162 PRACTICAL FORESTRY 
the trost in spring. Species may be divided into those 
which are frost-hardy and frost-tender. 
The severity of the damage depends upon the 
species, the age, and the nature of the locality. As 
a rule, valleys and lowlands suffer more than uplands. 
Holes or hollows in which there is poor ventilation 
suffer most. Areas treated in such a way that some 
shelter trees are left to cover the ground, are less 
damaged than those which have been cut clean and 
planted. Good drainage will reduce the danger of 
frost. By leaving shelter trees and planting strong, 
frost-hardy species, its injurious effects may be re- 
duced to a minimum. 
The conditions of weather favorable to frost are 
a clear sky, a dry air, and a still night. 
In the case of normal leaf-fall, a corky layer forms 
at the base of the petiole of the leaf. It is during this 
process that the changes occur in the leaf which pro- 
duce the gorgeous tints of autumn. When the ten- 
der leaf or shoot is nipped by early frost it is killed at 
once, and in a black and withered condition clings 
to the twig for a long period of time, often through- 
out the winter. 
Tn our North a large percentage of hardwood tim- 
ber is defective in consequence of frost-cracks and 
ribs. In the case of sudden extreme cold, especially 
in the spring, the trunks of trees crack open with a 
