752 TEXTBOOK OF BOTANY 



from early postglacial times, when the boreal forest dominated this 

 whole region. Likewise, there are areas of valley slopes and lake shores 

 where deciduous forest plants and communities have moved in and be- 

 come established. The hemlock-hardwood forest on its southern borders 

 has been most frequently replaced by deciduous forest communities 

 on the better soils. 



The climatic conditions within this region are less rigorous than in the 

 boreal forest. The growing season is usuallv four to five and a half 

 months. Increased rainfall and deeper snows limit or prevent the freez- 

 ing of the soil in forested areas. Frosts seldom occur between June and 

 August, although night temperatures may frequently approach 40° F. 

 Winter temperatures of 20-25° below zero are not uncommon, and at 

 rare intervals temperatures 40° -50° below zero are recorded. 



The period of dormano' thus is somewhat longer than the period of 

 growth, and the extreme winter temperatures limit the establishment 

 of many species common southward. For example, in some valleys of 

 the western Adirondacks during the winter of 1935-36 beech trees more 

 than 200 years old were killed bv extreme low temperatures, and of 

 course many of the younger trees died also. This is an example of the 

 fact that elimination of species from climatic formations may be effective 

 even if the extreme of a limiting factor occurs but once in many years. 

 The beech and maple trees near their northern limits are scrub-like trees 

 with numerous dead branches, 



Agriculturalh' the area occupied by the hemlock-hardwood forest is a 

 region of hay crops, interspersed locally with potato and root crops and 

 certain northern varieties of apples. Vegetables, grapes, and other fruits 

 are cultivated near the larger lakes where in autumn killing frosts are 

 delayed by the heat liberated from the water. 



Plant associations. As one studies the local diversity of plants and 

 plant communities within a climatic plant formation, he finds certain 

 communities with a similar appearance repeated again and again, 

 much the same group of species, and a similar internal organization. 

 They may occupy large or small areas, depending on physiography and 

 soils. These communities within a fonnation are called plant associations. 

 They are the smaller units of the vegetation of a great climatic region 

 which includes many topographic situations with difl^erent exposures to 

 light, wind, temperature, and precipitation. They are also limited by soils 

 of different depth, chemical composition, water content, aeration, and 

 texture. 



