of the slow heating of the water in the leaves. To a certain slight extent, then, a forest cover ought to behave as does a water surface; it ought to warm a little less rapidly and therefore it ought to cool less rapidly.
(h) The process of growth of the trees, and the chemical changes which are going on during their life, must require an expenditure of energy whose effect might possibly be observable in a difference of temperature between the forest and the open. The rise and return of the sap may also be expected to be accompanied by certain slight temperature effects resulting from the transfer of root temperatures upwards and of crown temperatures downwards.
In these, and perhaps in other ways, we may seek for the causes of forest influences upon climate. But, whatever may be the theoretical reasons for believing in such influences, we are here concerned only with the facts as they are at present known. One further word of caution is necessary. It is one thing for a forest to have a climate of its own within its own limits, under or above the trees. It is quite another thing for a forest to affect the climate of the surrounding country, or of distant regions. The latter effect is naturally the one in which the real interest centers. But it is also the one which is by far the most difficult to study. It is clear that nothing more than reasonably local modifications of climate ought to be expected. The special climate of the forest itself—so far as it may appear to have one—can only affect the surroundings by modifying the air currents which pass through or over it, by producing an ascending movement of the forest air to take part in the prevailing wind movement, or by causing, as may happen under especially favorable conditions, local air currents of its own. Most, if not all, of the above-mentioned theoretical effects of forests upon climate have been overestimated.
Forests as Wind-breaks
The most obvious effect of forests is that of the barrier, or windbreak. First, there is far less wind movement within the forest than there is outside. Second, friction on the tree-tops reduces the velocity of the wind blowing over the forest. Third, to leeward of the forest there is a belt of relative cairn which is roughly ten to fifteen times as wide as the forest is high, as has been determined by measurements in Iowa and in the Rhone Valley. More recently, in Roumania, Murat has shown that within 165 feet to leeward the decrease in velocity may be from four to eight miles an hour, and that the effect of the forest in decreasing velocity extends as far as 1,500 feet to leeward. Some years ago, comparative observations in the harbor, city and suburbs of New York and Boston showed a remarkable reduction in wind velocities with increasing distance inland, the velocities in the city being a little over three fifths, and those in the suburbs about one third, of those in the harbor.
