20 Charles E. Bessey 



cler becomes margined with tall-growing weeds which kill the 

 tenacious prairie grasses, at the same time affording a lodgment 

 for seeds of shrubs and trees. These grow, and gradually the 

 shrubs and trees retain possession of the belt of ground, at first 

 to the partial exclusion of the weeds, and later to their total 

 suppression. Still later the trees overtop the shrubs, and event- 

 ually the latter may be suppressed also. While this is happen- 

 ing, a new weed belt is forming in advance of the belt of shrubs 

 and young trees, and tbus the forest margin is continually 

 advanced. 



There are many such advancing forest borders in Nebraska. 

 In fact wherever the fires and domestic animals are kept out 

 such an advance is commonly taking place. The rate of ad- 

 vance varies from a few feet a year to a hundred feet under fa- 

 vorable conditions, and in exceptional cases to several hundred 

 feet. When it is remembered that an advance of but ten feet a 

 year along a forest border a mile long adds a little more than an 

 acre of woodland, even such a slow advance is seen to accom- 

 plish much. In this way in the course of a century the actual 

 forest area may be greatly enlarged. While such a steady ad- 

 vance of the forest margins is now actually going on, there is 

 another mode of distribution which is even more rapid. A seed 

 is carried by a bird or other means to a considerable distance 

 from the body of trees of its kind. It springs up in its new sta- 

 tion and eventually produces seeds, and becomes a centre from, 

 which further distribution takes place. A case of this kind has 

 been brought to my attention in the recent appearance of the 

 Linden tree {TUia aincricana) in the vicinity of Lincoln. 



In the western part of Nebraska the present spreading of the 

 Rock Pine (Pimis scopulormn) is quite noticeable. It is not 

 uncommon to find young trees considerably in advance of the 

 older trees of the sparse forest, around which are many small 

 trees which have sprung up from the young parent trees. 



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