THE RELATION BETWEEN SPRING PRECIPITATION AND 

 HEIGHT GROWTH OF WESTERN YELLOW- 

 PINE SAPLINGS IN ARIZONA 



By G. a. Pearson 

 Forest Examiner, Fort Valley Experiment Station 



An unusual opportunity for correlating plant activity with factors 

 of the habitat under field conditions is afforded by young growth of 

 western yellow pine (Pinus ponder osa, var. scopulorum, Engelm.) at 

 the Fort Valley Experiment Station, near FlagstafT, Arizona. Indi- 

 vidual saplings have been observed to exhibit a marked variation in 

 height growth from year to year. This variation is especially con- 

 spicuous for the three years 1915, 1916, and 1917. In 1915 the growth 

 was abnormally great, in 1916 there was a decided falling off, but in 

 191 7 it rose again to about the same level as in 191 5. Thus we find 

 the prevailing occurrence of a short internode, representing 1916 

 growth, separating two unusually long internodes, representing the 

 growth for 191 5 and 1917. 



Measurements were secured on 95 saplings, 5 to 8 feet tall, within 

 one-half mile of the Experiment Station. The area is practically level 

 and is typical of the mesa type of pine forest in this region. Logging 

 operations some 20 years ago removed the bulk of the timber, leaving 

 an open stand of mostly immature trees. Young growth occurs mainly 

 in groups in the openings. These groups are rarely so dense as to re- 

 sult in excessive competition for light and moisture ; in fact, they are 

 usually so open that every individual receives practically full sunliglit. 

 The measurements determine the length of each internode, or the dis- 

 tance between whorls of branches on the main stem. Since an inter- 

 node represents a year's height growth, the year corresponding to each 

 internode is readily ascertained. An effort was made to secure meas- 

 urements for 9 consecutive years, representing the period covered bv 

 meteorological records at the station. This proved to be difficult, be- 

 cause few saplings show uninterrupted growth during the entire period 

 on account of injuries and other abnormalities. As far as possible all 

 the saplings between 5 and 8 feet high in each group were taken, but 

 for the reason just stated many had to be rejected. The 5 to 8 feet 

 height class was chosen mainly as a matter of expediency, since this is 



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