ECOLOGY. 337 



the grassland formations, ecologically at least, and the equivalence of 

 its dominants, their competitive relations , and their behavior under 

 disturbance, particularly grazing, have thrown a flood of light upon 

 the relations of the several grassland associations. The response of the 

 mixed prairie to grazing is so unexpected and so striking as to explain 

 many popular and scientific errors concerning the prairie and plains 

 during the past century. Moreover, it furnishes the best of illustra- 

 tions of the essential interdependence of plants and animals in the biotic 

 community, and promises to provide an invaluable key to the great 

 changes of grassland in prehistoric and geologic times. 



Climatic Cycles, by F. E. Clements and A. E. Douglass. 



The investigation of climatic cycles has progressed sufficiently to 

 permit the publication of a complete summary under the title, "Ch- 

 matic Cycles and Tree Growth." The further studies of the year have 

 dealt with three additional problems in Sequoia cycles as follows: 



(1) The doubtful ring 1580a has been studied and promises to indi- 

 cate a separate year. 



(2) Short-period cj'cles, less than the sun-spot cycle of 11 years. 



(3) Gross rings. 



Records of the last 500 years were obtained from 12 different trees, 

 carefully selected 'with reference to topographic location, water-supply, 

 and nearness of other trees. A collection of 15 ''rubbings" was made 

 from old weathered pine and fir stumps, nearly all of which show 

 remarkably fine rhythm. One cutting was obtained from a sugar pine 

 that has an extraordinary rhythm and seems to show a relationship 

 to the sun-spot cycle. 



In order to test the cyclic growth of trees in other regions, sections 

 of pine, fir, or spruce have been obtained from various localities, such as 

 Pike's Peak, the Black Hills, eastern Oregon, etc. A signal advance 

 in the understanding and interpretation of cycles has come from the 

 recognition of the unique importance of the excess-deficit balance in 

 space as well as in time. While such a balance is of the very essence 

 of climatic cycles, its significance has just begun to be appreciated. 

 It is seen better in the double cycle of 22 years than in the sun-spot 

 cycle of 11 years, but it shows most strikingly in the 2 to 3 year cycle 

 because of the short periods involved. It explains why cycles are often 

 less obvious in humid regions, reconciles the discrepancy between 

 neighboring localities, and harmonizes the marked variations in the 

 rainfall of mountain and plain during the same year. Moreover, there 

 is considerable evidence that the principle of the excess-deficit balance 

 can be applied to the rainfall of the successive months of the year. 

 The study of native vegetation as well as of crop production leaves no 

 doubt that they reflect the cychc nature of climate, and that this basic 

 effect extends to the animal communities and human society. 



