19 June 2008

Mistletoe: The Water-sucking Vampire of the Desert

The dominant (and in many places only) tree at Craters of the Moon is the limber pine. No, not lumber: limber. Its smaller branches are extremely, even ridiculously, flexible. You can bend them just about any which way without hurting the tree, and even tie them in a knot. This is a tree that usually grows near the tree line in alpine environments, not in lava-encased deserts. According to the guide on the morning hike, the seeds were deposited in the area by glaciation during one of the ice ages, and found loess in which they could germinate. Almost all of the soil in Craters of the Moon gets blown in as loess. The rocks haven't deteriorated enough yet to produce soil locally.

However, the limber pines have a parasite: the dwarf mistletoe. It doesn't look like much to start with, but it is rather hard on the trees. What happens is that it sends chemical messages into the host tree, telling it to send more nutrients down towards where the mistletoe is growing. In turn, this causes the tree to sprout massive amounts of small branches around the mistletoe. The collection of branches is known as witch's broom, as it tends to resemble the rough bristly shape of a classic witch's broom. Additionally, the mistletoe draws water, sucrose and other nutrients out of its host. This can sometimes kill the tree, but I saw at least one tree where the infected part had died while the rest of the tree was still viable.

Early in the park's history, the powers-that-be decided that witch's broom should be eradicated. To that end, they cut down or poisoned thousands of infected trees, many of whose carcasses are still standing. Here it suggests that the purpose was "to 'control' a dwarf mistletoe outbreak," but the signs in Devil's Orchard imply that it was more of an aesthetic decision. They didn't think the trees were as pretty with the mistletoe on them. Nowadays, there seems to be more of a live-and-let-live attitude, and an acknowledgement that the mistletoe is a part of the ecosystem, too.

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