TOLKRANCK OF FOREST TREES G13 



ence to tree growth. Whatever the casual factor for the variation in 

 leaf structure, whether it be light or soil temperature, the plant is 

 adjusting itself to its water supply, and the structure of the leaf cannot 

 be used as a standard of judgment for tolerance if tolerance is a "light 

 relationship and nothing more." 



Still another view of tolerance is expressed by Bates. He empha- 

 sises the importance of the large amount of light which strikes the 

 leaf and is used by the plant as heat. That heat is one of the most 

 important limiting factors in photosynthesis is well known. The argu- 

 ment runs something as follows : Intolerant trees receive a large 

 amount of light, but owing to intense transpiration, due to the wind, 

 the temperature is lowered and photosynthesis proceeds slowly ; tol- 

 erant trees receive less light, but in their sheltered position they trans- 

 pire less, hence their temperature is lowered less. Accordingly the 

 weakened light, plus the higher temperature, is about as effective as 

 the stronger light and the lower temperature. We might add to this 

 that plants in the shade have a decreased respiration, and hence need 

 less food to maintain themselves. Apparently to Bates, tolerance is 

 not a light relationship but a heat relationship. This phase of toler- 

 ance should not be overlooked, and Bates deserves great credit for his 

 suggestion. 



If tolerance is a matter of the sensitivity of the chloroplast to light 

 or to heat, how can these be modified by other factors of the site or 

 habitat? I believe that the great amount of confusion in the field is 

 due to lack of sharp definitions. No one doubts the fact that a 

 chloroplast has its maximum-optimum-minimum intensities but it is a 

 serious question ho\V often the light in the forest gets below the mini- 

 mum at least to such an extent that it is the limiting factor in forest 

 succession. This last can only be determined by experimental work 

 done under control conditions. What are the factors in the field which 

 influence tolerance or the light relationship of trees? We find in the 

 literature the statement that, due to weaker light, the elm and ash arc 

 intolerant in north Germany but tolerant in south Germany. Again 

 trees are intolerant at high altitudes where the light except for clouds 

 is strong, on north slopes, and in raw climates. It would seem that 

 temperature might be the controlling factor in these cases. That is, 

 the tree in north Germany, on the mountain top, on the north slope, 

 and in the raw climate would all be suljjected to low air temperatures 

 and the length of the growing season would be the all-important factor. 



