatic factor limits the distribution of this parasite.

Hemlock dwarf mistletoe commonly causes branch distortions that are called witches’ brooms. The mistletoe has altered the growth-regulating hormones of the tree, producing a dense proliferation of twigs. Sometimes the mistletoe infection starts on the hemlock trunk, where it can eventually create a large swelling called a burl. Mature infections often send up aerial shoots, especially in the upper part of the tree crown. The shoots are typically yellowish-green and leafless, capable of synthesizing little carbohydrate on their own, so the parasite depends on its host plant for food and water.

Hemlock tissues infected by hemlock dwarf mistletoe may be attacked by fungi or insects that can kill the parasite. Surviving mistletoe infections often live for decades and can alter the water needs and shade tolerance of the host tree, as well as reducing growth. Severe infections may kill the tree.

Hemlock dwarf mistletoe is a flowering plant. Male and female flowers are borne on separate plants. The flowers contain nectar and are presumably pollinated by insects. After pollination, seed development takes more than a year. Each seed is enclosed in a fruit that explosively discharges the mature seed. The discharged seed is coated with very sticky material that gums down the seed wherever it lands. If it lands on hemlock bark, on the same or a different tree, it can germinate. Most sticky seeds are flung only for short distances, but some adhere to birds or squirrels and get carried longer distances. The germinating seed sends root-like structure into the hemlock and captures nutrients from the tree’s vascular system. After establishing itself, the new mistletoe may send up its own aerial shoots and eventually flower.

The bark around hemlock dwarf mistletoe infections is sometimes eaten by squirrels. The “brooms” provide nesting platforms for marbled murrelets and other birds, and songbirds often feed on invertebrates that hide in the brooms.

What about the huge burls we sometimes see on sitka spruce trees? Wood-turners love them for the intricate figures exposed when the wood is carved. But what causes those exuberant growths? I was astonished to learn that apparently no one knows!

Many thanks to helpful local plant pathologists, who provided information and photos!

• Mary F. Willson is a retired professor of ecology.

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