ECOLOGY. 337 



ECOLOGY. 



Clements, F. E., Tucson, Arizona. Associate in Ecology. (For previous 

 reports see Year Books Nos. 16-20.) 



As usual, the experimental work of the growing-season has been centered 

 at the Alpine Laboratory from June 1 to September 15, while studies of the 

 volume and composition of soil-air and plant-air have been continued during 

 the autumn and winter. Further investigations in the water-cycle series 

 have been made at the Desert Laboratory during winter and spring, and the 

 ecological relations of latex have been under examination at the University 

 of California. Statistical and garden studies of the Madiese, Haplopappus, 

 and other genera have been continued at the same institution, and a Sierran 

 transect for experimental evolution, a complement of the Petran one at 

 Pike's Peak, has been installed from Mather through Tioga Pass into the 

 sagebrush desert at Benton. The absorptive processes of roots have been 

 further investigated at the University of Nebraska, and a field station has 

 been installed at Greeley, Colorado, for experiments in root behavior under 

 irrigation. The series of stations for experimental vegetation and crop ecology 

 has been maintained as usual. 



Vegetation studies were carried on actively throughout southern Arizona 

 during the winter and formed the chief objective of several field expeditions. 

 The first traversed the Colorado Desert and the coastal region of California, 

 and dealt especially with relict dominants, the grasses in particular, and with 

 other evidences of climatic changes. The second crossed the Great Valley 

 of California and the Sierras and passed through central Nevada and Utah, 

 devoting the chief attention to the structure and relations of the climaxes, 

 the serai position of the sagebrush dominants and the significance of relicts 

 as climatic indicators. The third expedition traveled through eastern Utah, 

 southern Wyoming, and western Nebraska and made a study of the general 

 relations of climate and sedimentation in classic bad-land regions of the Eocene, 

 Oligocene, and Miocene. The route of the fourth expedition was through 

 northern Kansas and southern Nebraska to eastern Kansas, Oklahoma, and 

 southeastern Texas, and then westward along the border to Arizona. Its 

 primary objectives were the ecotone between forest and grassland on the east, 

 the contact between postclimax scrub and the desert plains on the south, and 

 the movement of grassland dominants in relation to grazing and climatic 

 shifts. 



FACTOR STATIONS. 



Four series of factor stations have been maintained during the year. The 

 most extensive is the one established for the past 5 years in the associations 

 of the grassland formation from the subclimax prairie of eastern Nebraska 

 to the mixed prairie of eastern Colorado and devoted to the measurement of 

 the factors efficient in crop production and ecesis. The second is installed at 

 the Alpine Laboratory in the montane zone for the study of the factors respon- 

 sible for the differentiation of vegetation on north and south slopes. The 

 third has been established at Greeley, Colorado, for the interpretation of the 

 conditions under which roots develop and crops are produced in dry land and 

 under half and complete irrigation. The fourth series is located in the desert 



