Tuesday, June 01, 2010

A Light in the Forest: Aspens and Fire

The morning after fire, burned soils are slowly cooling. Lodgepole pine seeds drift down from cones opened by the intense heat. Light breezes stir the layer of ash on the ground. Survivors—birds, mammals, and other forest life—are already seeking new homes. The forest is quieter today and smells strongly of fire. For us humans, fire is a terrifying and uniquely threatening experience—one we cannot always control or appreciate. We see only a charred landscape, and grieve. But while we continue grieving, nature simply begins again.

Already the shallow lateral roots of aspen are feeling the effects of auxin deprivation. The aboveground stems, now absent, no longer send their chemical message of living trees above, a message that has held potential new shoots at bay. Thousands of tiny buds, the primordia of aspen trees to come, are awakening to start the long process of recovery. Long, that is, by our standards. In the life of a forest, events will proceed quickly. In a year or two, there could be thirty young shoots for each mature stem that existed before the fire. Within 20 or 30 years, superficial evidence of fire disappears, leaving the aspen themselves as the primary clue.

In an aspen woodland, the life below ground is older than the forest we see, and it may well have seen fires before. In fact, aspen has an ancient relationship with fire, but it is an unusual one. With a lifespan comparable to ours (say 60 to 120 years), individual stems of aspen are short-lived as trees go. The forests they create thrive on the rebirth that fire provides. That is, until we came.

Today, aspen forests are as often rejuvenated by logging as by fire. Any method that removes the overstory trees without damaging the all-important root system is effective. In fact, researchers have found root density to be a key indicator of successful regeneration in aspen stands. Too few roots, and the aspens give way to grasses or conifers.

Of all the forest types adapted to fire, aspen is the least flammable and in many ways the most dependent on fire. Our other forests—of ponderosa pine, spruce and fir, or other conifers—burn more readily. In most seasons, aspen forests are either snow covered or too green to carry fire, and may even act as a firebreak, slowing or stopping fires that reach them. This trait is used to advantage by firefighters. In fall, however, when the lush understory has been killed by frost and the thick layer of aspen leaves has dried to a fine tinder, then aspen can burn and burn brightly.

Most fires in aspen woodlands occur in October under just such conditions. Such fires during the mid- to late-1800s gave us the aspen glories we drive to the mountains to enjoy today. We have so few deciduous trees: what would we do without the golden glow of October's aspen? Some years, extra hot and dry, give us Octobers to watch with caution, mindful of the long-standing relation between aspen and fire.

That dependence of aspen on fire has its own dynamic. Although they need fire, aspen forests are very vulnerable to it. Individual stems, with their thin bark, are girdled and killed by even moderate fires, but remember that the deeper life is underground. The parent root system must start over, each time, from scratch. Even a severe fire kills lateral roots only in the top two inches of soil. Suckers can sprout from laterals five to six inches deep if necessary.

If fires are too frequent, however, the roots will be exhausted from the efforts of renewal. If fires are too rare, occurring at intervals of more than a century or two, the aspen may grow old and die out beneath a dominant overstory of conifers, leaving no roots to replenish the site. Fire every 50 to 100 years is about right for continuous aspen, if that's the goal.

In comparison, ponderosa pine forests use a different strategy to achieve persistence through time. Frequent fires, every five to twenty years or so, thin the understory while individual trees live on, encased in their protective layers of bark. Fire delayed is made deadly. As we live with and in Colorado's forests, we are not required to choose whether or not we will have fire: we will. Our challenge is to decide whether we prefer fires to be frequent—or intense. Choose wisely, be careful, and enjoy the gentler side of aspen's October glow.

Copyright, S.L. White, 2010. Illustration copyright Jan Ratcliffe.
Originally published in Upbeat, October 1994.

No comments: