Trees of New York State 393 



level, etc.), and terrain cause further modification of those types. 

 It is generally conceded that the rainfall necessary for forest 

 growth is about the same as that required for agriculture without 

 irrigation, that is, from 20-24 inches. But precipitation is de- 

 pendent on such physiographic features as (a) proximity to the 

 ocean or other large bodies of water, (b) mountain ranges athwart 

 the rain-bearing winds which necessitate a cooling of the air and 

 heavier rainfall, and (c) location within or near the track of 

 cyclonic storms. "With equal temperature the kinds of trees and 

 forest types vary with precipitation. This accounts largely for 

 the fact that the Great Plains and the Great Basin of the United 

 States are forestless though not treeless. The plant belts which 

 owe their origin primarily to latitude are greatly modified in the 

 United States owing to varying temperatures brought about by 

 topography and by wide extremes in annual precipitation. 



TRANSCONTINENTAL BELTS OR LIFE ZONES 



In the development of the natural resources of the United Statas, 

 particularly the agricultural resources, it was early recognized 

 that North America contained seven transcontinental l)elts or life 

 zones, each again divisible into a large number of minor floral- 

 faunal areas. These zones, as defined by Merriamt are as follows: 



I. The Boreal Region 



1. The Arctic-Alpine Zone — The far north beyond the 

 limit of tree growth and in the United States, liigh moun- 

 tains above timber line. Zone of the polar bear, musk ox, 

 reindeer and of arctic popp}*, dwarf willow, etc., in north 

 polar regions. 



2. The Hudsonian Zone — Includes the northern part of 

 the boreal conifer forest stretching from Labrador to Alaska. 

 In eastern United States it is limited to the cold summits 

 of the highest mountains from northern New England to 

 western North Carolina. 



3. The Canadian Zone — Includes the southern and most 

 valuable part of the transcontinental boreal conifer forest 

 in Canada and parts of Maine, New Hampshire and 

 Michigan, extending southward along the Appalachian 

 highlands to western North Carolina and Tennessee. Zone 

 of red spruce, balsam fir, paper birch and mountain ash. 



t Merriam, C. Hart. Life Zones and Crop Zones. U. S. Biol. Survey. Bull. 10. 

 1898. 



