352 FOREST INDICATORS. * 



activity, the trees are dependent almost entirely upon moisture stored in the 

 soil during the preceding winter and spring. Normally the great bulk, and 

 in some years all of this moisture, is stored during the winter months, De- 

 cember to March. When winter precipitation constitutes the sole supply, 

 height-growth in young saplings is apt to be small. If winter precipitation is 

 supplemented by 2 inches or more in April and May, a pronounced stimulus 

 to height-growth results. It may be stated as a general rule for the sites 

 covered by this study, that 2 inches or more of precipitation between April 1 

 and May 31 is several times as effective as the same amount in exce^ of the 

 normal precipitation between December 1 and March 31. Factors reflecting 

 atmospheric moisture conditions, including evaporation, wind movement, 

 relative humidity, cloudiness, and length of rainless period, from April 1 to 

 June 30, show a close, though not entirely consistent, relation to height- 

 growth. Temperature on the sites studied appears to be important only in 

 so far as it affects moisture conditions. Since the increase in temperature 

 results in increased water consumption, height-growth, if, as is usually the 

 case, there is a shortage of moisture, varies inversely with temperature. Ob- 

 servations indicate that where moisture is abundant, height-growth increases 

 directly with temperature. Complete records of soil moisture, if available, 

 would probably show even a closer relation to height-growth than does pre- 

 cipitation. " 



It is highly probable that water-content is the factor that exerts the primary 

 control upon height-growth, and width-growth also. However, it seems 

 practically certain that the competition for water and food between the grow- 

 ing points and the cambium ring determines that height-growth shall largely 

 precede width-growth during each year as well as during the Ufe history of the 

 individual (Mitchell, 1918). The studies of Brewster (1918:869) indicate 

 that "the height-growth of larch seedUngs does vary in accordance with 

 variations in weather conditions from year to year, and that the most favor- 

 able conditions for rapid height-growth are produced in the North Idaho 

 region by a combination of temperatures somewhat above the average, 

 coupled with a high percentage of clear days, with an average amount of pre- 

 cipitation evenly distributed in the form of good rains at intervals of four to 

 ten days preceded and followed by lighter showers." The greater rainfall, 

 lower temperature, and greater cloudiness of northern Idaho in comparison 

 with northern Arizona readily explain the relatively greater importance of 

 temperature and Ught in height-growth, as well as the difference in the sea- 

 sonal occurrence. This must be expected for the various climax associations, 

 for which the task of correlation is primarily one of discovering the limiting 

 factor by the measurement of the habitat complex. 



In the determination and classification of sites, as well as in their discussion, 

 it will conduce to clearness to recognize that this is almost wholly a matter of 

 applying the indicator method. While the word site appears to refer to the 

 physical conditions, it does so only in so far as these are indicated by the 

 presence or growth of the species concerned. And while it is felt that the 

 species affords a better measure than instruments do, such a measure is one of 

 actual growth and not one of the controlling or hmiting factors. Hence, it 

 must be recognized that height-growth indicates habitat only in a general 

 way, and that its specific indications apply only to the productiveness of the 

 area in terms of a particular tree crop. 



