Page:Popular Science Monthly Volume 85.djvu/344

340 states and Canada printed since the invention of the half-tone process, about thirty years ago.

In the United States the jack pine prefers coarse sand and the other trees above mentioned are found mainly in peat bogs; but farther north they may grow in almost any kind of soil, wet or dry. (In Alaska even some of the glaciers are said to be partly covered with spruce forests.) The regions where they grow are characterized by cool and moderately humid climates, with an average temperature of 45° F. or less, and an average growing season (i. e., period free from killing frosts) of not

more than 150 days. The ground freezes several feet deep in winter, and temperatures of -30° F. or lower are likely to be experienced by each tree many times during its life.

The average annual precipitation is 20 inches or more, and in most places in the boreal conifer region there is more of it in summer than in winter, which tends to keep the soil moist throughout the year.

A climatic factor which involves both temperature and precipitation is the amount of snowfall; and it appears from statistics of the snowfall of the United States recently published that the type of forest under consideration can be correlated pretty closely with an average annual snowfall of 50 inches and upward. Although it would not be exactly correct from a biological standpoint to say that the narrow conical form of these trees is an adaptation to heavy snows, like the steep roofs of Norway, for example, it would be difficult to imagine any other