Clearly, then, wind-breaks such as those which have been recommended for, and are found in, much of our western treeless area furnish considerable protection, over a narrow strip to leeward of the trees, against the sweep of strong hot or cold winds. Such a reduction in wind-velocity may have beneficial effects in reducing somewhat the extremes of heat or cold, and in diminishing evaporation from soil and from plants, and perhaps also in checking the blowing away of the soil. On the other hand, frost is more likely to occur where there is less air movement. Deforestation, on a large scale, especially on extended level areas, will therefore favor a freer sweep of the wind, which may be hostile to the growth of crops. Over any extended treeless area, exposed to high winds and with a severe climate, the best protection will be found in the planting of narrow belts of trees, alternating with agricultural strips. It should be noted, however, that this very wind-break, by decreasing wind velocity, keeps the air of the forest interior from affecting the atmospheric conditions round about. In other words, the forest diminishes its own climatic influence.
Influence of Forests upon Temperature
There is comparatively little popular interest in any possible influence of forests upon temperature, attention being almost altogether focused on the rainfall factor. Upon their soil temperatures, forests have a slight cooling effect (up to about 5°) attributable to the shade and to the greater moisture of the forest floor; the extremes are retarded and reduced; frost penetrates less deeply. Between evergreen and deciduous forests there is this difference, that in the former sunshine has freer access to the ground, and warms and dries it better than in the latter. In general, a forest climate bears a faint resemblance to a marine climate in having a slightly smaller range of temperature than the open, the extremes being most moderated in summer. In central Europe the mean annual minima are about 2° higher in the forest, and the mean annual maxima are about 4° lower. Individual summer maxima may be 6° to 8° lower in the forest, and individual winter minima 3° higher (Prussia). Conditions in the United States are probably not very different, although our greater extremes of heat and cold here would perhaps lead us to expect a slightly greater forest effect in moderating these extremes. The sum-total effect is, therefore, a slightly cooling one, chiefly because the forest is a little cooler than the open in summer, and about the same, or very slightly warmer, in winter. But these temperature differences in the average of the year are very small, and even in individual cases are certainly usually inappreciable without the use of thermometers. The considerable difference in our feelings of heat and cold ("sensible temperature") within and outside of a forest is probably chiefly due to the combination of the other factors, such as wind movement, moisture, exposure to sunshine, etc. Indeed, a good many of the reported differences between field and forest are probably
