Aspen Ecology

By David Cronenwett

A cluster of gold-leafed trees sits in the center of a photo my son recently sent me. He lives in a northwest Montana valley dominated by a mix of grass and conifers. During fall, the sun’s low angle illuminates a subtle pallet of muted prairie color. But in the picture, just beyond stacks of fenced bee boxes, a familiar group of deciduous trees stands out like an incandescent beacon. Aspen are like that, drawing the most attention in September-October, but even outside of autumn, their brilliant green foliage differentiates them from nearby conifers in spring and summer. In winter, bare limbs and stark white boles lend a haunting aura to any landscape. Next to wetlands, aspen habitat is one of the most species-rich ecotypes in the West, a fact well understood by birders, hunters and photographers. Trying to quantify how much of our native wildlife use aspen forests for part of their life cycles would be a time consuming exercise; from butterflies to grizzly bears and much in between, aspen play a critically important ecological role wherever they occur.

Aspen (Populous tremuloides in the West) like all in their genus, are a water-loving, shade-intolerant tree. But simply defining aspen this way does not give a full picture of its remarkable ecological value and natural history. They are better thought of as a “super organism”; the group of trees you see are not individuals, but genetic replicas of one another joined together by a shallow, subsurface root network. This “clone” expands and contracts over time in response to natural disturbances like grazing and wildfire. Each clone is either male or female (dioecious) and can intermingle and overlap with others but are easily differentiated as the timing of leaf-out and autumnal color change is slightly offset between them. Where aspen occur, there is always water near the surface and stands thrive best in abundant sunlight. Healthy clones contain a mix of older, middle aged and young “trees” (stems) and support a great assortment of forbs and shrubs. The rich understory of an aspen clone is many times more diverse than the floor of a conifer forest.

This combination of moisture, light, and diversity of plant and insect species in an aspen clone will draw wildlife from miles away. On the prairie, an exposed, windy and generally arid environment, aspen clones provide food and refuge. Grizzly bears hit them in late spring to graze on cow parsnip and if lucky, may find a winter-killed elk carcass. For Montana birders, there is scarcely a more happening place to be than an aspen forest during spring migration; numerous species of warblers, raptors, thrushes, woodpeckers, vireos and others can be found there. Many “life lists” have been augmented by careful stalking among the aspen at this time.

While this species spreads most obviously by sending new root sprouts from existing trees within the clone, they can also reproduce via seed. For a long time it was believed that conditions for the establishment of new clones by seed were extremely rare because aspen are best adapted to germinate in a cool, wet climate, like it was just after the last glacial period. Evidence of new seed-propagated clones has since been confirmed in recent decades, most notably in Yellowstone after the fires of 1988. Fire and aspen are intimately tied together in the West. Without the periodic disturbance of wildfire, nearby conifer species will inevitably surround, invade, and starve a clone of both water and sunlight. But watching a stand rebound from a fire is an amazing experience; I’ve been monitoring the recovery of aspen in an area of the Teton River country severely burned in 2007. New aspen sprouts began to appear the following spring after the fire, and today many are now taller than I am. Evidence of elk, moose, bear, and raptors is obvious in these clones, which will likely expand their size substantially in the absence of conifers.

Aspen are highly resilient and can be incredibly long-lived. Many of the naturally occurring aspen we see today have probably been there a very long time. Evidence suggests that under the right conditions, it is possible for a clone to exist for thousands of years, ebbing and flowing across the landscape over millennia, making it one of the oldest living organisms on earth. However, there is new concern that rapid changes in climate could threaten aspen in some areas of its range. In the mid-2000s, clones in Colorado, Utah and elsewhere suddenly began to die. The problem, called Sudden Aspen Decline (SAD), was widespread in the West for several years and left ecologists scrambling for answers. It was eventually determined that extreme heat and drought conditions in the region beginning in 2002 were largely responsible. The disturbing piece of the puzzle is that the areas of aspen hit by SAD happened in habitats, at least according to one study, projected to become devoid of aspen by 2060 as a result climate change. However, other studies suggest that additional carbon in the atmosphere combined with more frequent wildfire in a warmer world could see aspen expand its range in some places. Whatever the case, more extreme weather events like drought, severe heat, and freak cold snaps will certainly change where this organism, and many of the species that rely on it, will live.

My son’s photo shows a vigorous aspen clone in its breathtaking but ephemeral fall colors. I imagine him exploring that little forest of gold as he grows up, observing the wildlife drawn to it and witnessing its many moods and changes over time. Maybe it has been there for a thousand years or more and with luck, will persist a great deal longer. I know that clone is bare now, its leaves long since taken by wind, but my son tells me he’s still up there, watching.