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125 Chapter 11 Natural and Unnatural Interference Thus far we have considered only the interactions between coniferous trees and other living things. Now we come to their interactions with the nonliving: fires, the weather, and machinery. (Humans without machinery are almost harmless. With it, they are the most serious forest pests on the planet.) Fire The effect of fire on individual trees is injury or death; its effect on forests is to bring about forest renewal. Fire, like decay, disposes of dead vegetation . Without fires, as noted in chapter 8, the forest floor would become an impenetrable tangle of fallen trees, broken branches, and withered and rotting vegetation of all kinds. Dead trees and their parts contain all the mineral nutrients they obtained from the soil while they were growing, and the nutrients remain locked up inside them until the dead remains decay or burn. In cool northern latitudes, decay happens too slowly to keep up with the continual supply of dead material, hence, the value of fires to keep the forests as a whole growing vigorously. Fires cause the sudden appearance of open, sunlit, ash-covered ground where before it was deeply shaded and the surface soil was moist and organic. Therefore, the tree species that invade and succeed have different requirements than those that were burned. This is the chief form of forest succession throughout the forests in our area, and it is discussed further later. First, let’s consider individual fires. 126 the world of northern evergreens There are three kinds of fires: surface fires, crown fires, and underground fires. Surface fires burn everything flammable lying on the forest floor but do not seriously injure full-grown trees of fire-resistant species, those with thick layers of cork under the bark such as red pine in the east and ponderosa pine and Douglas-fir in the west. The open ponderosa parklands in arid parts of the west owe their existence to frequent surface fires (figure 11.1). The fires consume fallen dead branches and kill young seedling pines, but the older pines are uninjured and have enough space to grow tall and stately in the absence of competing saplings. Most pines are fairly fireproof. Trees with less cork, such as firs and spruces, are killed by comparatively mild fires. The most impressive fires are crown fires. Some fires burn only through the trees’ crowns, as in figure 11.2. Other fires are combinations of surface and crown fires. Everything above ground burns, and if quantities of fuel (dead trees, branches, snags, and logs) have accumulated since the last fire, the result is a devastating holocaust in which winds generated by the fire itself can become strong enough to snap the trunks of large trees. The third kind of fire is the ground fire. Ground fires happen only where there is an accumulation of peat. All the action is subterranean, and the only evidence that a fire is in progress is occasional wisps of smoke emerging from small holes in the ground and the pleasant, if disturbing, smell of smoldering peat. A ground fire that was started by campers but not completely doused can spread a long way under the surface without changing the appearance of the ground above. When this happens, a normal-looking forest floor may be only a thin roof over large, blackened caverns where the peat has been burned away. Ground fires are also started by surface fires. They can continue undetected for months, smoldering slowly and invisibly below innocent carpets of needles. Nowadays it is realized that it is not necessarily wise to fight all fires. A mild fire that is no threat to camps, cottages, and towns, and that is burning up a modest amount of accumulated fuel, is best left to burn itself out. If it is needlessly extinguished, the next time fire strikes there may be so much fuel that the fire becomes dangerously uncontrollable. Fire is a natural event. In wilderness country, fires started by lightning have recurred more or less regularly ever since the land became forested. Fire tends to recur in regular cycles, whose periods depend to a high degree on the climate and the species of trees forming the forest (these two factors are obviously closely related). The average cycle length for many forests is natural and unnatural interference 127 100 to 200 years; it is much shorter, about 25 years, where the climate is dry and forests of two-needle pine (jack in the east...


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