THE XOETHEBX HARDWOOD FOREST. 15 



height, secures crown space and light through the aggressive spread 

 of its slender, flexible twigs and small branches. (PI. IV, figs. 2 and 3.) 



Resistance to various kinds of injury contributes to length of life, 

 and since it varies more or less with climate and soil, it is doubtless 

 at least partly responsible for some regional variations in the compo- 

 sition of the forest. Thus, sugar maple in northern Wisconsin is 

 apt to be inferior to yellow birch in soundness, and is not so abundant. 

 (PL II.) Unsoundness does not always influence the forest composi- 

 tion, however. Basswood is extremely unsound, even in the region 

 of its greatest abundance. Its soft wood falls an easy prey to 

 wood-destroying fungi and insects which eat out the hearts of the 

 trees, so that nearly all large basswoods in the old-growth forests 

 are hollow. In spite of this the trees attain great age and size. The 

 reproductive power of large basswoods is apt to be considerably 

 reduced by the breakage of branches in the top, due to snow and 

 wind. 



Climate and soil. 1 — Climate has an undoubted selective influence 

 on the composition of these forests, but its precise effect can not easily 

 be disassociated from the other elements determining the composi- 

 tion. In general, however, it appears to restrict the growth of yel- 

 low and paper birch and the aspens to the cold, humid air and soil 

 of the north and of fairly high altitudes; the paper birch and aspen 

 extend beyond the Arctic Circle. The wide north and south ranges 

 of most of the hardwoods show that they are less influenced by 

 climate. Some, however, are influenced more than others; for ex- 

 ample, in the mountain regions white elm, ash, and basswood are 

 practically confined to warm, lower slopes and sheltered valleys, but 

 beech and sugar maple grow at altitudes as high as those reached by 

 yellow birch. The white elm, ash, and basswood are at their best in 

 the continental climate of the Lake States and southeastern Canada, 

 where they hold their own against the more tolerant beech, maple, 

 and yellow birch. Black birch, though essentially a northern hard- 

 wood, is scarce in New England, and its range indicates less hardi- 

 ness than that of yellow birch. Beech apparently endures greater 

 air dryness than the other northern hardwoods. South of the north- 

 ern hardwood region it is often a prominent associate of white oak 

 and hickory in relatively dry situations. 



As compared with the pines and the hardwoods of the oak- 

 hickory-chestnut types, the northern hardwoods are exacting in their 

 soil requirements. In common with most tree growth they are best 

 suited by deep, fresh, well-drained, fertile loams, mixed with sand or 

 with clay, and kept porous and moist by abundant, well-decomposed 

 humus. It is probable that mycorrhiza and nitrifying soil bacteria 



1 The general climatic conditions within -which the northern hardwoods forests grow have been outlined 

 on pp. 3 and 4. 



