Page:Popular Science Monthly Volume 85.djvu/345

Rh form of evergreen tree with the same amount of wood and foliage which would be less liable to injury from snow and ice than the spruces and balsams. The tamarack has an additional safeguard in that it loses its leaves in winter; and at the northern limit of the forests it is said to grow comparatively tall and straight while the spruces around it are much stunted.

Burned areas, in which all the trees have been killed by fires sweeping through their crowns, are and always have been, from all accounts, common throughout the spruce region (not only in the East, but also in the Rocky Mountains, where forests of different species but similar aspect predominate); and many great fires, involving loss of life and much property, have become historic. In northern Michigan and doubtless in many other places where spindle-shaped conifers abound posters warning against the dangers of allowing fire to spread greet the traveler at every turn; and some of the western railroads print similar advice in their time-tables.

Although at the present time the origin of most of the northern forest fires can be ascribed to human agencies, lightning is known to cause a considerable proportion of them (estimated by Plummer at 15 per cent.), and in prehistoric times it must have been the principal cause. From all the evidence available it would seem that the normal frequency of fire at any one spot in the boreal conifer forests is about once in the average lifetime of a spruce tree, which may be between 50 and 75 years. The average extent of a single fire must be several square miles.

In the untold ages that fire has been a factor in the life-history of these forests there has developed a class of plants known as fireweeds, consisting of a score or more of herbs, shrubs, and short-lived deciduous trees, such as birch and aspen, which quickly take possession of burned areas and flourish until the dominant, but more slowly growing conifers have time to reestablish themselves. When the foliage of the conifers is consumed by fire the potash and other mineral nutrients stored up in several years' growth of evergreen leaves is returned to the soil in readily available form, and this must be a significant factor in the rapid growth of the fireweeds. Quite a lengthy chapter could be written about this