SPRING PRECIPITATION AND HEIGHT GROWTH 



683 



the product of all the other factors mentioned, in addition to tempera- 

 ture. The graphs for these factors (figure 2) show a general, though 

 not entirely consistent, relation to one another and to height growth. A 

 notable exception to the rule is found in the trend of the graphs from 

 1914 to 191 5. Both growth and precipitation show a pronounced rise, 

 but the other moisture factors, with the exception of length of rainless 

 period, show a decline. A plausible explanation for this discrepancy 



•ygp: -jsva ?*7« wn TSm. — ~ — WW isn ms~ 



Figure 2. — Factors affecting atmospheric moisture as related to height growth of western 

 yellow pine saplings. a, F.vaporation from free water surface, total inches, May-Tune 

 (records for April not available); b. Wind movement, mean miles per day -i- 10, April-June; 

 c. Number of prevailing cloudy days -^ 10, April-June; d. Annual height growth, inches; 

 e. Relative humidity, mean daily per cent -=- 10, May-June; f. Precipitation, April-June, inches. 



is that although the influence of evaporation, wind, relative humidity, 

 and number of cloudy days in 19 15 was to intensify drought conditions, 

 this influence was counterbalanced by the extremely heavy rainfall of 

 April and May; in other words, while evaporation in June and the 

 latter half of May, 191 5, indicates a high water loss, the soil was so well 

 saturated as a result of the heavy precipitation during the preceding six 

 weeks that the loss by evaporation could be borne without retarding 

 tree growth. 



