DEPARTMENT OF BOTANICAL RESEARCH. 65 



principal of these is the altitude of the plain from which a given mountain 

 rises. In the region under consideration the lower limit of encinal and forest 

 is determined by the elevation above the basal plain more nearly than it is 

 by absolute elevation. Also, the total elevation attained by a mountain 

 determines the vertical limits of its types of vegetation. The upper slopes of 

 a small mountain will be covered with a vegetation which reaches its upper 

 limit from 1,000 to 2,000 feet lower on a large mountain. 



These features are related to differences in rainfall, movement of convec- 

 tional storms, cold-air drainage, and other factors as to the operation of which 

 considerable is known from earlier work in this region. With them are 

 associated differences in the altitudinal limits of vegetations and species due 

 to differences in soil character. If a mountain is taken as the norm which has 

 a loam soil of granitic origin, it will be found that the lower limits of encinal 

 and forest are higher on rhyolitic and other volcanic soils (except basaltic 

 clay), and are still higher on limestone soils. 



Measurements of Erosion and Deposition as related to Vegetation, 



by Forrest Shreve. 



Biennial measurements have been made for two years on four series of 

 metal bench-marks established permanently in the Avra Valley and on the 

 bajada of the Sierrita Mountains near Tucson. There are two series of 

 bench-marks in each locality, one running parallel to the drainage in small 

 streamways, the other transversely to the lines of drainage. The streamway 

 in the Avra Valley has a very low gradient and is surrounded by a region 

 of indeterminate drainage. The streamway on the bajada of the Sierrita 

 Mountains has a fall of about 100 feet to the mile, and the region which it 

 drains is gently rolling in transverse section. 



In both of the series of bench-marks placed in the streamways there has 

 been an irregular alternation of erosion and deposition, such as is common to 

 all streamways, of whatever size, that are fed only by torrential desert rains. 

 The nodes at which there was no change of elevation are from 50 to 100 

 meters apart. The bench-marks placed transversely to the present drainage 

 in the Avra Valley have shown little change other than a slight deposition 

 near the largest streamway and a slight erosion at one spot where a new 

 streamway is developing. On the bajada of the Sierrita Mountains the trans- 

 verse series has shown a consistent cutting down of the elevated spots and a 

 filling of more than half of the very small drainageways comprised. 



Great irregularity in erosion and deposition is to be expected from year 

 to year in all of these series, and it will require a number of years to determine 

 the trend of events at the places under investigation. The vegetation of the 

 Avra Valley bears abundant evidence of the abrupt changes of physical con- 

 ditions that have accompanied heavy deposition of very fine material or the 

 rapid development or filling of drainageways. It is hoped that some evidence 

 can be secured through the bench-marks as to the magnitude and chronology 

 of these changes. 



Stem Analysis of Monterey Pine and Redwood, by Forrest Shreve. 

 Further study has been made of the longitudinally bisected pine (Pinus 

 radiata) cut in 1921, and a redwood {Sequoia sempervirens) of similar size 



