155 



4- Destruction of trees by animals : the processes of defolia- 

 tion, borings in branches, bark, trunk, or roots, and the girdling of 

 trees. Fires started by man, depending on the degree of destruction, 

 cause new cycles of succession. Both beavers and man build dams, 

 flood areas, and thus kill trees. 



5. Combinations of physical and organic processes; the flood- 

 ing of river bottoms by driftwood rafts which become converted into 

 dams and thus submerge large areas. 



Since it is most usual for these causes to act, not singly but in 

 various combinations, and since they also vary greatlv in their degrees 

 of influence, their operation is extremely complex. The drowning of 

 the forests along the Mississippi River through the sinking of the 

 land by the New Madrid earthquake, is a good example, showing how 

 a large tract of forest may be killed and much dead and decayed wood 

 formed, as has been shown by Fuller ('12) — (Plate XXXII). Tarr 

 and Martin ('12) have shown how destructive to forests the earth- 

 quakes are in Alaska. The influence of the New Madrid earthquake 

 upon animal life has not been investigated, but it is not too late even 

 today, after more than one hundred years, to make important studies 

 on this subject. On the other hand, the processes of erosion operate 

 more continuously than the periodic earthquakes, and tend to degrade 

 the land, lower the water-level, and to change the habitats in swamp 

 and other forests. 



The results of climatic influences are seen in the amount of injury 

 done by sleet, which, weighing down the branches, breaks many of 

 them and leaves the fractured stubs as favorable points for attack 

 by fungi and insects. Webb ('09) has shown that when a tornado 

 passed through Mississippi and Louisiana the felled pine forests were 

 from one to three miles wide. Practically all of this timber became 

 infested with the larv?e of Monohauimus titillator Fabr. After a 

 severe frost in Florida the dead wood of the orange-trees became in- 

 fested bv wood-boring larv?e, which spread from this wood to the 

 enfeebled living wood, as Hubbard (Howard, '95) observed. Light- 

 ning (Plummer, '12) kills and maims many trees, producing dead 

 wood, and through fires started in the same manner much more dam- 

 age is done. Hopkins ('09) considers that much of the injurv at- 

 tributed to fire is primarily due to insects which made the dead and 

 dry fuel for the destructive fire work. 



That competition among trees weakens some of them is well 

 known. This weakening makes them more susceptible to attack by 

 fungi and insects. In a forest where the shade-enduring trees can 

 shade out all competitors, the shrubs and trees which are intolerant 

 show just such a lack of resistance. As an example of this process 



