Ascending the West Glacier Trail

Secret: Domatia act as houses for dwelling mites in the leaves of the alder

Posted: Sunday, April 23, 2006

The trail starts in young conifer forest, mixed with alder, in an area that was still covered by glacier ice about 100 years ago.

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The first part is flat and crosses several small streams that are used by spawning salmon in fall. The trail then ascends gradually through forest that is mostly composed of alder and willow - an earlier stage of vegetation succession on land that emerged from the ice during the 1930s and 1940s. If you look at the West Glacier Trail and Mount McGinnis from the visitor center, it is easy to pick out the "trim line," where deciduous vegetation on the lower slope and conifer vegetation above meet and mark the transition from older to younger forest.

A mile or so up the trail, there's a sheltered bench. Here an unimproved side trail leads down toward the lake and up on "The Rock," the big rocky peninsula that sticks out from the west shore.

The ice began to leave the south side in the early 1960s, and by 1990 about half of The Rock was exposed; the north side has been exposed for less than 10 years. Look for a beaver lodge in the northwest corner of the lake at the base of The Rock. The beavers have been actively cutting deciduous trees nearby, and in fall the submerged pile of cut branches builds up in front of the lodge. Bark from this pile of branches is used for winter food, accessible below the ice.

The south face of The Rock is still used by numerous nesting gulls, but as the vegetation increases over time, the gulls will eventually move out. The open areas of The Rock are good places to look for the early-blooming purple mountain saxifrage (the name means "stone-breaker").

Queen bumblebees emerge early from hibernation, gather food to start their annual brood, and pollinate the purple flowers.

The alders on The Rock and along the main trail may exhibit one of the best-kept natural history secrets in Juneau. Some of the alder leaves have tiny tufts of hair where the side veins meet the midrib. These miniscule structures, called "domatia," are houses for ridiculously small mites. The mites are defenders of the leaf, eating fungi and small insects that attack leaves, but they are more numerous in the warmer sites in Juneau. The domatia are not galls, which are induced by an insect laying eggs; instead, they are produced independently by the plant and can be seen, with a microscope, in the unopened leaf bud. Mites use these houses for all domestic purposes - molting their exoskeleton, mating, egg-laying, and shelter.

The relationship between mites and plant is mutualistic, in that both parties benefit. Why only some of the leaves produce domatia is still a mystery. The mite-plant relationship is far more common on deciduous trees at lower latitudes in North America, but it is found from the treeline in the north to the treeline in the southern hemisphere.

Mountain goats in the Pacific coastal ranges commonly have a seasonal altitudinal migration. Here, they often descend from their summer haunts high on McGinnis and Stroller White to the forests near the trail and even go out on The Rock. They eat grasses, sedges, moss and lichen, various herbs, and some woody vegetation, especially in winter. Both male and female have sharp horns used for stabbing opponents; they do not usually fight by crashing together headfirst, like Dall and bighorn sheep. Although they are uncommonly agile, and their hooves are designed to provide exceptionally good traction, the main causes of natural mortality are probably avalanches and falls.

The main trail continues to ascend over rocky terrain to several vantage points with views of the Mendenhall Glacier. A landslide in one stream drainage has created a broad expanse of rock and (usually) shallow water that must be crossed if the goal is access to glacier views. Near the end of the trail, an unofficial route up Mount McGinnis branches off the main trail.

• Mary Willson is a retired ecology professor and a Trail Mix board member.



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