90 



served to hold its mineral particles together, and to retain the water of preci- 

 pitation, and thus loosens, pulverizes and dries the earth ; it destroys reptiles,, 

 insects, and worms, with their eggs and the seeds of trees and of smaller 

 plants; it supplies, in the ashes which it deposits on the surface, important ele- 

 ments for the growth of a new forest clothing as well as of the usual objects 

 of agricultural industry ; and by the changes thus produced, it fits the ground 

 for the reception of a vegetation different in character from that which had 

 spontaneously covered it. These new conditions help to explain the natural 

 succession of forest crops, so generally observed in all woods cleared by fire and 

 then abandoned. There is no doubt, however, that other influences contribute 

 to the same result, because effects more or less analogous follow when the trees 

 are destroyed by other causes, as by high winds, by the woodman's axe, and 

 even by natural decay. * 



When the forest is left to itself the order of succession is constant, and its occa- 

 sional inversion is always explicable by some human interference. It is curious 

 that the trees which require most light are content with the poorest soils, and 

 vice versa. The trees which first appear are also those which propagate them- 

 selves farthest to the north. 



The birch, the larch, and the fir, bear a severer climate than the oak or the 

 beech. 



The difficulty of protecting the woods against accidental or incendiary fires- 

 is one of the most discouraging circumstances attending the preservation of 

 natural and the plantation of artificial forests. In the spontaneous wood the 

 spread of fire is somewhat retarded by the general humidity of the soil, and of 

 the beds of leaves which cover it. But in long droughts the superficial layer of 

 leaves and the dry fallen branches become as inflammable as tinder, and the fire 

 spreads with fearful rapidity, until its further progress is arrested by want of 

 material, or more rarely, by heavy rains, sometimes caused, as many meteorolo- 

 gists suppose, by the conflagration itself. 



* Trees differ in their power of resisting the action of forest fires. Different woods vary greatly in 

 their combustibility, and even when the bark is scarcely scorched, trees are, partly in consequence of phy- 

 siological character, and partly from the greater or less depth at which their roots habitually lie below the 

 surface, differently affected by running fires. The white pine, Pinus Strobus, as it is the most valuable, is 

 also perhaps the most delicate tree of the American forest, while its congener, the northern pitch-pine, Pinus-. 

 Rigida, is less injured by fire than any other tree of our country. Experienced lumbermen maintain that 

 the growth of this pine was Hven accelerated by a fire brisk enough to destroy all other trees. 



