174 Temperature 



slopes at the upper limits of forest vegetation. We may consider first 

 the northward boundary of timber in Canada as an example of a 

 continental tree line. 



A traveler going north in the eastern part of North America passes 

 from the southern region of palm trees into the belt of southern 

 pines and then into the "central hardwoods, " dominated by white oaks 

 and hickories. The central hardwoods extend into southern New 

 York, Connecticut, Rhode Island, and maritime Massachusetts. In 

 east central Massachusetts and up the valleys in New York, Vermont, 

 and New Hampshire the vegetation changes to the "transition forest," 

 in which the maple, red oak, ash, and black birch are the most 

 abundant species. In northern New York, central Vermont, and 

 New Hampshire the forest becomes the "northern hardwood" type, 

 with maple, beech, and white and yellow birch predominating. 

 When the traveler reaches northern Maine and the corresponding 

 parts of Ontario, he finds himself in the spruce forest. Here red 

 spruce, firs, and white and yellow birch are the common trees. Still 

 farther north, about at the southern edge of Hudson Bay, the trees 

 have become so small that the Indians referred to the area as the "land 

 of little sticks." Then the forest stops. 



Although temperature is believed to play a major role in determin- 

 ing the poleward boundary of forest vegetation, it is difiicult to de- 

 termine just how this factor operates. We know that tree line is not 

 related primarily to the coldness of winter since even in Siberia, 

 where extremely low temperatures are recorded, heavily forested 

 regions are found. Nor is the tree growth determined directly by 

 the occurrence of certain maximum temperatures in summer, since in 

 Labrador and in Alaska, far beyond the tree line, temperatures of 

 32 to 38°C have been recorded. High daily maxima are beneficial 

 since they hasten the melting of snow. In order to survive trees must 

 have sufiicient heat for growth in the interval between the arrival 

 of effective temperatures in the spring and their disappearance in the 

 fall. In Alaska, and also on the San Francisco mountains, the conifers 

 reach but do not pass the July isotherm of 10°C ( Griggs, 1946 ) . On 

 the other hand, the position of the tree line in Scandinavia appears 

 to be chiefly influenced by the occurrence of a season with sufficient 

 heat for seed germination, especially of the birch. 



In some regions the curtailment of tree growth is brought about by 

 excessive solifluction— the heaving of the soil by frost action. Trees 

 are tilted and their roots broken by severe solifluction, and in such 

 situations this indirect action of climate may be more significant 

 than its direct effect. The occurrence of permafrost (a stratum of 



