56 



SCIENCE. 



[N. S. Vol. XXII. No. 550. 



Dakota, Nebraska, Kansas, etc., he found that 

 most of the fires occur there in the fall when 

 the prevailing winds are from, the west, and 

 the vegetation of the plains lying to the wind- 

 ward of the area now prairie is exceedingly 

 dry and combustible. The habit of the In- 

 dians of setting fire to the grass of the plains 

 at that time of the year and its annual occur- 

 rence are matters of history. These annual 

 fires, driven by strong westerly winds and find- 

 ing no obstacle to their progress in the flat or 

 gently rolling land, spread eastward until they 

 reached the green timber. Year after year 

 these fires ate their way a little farther into 

 the forest, making the dense forest first more 

 open and eventually entirely consuming it. 

 The irregular projections of the prairie east- 

 ward into the forest are considered by Mr. 

 Gaskill as the result of the work of the fires. 

 Another evidence of its influence in causing 

 the prairie he finds in the fact that with the 

 settlement of the country and gradual elimina- 

 tion of fires the forest commenced to gain 

 again on the prairie. 



The prairies in other cou^ntries were also 

 considered from the same point of view, and 

 the speaker suggested that local students would 

 probably confirm his opinion that fire has 

 been an active agent in all such regions. 



PRINCIPLES INVOLVED IN DETERMINING FOREST 

 TYPES. 



On February 23 Mr. Eaphael Zon read be- 

 fore The Society of American Foresters a 

 paper on forest types. These he identified as 

 tree associations in the ecological sense, and 

 not mere aggregations of trees as they often 

 are conceived to be. Few will dispute the 

 statement that the forest type is the comer- 

 stone of silvics, and that the proper recogni- 

 tion of types in any forest is the first and most 

 important question that a practical forester 

 has to answer. " * * * The division of a 

 forest into natural types of growth, however, 

 is not such a simple thing as it may appear at 

 the first glance. Stands differ from each other 

 in many respects ; they may be pure or mixed, 

 even-aged or irregular, dense or open, of seed- 

 ling or sprout origin, etc. Which of these 

 features jvistifies the subdivision of the forest 



into types of growth, and what must we call 

 a natural forest type? * * * When we at- 

 tempt to trace to sopae definite causes the 

 difl'erences between stands composing a large 

 forest, we finally come to two main ones : first, 

 external physical conditions, such as climate, 

 soil, moisture in the ground, topography, ex- 

 posure, etc. ; and second, interference by man, 

 and natural accidents, such as fire, wind and 

 so on. * ^ * It does not take very long to 

 realize that segregating stands into types 

 based on density, age or mode of origin can 

 not be justified, since such features are not 

 I^ermanent and can not be characteristic of any 

 definite forest type. * * -» A forester who 

 mistakes any such temporary forest growth 

 for the original natural types of growth, thus 

 failing to understand the natural evolution of 

 the forest, will always have nature against 

 him in all his operations, instead of being 

 aided by her. * * * The physical conditions 

 of the situation then are the main factors 

 which determine the whole character of a 

 forest type. Of these physical factors, climate 

 undoubtedly has a marked influence upon 

 plant life, if we compare vegetation of differ- 

 ent latitudes. * * * Soil, moisture in the 

 ground, and topography, to which in mountain 

 coimtries must be added altitude and exposure, 

 are the main factors which determine the char- 

 acter of forest growth in a forest region, and, 

 therefore, must be accepted as the basis for 

 the division of the forest into natural types of 

 growth. A natural forest type then is an 

 aggregation of stands which may differ from 

 each other in age, density and other secondary 

 features, but have the same physical condi-- 

 tions of situation, like soil, topography, ex- 

 posure, etc. * * * The relationship between 

 the physical conditions of the situation and 

 the character of growth upon it is so constant 

 and characteristic that by the given physical 

 conditions of a situation, like soil, topography, 

 and so on, one can describe the general char- 

 acter of its forest, the predominant species, 

 habit of trees, reproduction, undergrowth and, 

 vice versa, by a given type of the forest growth 

 one can describe the physical conditions of 

 growth, soil, situation, etc. * * * A forest 



