LECTURE IV. 



THE EVOLUTION OF A FOREST GROWTH. 



Last night we took a glimpse into that part of dendrology the study 

 of trees which concerns itself with the development of the single individ- 

 ual. To-night we will look into the communal life the sociology, as it 

 were, of trees, as exhibited in forest growth. 



For, in order to practice forestry, there is, first of all, need to under- 

 stand the natural history of the forest. How does nature produce her for- 

 ests ? What are the laws, what is the progress in the evolution of a forest 

 growth ? These questions i shall endeavor to answer to-night. 



The earth may be said to be a potential forest. A cover of tree growth 

 more or less dense is, or has been, the natural condition of at least the lar- 

 ger portion of the habitable earth, and, of the entire land surface, not less 

 than 60 per cent, may be classed as actual or potential woodland; 7 per 

 cent prairie, and 33 per cent, plains or barrens. (In North America the 

 proportion is about 45-5-50; in Asia, 45-3-52; in Europe, 84-10-6). 



In the struggle for existence and for occupancy of the soil between the 

 different forms of vegetation, tree growth has an advantage in its perennial 

 nature and in its elevation in height above its competitors for light, the 

 most essential element of life for most plants. These characteristics, to- 

 gether with its remarkable recuperative power, assure to the arborescent 

 flora final victory over its competitors except where climatic and soil con- 

 ditions are not adapted to it. 



The entire absence of the tree growth from some localities, such as the 

 northern tundras, the high peaks above timberline, and the arid plains, is 

 due to temperature, moisture and soil conditions, either one or the other, or 

 the unfavorable combination of them. On the high peaks, the two charac- 

 teristics, of perennial life and persistent height growth, become unfavor- 

 able, since the extreme winter temperatures above the snow cover, drough- 

 ty winter storms, and frosts every month in the year can be endured only by 

 those plants which have a rapid cycle of development, or are sheltered near 

 the ground by the snow cover. The wet soil on the tundras, frozen for 

 most portions of the year, or the thin soil on the Alpine peaks, add to the 

 difficulties for deep-rooting species in their contest with the lower vegeta- 

 tion. Again, in the interior of continents and other localities unfavorably 

 situated with reference to the great sources of moisture and moisture-bear- 

 ing currents, deficiency of water, scant rain-fall or low relative humidity, 

 or both, and excess of evaporation, are inimical to tree growth, occasioning 

 plains, which although not always and by necessity treeless, do not permit 

 any forests to establish themselves unaided. 



