486 forestry Quarterly 



to a serious extent. For instance, the foresters of central Europe 

 successfully cultivated the larch only in the highlands at first. 

 Later it was found that the tree would reach a greater and 

 quicker development in the lower levels and was accordingly 

 planted in pure forests in damp localities. A fungous parasite 

 closely associated with the larch in its mountain home and only 

 producing reproductive organs in damp situations followed the 

 tree into the new habitat with the result that great damage was 

 done and special methods were necessary to counteract the rav- 

 ages of the disease. 



A method successfully practised in many forest regions of the 

 Old World is the introduction of other tree species possessed of 

 different qualities from those occupying the original stand. The 

 old forest composed of species to which numerous parasitic 

 fungi had become adapted, not merely because the tree had in 

 any way become decadent, but because a favorable environ- 

 ment was afforded a particular disease through the evenly bal- 

 anced condition of a pure forest. Pure forests, with few excep- 

 tions, always afford favorable conditions for parasitic tree fungi, 

 and the trouble is intensified if the forest is even-aged. It is easy 

 to understand this. Uniform crowns, similar branching and 

 natural pruning, causing wounds at same level, equal depth and 

 extent of root system, equal annual increment with same amount 

 of sap wood ; constant temperature, moisture and light relations 

 produce a condition, by increasing or maintaining a funguous 

 activity with no unfavorable influences, unequaled in mixed 

 forests. The constancy of such factors in the life history of the 

 principal attacking fungus introduces a regularity in its growth 

 and the amount of wood decayed, so that it would enable definite 

 pathological units to be established in estimating the amount of 

 merchantable timber to be had. The rotation of fruits, vege- 

 tables and field crops in general has brought good results in com- 

 bating plant disease. The principle involved is the destruction of 

 the spore of the fungus by a long exposure to the soil before 

 the newly introduced host has made its appearance. Moreover, 

 the alternating crop is chosen because of its resistance to the 

 diseases of the preceding ones. To what extent the idea of the 

 immunity of forest trees, other than hardiness under certain cli- 

 matic conditions, is to be entertained is an important problem. In 

 proof of this the sole salvation of retaining chestnut forests in 



