Sunday, June 14, 2009

Hard-luck hemlock.

Crazy tangles of conifer branches are not uncommon in local western hemlock-dominated forests. These aptly-named "witches' brooms" are the result of parasitism by a dwarf mistletoe, Arceuthobium tsugense, an obligate parasite of hemlock and shore pine. The roots of the mistletoe are endophytic (= inside plant); they penetrate the xylem and phloem of their host, taking up both water and nutrients. Externally, the mistletoe produces short shoots with male or female flowers. The fruit that form contain single seeds, which are ejected using hydrostatic pressure. (Ptooi!)

The seeds are sticky, and hopefully, from the mistletoe's point of view, will adhere to the growing shoot of another potential host. Typically, seeds are jettisoned from the upper branches of infected trees and land on the lower branches of adjacent trees, or near the crowns of smaller, younger hemlock, which, if not too incapacitated by the parasite, will grown to launch seeds onto the next generation. In this way, the mistletoe is maintained within a host population. Clever!

Witches' brooms are a reaction of the hemlock to the mistletoe, a hypertrophic attempt to bypass the parasitic roots, causing wild and unruly shoot growth. Large, burl-like growths on the bole (trunk) can also occur. Branch loss and bole failure can follow, and parasitized tissue can also become entry points for secondary infections (fungal or insect).

I don't know if dwarf mistletoe is responsible for this hemlock losing its top, but in any case it gave it a rough time.

A detailed account of dwarf mistletoe is given at this US Forest Service site.

3 comments:

Tim said...

I like your posts because they teach me new biological terminology in everyday language. But I (and Google) can't seem to figure out what "potenital" means.

Marvin said...

Around here the witches brooms would accumulate a lot of ice during ice storms and make a hemlock truck prone to snapping, but we have no hemlock and, therefore, no hemlock dwarf mistletoe. One less thing to worry about.

Hugh said...

Thanks Tim, I fixed it.

Marvin, They tend to snap somewhat easily (or fall over, roots and all) even without the brooms. They're definitely not tough enough for your weather.