188 Nebraska State Horticultural Society. 



THE MIGRATION OF OUR FOREST TREES, 



By Charles E. Bessey. 



It is a familiar fact that new species appear from time to time 

 among the native plants of a region. Such newcomers turn out on 

 examination to be new only in the sense that they have not previously 

 lived in the region, and in every instance these new plants are found 

 to have come from other regions where they had existed for a longer 

 or shorter period of time. In some cases the new species remain for a 

 time and then disappear, or at least become inconspicuous, but more 

 commonly they crowd in among the former plants and become perma- 

 nent members of the plant community. Whenever such an addition is 

 made to the flora of a region there is a readjustment of the former 

 species, with a necessary change in the relative members of the in- 

 dividuals, and the particular habitat of each. In the case of annual 

 plants these adjustments are made rapidly, so that in a. snort time the 

 prominent features of the plant community may be entirely changed. 

 On the other hand, in the case of perennial plants there is greater sta- 

 bility, new species finding greater difficulty in entering, and the old 

 species giving away, if at all, only after the lapse of a much longer 

 time. A vegetation which is fvell rooted in the ground is much less 

 easily disturbed than one whose roots live for but a single season and 

 then abandon the particular plot of ground where they grow. Forests 

 are therefore conservative plant communities, into which new species 

 gain entrance with difficulty, and which change very slowly after such, 

 entrance has been effected. There is only one other plant community 

 whose stability approaches that of the forest, namely, the grassy vege- 

 tation of the prairies and plains, which is composed of perennial- 

 rooted grasses, sedges and rushes. Where these form a close sod new 

 species are almost wholly excluded, and but little change takes place 

 in the character of the vegetation. It is only where the surface is not 

 closely covered that the grassy vegetation is more easily modified by 

 the incoming of new species. W^here accident, or disease, or some 

 other cause has destroyed the grassy covering new species promptly take 

 possession. A fine example of this is to be seen in the growth of 

 Helianthus animus on the mounds made on the prairies by such burrow- 

 ing animals as gophers and prairie dogs. Where the tough sod was 

 broken by the freight wagons which crossed Nebraska by various 



