HERMAN H. CHAPMAN 23 



Effect of Heat on Distribution of Trees. — The site or lo- 

 cality determines the species of trees which can grow there. 

 The climate has a profound effect upon the range of tree 

 species. The total amount of heat in the growing season 

 diminishes with increasing latitude and with elevation. 

 The effect of increased altitude on the total heat is espe- 

 cially noticeable and in mountain ranges is chiefly respon- 

 sible for the distribution of trees in distinct zones, corre- 

 sponding to the altitude. Species which grow well in a 

 climate with a certain average and total heat, would per- 

 ish in a warmer or colder climate. The northerly range 

 of a species is limited by its ability to withstand frost and 

 to ripen its wood in the growing season. Early fall frosts 

 destroy the shoots of slow maturing species and extreme 

 winter cold combined with evaporation and the deep freez- 

 ing of the soil causes the death of species not acclimated 

 to such conditions. The southern range of northern species 

 is limited by excessive heat which stimulates the activities 

 of the plant beyond its natural capacity, and causes rapid 

 production of weak and coarse wood, and early decay and 

 death. But the real factors which limit the range of a spe- 

 cies are those which prevent its natural reproduction. 

 Seedlings are always more sensitive to extremes of heat, 

 cold and drought than mature trees. So it often happens 

 that if trees are artificially planted and protected they 

 may grow much farther north or south than their natural 

 range. But even then they will not be as healthy and vig- 

 orous as when they grow within the range to which they 

 are acclimated. So ingrained are these characteristics of 

 climatic requirements that it is not possible for a species 

 to thrive much beyond its natural range, and the acclima- 

 tization of a tree species involving a change in the heat 

 requirements of the species, has never been successfully 

 accomplished. Seeds or seedlings may be brought long dis- 

 tances and thrive, as for instance natives of Japan such as 

 the Ginkgo tree, which grows well on our eastern coast, 



