REPORT ON FORESTS. 183 



flora are always ready to seize upon any advantage which will 

 give them a foothold further south, while the elements in the 

 coniferous flora are always ready for an advance in the opposite 

 direction. The former may, therefore, be considered as a south- 

 ward-moving and the latter as a northward-moving flora. 



In consequence of these conditions the vegetation in the ten- 

 sion zone is in a state of unstable equilibrium, in which the 

 strain is constant, and any change in or interference with the 

 conditions releases the strain and causes a disturbance of the 

 relations between the floras until new conditions have become 

 established. Each such disturbance is marked, to a greater or 

 less extent, by changes of species, and lines or areas of limitation 

 and distribution thus vary from time to time. 



The influence of civilization has been the most marked factor 

 in this connection in recent times. Fire and cultivation have 

 caused, directly and indirectly, great changes in the inter- 

 relations of species. In places certain species have been 

 removed for economic purposes and others allowed to remain, 

 which, of course, gives a direct advantage to the latter in any 

 subsequent struggle for place. In other localities all species 

 have been removed and such a change made in the environment 

 that foreign species, better able to adapt themselves to the new 

 conditions, become established there. 



One corollary to the above proposition is that changes 

 wrought within the limits of the deciduous or coniferous zones 

 would not normally lead to such extensive subsequent changes 

 in species as would be the case under similar circumstances 

 within the limits of the tension zone. 



In the deciduous zone, for example, where clearings have 

 been made, these are often occupied at once by pines and cedars, 

 which have been awaiting the opportunity to establish them- 

 selves where they will be free from competition. The soil 

 favors their growth, but they are almost always crowded out 

 subsequently when the angiosperms regain their foothold, and 

 the ultimate result is practically a return to the old conditions, 

 due to the higher and more aggressive type of life represented by 

 the latter. 



In the coniferous zone also the same species would be likely 

 to re-appear after a tract has been cleared, for the reason that the 

 soil conditions are usually barren and unfavorable for the angio- 



