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VOL. IV

APRIL, 1917

No. 1

EFFECTS OF FORESTS ON CLIMATE, STREAM FLOW, AND SOILS.

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This article is a portion of a recent report to the Association by its
committee on Forest Influences, of which Prof. Ralph S. Hosmer
of Cornell University is chairman.

HE TERM Forest Influences as it is used by foresters includes all effects resulting from the presence of the forest upon climate (including temperature, rainfall, wind, etc.), upon streamflow, upon the mechanical condition and erosion of the soil, upon health and recreation, and even upon the ethics of the people. The purpose of this statement is to show some of the ways in which the forest exerts these influences, and to connect this phase of the subject with the other branches of forestry, especially with reference to New York State.

The premise on which all forestry work is based is that forests are of value to man in that they are of use to him, in one or more of three ways: (1) as a source of wood or other forest products, (2) as protective covers on watersheds or where forest is required to restrain torrents or to control shifting sands, and (3) as agencies of health, recreation or beauty. Forests devoted exclusively to these uses are termed, respectively, supply forests, protection forests, and (for want of a better term) recreation forests. The names aesthetic or luxury

forests are also applied to the last class. Each class in its own way is of service to man. Not infrequently a single forest may serve all three functions sometimes it is found advisable strictly to limit the use of a forest to only one use. One of the problems now before the people of the State of New York is to determine how far it is necessary and desirable to carry out this segregation in the case of the state owned forests.

It must never be forgotten that the essential and basic idea in forestry is the perpetuation of the forest through wise use. Some of the

natural resources, like coal, gas, and the minerals, must ultimately be exhausted; others like forests, can be renewed. The object of forestry is to discover the method by which each kind or class of forest can be made to be of the most service to man, first of all to the people now using it, but also with due regard to those who are to come after us. The term "wise use" embraces both the present and the future. It all sums up to finding the best way to make the forests—as well as all other natural resources-serve the greatest good of the greatest number for the longest time.

The highly complex conditions of the forest and of its existence are among the most fascinating of natural phenomena. Here are interrelated physical and biological elements of great variety. The chemical and mechanical characters of the soil, the amount of rainfall, the atmospheric humidity, the temperature, the surface gradients, the elevation above the sea, with its accompanying relative air density, are physical factors which are intertwined with the adaptability of plants to their environment, resulting in forests of many different kinds as regards their dominant tree species, density, and undergrowth. Some of these interrelations we crudely understand; others are as yet wholly obscure. The functions of the animal and vegetable life of minute size, low organization, and great variety which inhabits the forest floor are probably much more important than we know.

In the State of New York, we have forests of variety, both of evergreen (coniferous) and deciduous (hardwood) trees, and their plant and animal populations are diverse. The evergreen forests are mostly in the north, where white pine, red pine, spruce, balsam, hemlock, and red cedar are the dominant trees, and on Long Island, where there are areas of pitch pine forests; the hemlock is also dominant in some areas along the rivers. Forests of hardwood trees occur locally nearly throughout the state, a large number of different trees being dominant in them at one place or another. There are also mixed forests, made up of both evergreen and hardwood species. Northward, we find in swampy areas forests of tamarack or larch, a coniferous tree whose leaves are deciduous.

Forests and Climate

Taking up one by one the several divisions that are recognized under

the head Forest Influences, as enumerated above, the first to be considered is the forest in relation to climate. The possible influence of forests on climate is a very different thing from the influence of climate upon forests. This statement deals only with possible modifications in local climate that result from the presence or absence of large bodies of forest, for to exert any appreciable influence the forested area must be extensive. This subject is one that contains many moot points and that has led to great discussion, much of which has been based on entirely inadequate data. But leaving on one side some of the points over which there is the most marked difference of opinion, it may be stated as the concensus of opinion among those best informed, that forests when in sufficient area, do exercise an influence on climate.

In the matter of temperature, the researches of the European Forest Experiment Stations prove conclusively that the temperatures both of air and soil within the forest fluctuate less and do not go to the extremes reached under corresponding conditions in the open. In other words the forest tends to equalize temperature and also to exert a beneficial effect on the open land immediately adjacent thereto. The temperature of the air over a forest is lower than that over open country.

So too with winds. Forests break the force and drying influence of winds and also serve as shelters for man and beast. Everyone is familiar with the use of rows or belts of trees as windbreaks, and the inestimable value that such plantations afford to the farm or the orchard.

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Humus cannot be produced to the best advantage except where the canopy of tree tops is kept dense and sun and wind excluded from the forest floor. This is one of the reasons why a wind mantle," or dense growth of underbrush, is a desirable feature to maintain on the edges of all woodlots.

With regard to the influence of the forest on precipitation one enters on debatable ground. Forests of themselves do not cause rainfall. That rests with the various meteorological factors that induce precipitation, such as temperature, humidity, and the presence of moisture laden clouds brought in by winds and air currents. But under certain conditions, not as yet well understood, the mechanical obstruction offered to the clouds by the forest, plus the cooler temperature that exists over

a large body of forest, may in some cases cause clouds which otherwise would pass over, to drop their moisture. Very many claims have been made for this influence of the forest without any warrant. It is not a topic on which to be dogmatic, but the evidence available does seem to point in the direction that in certain localities, where the balance between the meteorological factors is very delicately adjusted, the influence exerted by a large body of forest may be enough to tip the scales in favor of precipitation. In any event the influence is a local one. Under the conditions which obtain in New York State this phase of the subject is more a matter of theoretical interest than of practical importance.

While forests, especially those on level country, give off much moisture into the air through transpiration from the foliage, a forest cover retards evaporation from the soil and in hilly and mountainous country helps materially in retaining in the soil the portion of the rainfall that reaches the ground. This influence becomes more and more marked on hills and mountains as steepness of slope increases. It is one of the reasons why such areas should be maintained under forest cover.

Forests and Stream Flow

With regard to the beneficial influence of the forest on that portion of the precipitation that reaches the ground, there can be no possible question. Forests retard runoff, first through offering mechanical obstruction to the runoff, second, through providing a reservoir in the water holding capacity of the litter and humus that make up the forest floor. This layer of decaying vegetable matter can hold many times its own weight of water, so that instead of running quickly away down the slope, and incidentally causing damage through erosion, the rain that falls on a forest is retarded in its escape and so tends to equalize the flow of springs and brooks that have their origin in the forest. Further, snow is slower in melting in the forest than in the open, which also helps in maintaining the supply of available ground water.

Forests do not prevent floods, for after a time, in periods of heavy precipitation, the ground becomes saturated, after which the surplus water of course runs off somewhat as it would from a bare surface. But they do help very materially to control the amount and degree of the runoff and render a large percentage of it available for some form of human use. With the increasing demands for an adequate supply of

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