There are few autumn shows that exceed golden-leaved aspen groves, imbedded in emerald-colored conifer forests, against an azure sky. The aspen's one- to three-inch leaves are nearly round except for a point at the apex. Flattened leaf stalks (petioles) are attached to the blades at right angles, allowing the leaves to flutter or tremble in the slightest breeze. This creates the attention-attracting flash between the leaf's shiny green upper surface and silvery lower surface. The fluttering helps to keep the leaf from over-heating in the summer sun. Aspen's smooth greenish to creamy-white bark (which may become fissured on old trunks), is marked with eye-shaped black scars where shaded branches have been shed.
Aspen is very shade intolerant. Some trees reduce the rate of carbohydrate consumption when shaded to match that produced by photosynthesis in reduced light. Aspen does not. In order to prevent shaded branches from consuming more carbohydrate than produced, aspen and many other shade-intolerant trees (e.g., ponderosa pine, lodgepole pine, and larch) shed the shaded branch by forming a breakaway layer where the branch meets the trunk.
Aspen and wildlife
Sunlight streaming through the open canopy of an aspen grove enables the growth of luxuriant understory that feeds and shelters a variety of wildlife. The trail has been worn into the ground by the hooves of both wild and domestic animals.
The open canopies of aspen groves allow sunlight to penetrate through to the ground, creating rich and diverse understories of shrubs, forbs, and grasses. This richness and diversity, along with aspen's palatability, make it the most wildlife-friendly tree in the west. Moose, elk, deer, black bear, and snowshoe hare browse its bark, shoots or buds. Thirty-four species of birds are found in aspen groves. Aspen is a keystone species for ruffed grouse where their ranges overlap. Keystone species irreplaceably provide something for another species at a critical moment. Ruffed grouse chicks feed on insects in a grove's floor, and during the summer adult grouse feed on a variety of seeds, leaves, and buds. But in winter, when snow covers the ground, the grouse is dependent on the floral buds of the aspen's male (staminate) catkins.
Scar made by a young buck deer scraping from his antlers their vascular skin called velvet.
Grouse may subsist on these catkins alone, but rose hips and willow buds, which develop in the aspen grove understory, are good supplements.
Beaver are aspens' "sustained yield" foresters. A beaver requires the bark from 200 trees per year for food, and by thus harvesting the trees releases buds for sucker growth and stand replacement for a future generation of beaver.
Aspen forests along the Trail
It is not likely that Lewis and Clark encountered many aspen groves as extensive as those found throughout Colorado and Utah. Aspen distribution along the Lolo trail is pretty much as they described it (July 2, 1806)—near watercourses and in surrounding meadows. The aspen pictured here have been growing near Lahrity Lake in the Prairie of the Knobs since the end of the last Ice Age. Lewis might have seen them as he passed by in early July of 1806, but he didn't mention them in his journals.
Small groves occur in the moist areas of conifer forests, along with occasional colonies that occupy gaps in the evergreen forest canopy. Forest fires burn through the conifer forests of this region every 100 to 300 years. When this happens aspen quickly establishes an expanded stand from sucker shoots, doing so because sucker shoots grow much faster than the conifers start from seed. In time conifers will overtop the shade intolerant aspen, converting the forest back to conifers, but leaving a few aspen survivors in sunlit canopy gaps or other favored places, awaiting the next post-fire episode to re-establish by sucker shoots another moment in the sun.
Aspen groves elevate themselves above water tables at the "capillary fringe," where they can draw water up from water table, but are not saturated to the extent that their roots drown. Plants such as cat-tails have systems to transport oxygen to their roots, so they can live in anaeroboic mud and prosper, whereas most plants do not. Note the dead snags in the foreground; the lifespan of an aspen is from 80 to 100 years.
Funded in part by a grant from the Montana Cultural Trust.