i 9 2o] CLAYBERG— UPLAND SOCIETIES 39 



American marsh, finding evaporation rate proportional to height 

 above the soil. These suggest that data on the levels of the climax 

 forest of this region would be significant. Gates (8) compares 



in 



not at different levels. He 



cause, of succession. 



Environment 



Competition is affected by several influences: physical and 

 chemical factors, parasites, and individuals of the same or an 

 older generation. Scattered among the herbage are tree seedlings, 

 many of them dead or dying. In fact the younger the group, the 

 more die. No competition between seedlings occurs except as 

 two are found within short radius of each other. The critical 

 competition for them occurs with the older trees in the form of 

 light interception (most important) from above and nutrient inter- 

 ception from below. Since the lifting of the light inhibition is 

 very slow in terms of potential seedling growth, the plasticity of 

 seedlings becomes a factor. Being so adaptable, one can fit itself 

 to any rift by lateral growth; occasionally one with over 90 per cent 

 of its leaves on a far side branch will be found. Maximum spatial 

 crowding is reached in the sapling age, and consequently the most 

 critical competition of the life cycle occurs here. 



Approaching the climax of elimination, the first to go are those 

 with too few leaves in the light. Among other causes this may be 

 due to shortness, distortion, slow growth, or accentuated crowding. 

 There are more weaklings and distorted trees at this age than at 

 any other, and in their removal comes the critical stage in spacing 

 evolution; for removal of the very old trees above results in intensi- 

 fied elongation and more rapid destruction, since the spacing 

 interval is increased 20-100 times before the third life age is reached. 

 In general, the sapling race is not only a struggle for life by vertical 

 elongation, but it is one in which the time element is crucial. 



Having reached the third age, the tree is nearly immune from 

 lateral competition, the permanent stand being formed here. 

 Future struggles are against rot, parasites, w r ind, and weather, both 

 root and branch systems now being amply competent to maintain 

 life processes. Since the tree's juniors must be limited to what it 



