Burning Issues

Fires of Old
  |  Gorp.com

The Ponderosa pine forests near Flagstaff, Arizona, bear almost no resemblance to their ancestors.

The ponderosa pine is among the more fire-dependent species of the West. Ponderosa pine forests evolved with a cycle of low-intensity ground fires every two years. Mature ponderosas develop thick bark and “self-prune” their lower limbs to mitigate the ladder effect. This forest type featured stately groups of old-growth and mature pines, with only a small number of new saplings each year. The very small survival rate of young pines was due to several factors.

Competition with a vigorous understory of wildflowers and Arizona fescue, a native grass, made it difficult for pine seedlings to even get into the ground. The very regular fire cycle incinerated most saplings that did sprout. The ponderosa pine is also finicky as far as conditions for seed germination. It needs a very clean piece of earth free of competing vegetation, followed by above-average precipitation in the fall, after its seeds are sown.

Timber harvesting has removed all but a fraction of old-growth ponderosas. Livestock grazing steadily weakened the understory community of grasses and wildflowers, allowing more sapling pines to survive, along with other more competitive shrubs. Fire suppression has allowed forest litter to build and the understory to grow ever more tangled. As a result of both logging and fire suppression, the ponderosa pine has suffered a reduction throughout its historic range. Dense thickets of shrubby growth or shade-tolerant firs now dominate lands where ponderosas have been removed by logging or severe wildfires. Bringing ponderosa pines back to these areas is extremely difficult.

In California's Lake Tahoe Basin, a similar one-two punch of logging and fire suppression demonstrates yet another problem in forests today: disease- and insect-ridden trees.

After mature western larch and ponderosa pine were logged off in the basin, fire suppression commenced. Left to its own, a mixed forest of larch, ponderosa pine, and Douglas-fir would ignite in a fire cycle averaging 5 to 30 years.

Without fire, Tahoe Basin forests today feature an unnaturally heavy concentration of fir trees—grand fir, white fir, along with incense cedar—all competing for limited space, water, and nutrients. The result is a forest of smaller, weakened trees that have become magnets for insects such as spruce budworm and bark beetles. Fir trees are not fire-resistant; they are fire-prone and burn hotter than pine. Smaller, periodic fires would have thinned these stands. Instead, a forest in this condition is loaded and primed for a major blaze. The U.S. Forest Service estimates that some 20 million acres of public forests will face higher than average mortality levels over the next 15 years as a result of disease and insect infestation.

This Tahoe-area forest resembles another forest in Idaho, which in the past supported an average of 28 trees per acre. Following the same process of clearcutting and fire suppression, the area now has 533 trees per acre—60 percent of which are dead.


Published: 28 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication

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