7S BULLETIN 1233, V. S. DEPARTMENT OF AGRICULTURE. 



not been determined, but other evidence points to a very serious 

 deficiency in the growth activity, with a resultant lack of vigor in the 

 new growth of 1921, notwithstanding an unusual abundance of 

 water in 1921. 



It would seem that for the region and flora that are being described, 

 an increasing ratio of precipitation to evaporation might be considered 

 favorable until the ratio reaches about 2:1, after which any increase 

 is probably detrimental. This should be definitely determined by a 

 study of the width and structure of rings. 



A succession of years such as 1916 and 1917 is evidently a very 

 serious strain on the forest, and probably this period has done more 

 toward weeding out mature trees in the Pikes Peak region than any 

 other set of conditions covered by this series of observations. This 

 has been particularly noted in relation to yellow pine, both in pure 

 stands and where the tree competes with Douglas fir. The yellow 

 pine suffered more than any other species from the unusual drying 

 winds of January and March, 1916. Further weakening through two*^ 

 seasons of drought no doubt gave its natural enemy, the mistletoe, a 

 very decided advantage, and the attacks of this parasite became 

 most evident in 1920 in a very marked deterioration of old trees, ■ 

 wherever mistletoe was at all abundant. As elsewhere pointed 

 out, this loss of yellow pine is giving Douglas fir in many places a 

 decided advantage. These casually observed facts are stated in 

 order to show the importance of unusual conditions in the life of the 

 forest, and the danger arising from short-period comparisons of the 

 conditions in the several forest types. Without doubt this com- 

 parison of years, which has been attempted, demands as much con- 

 sideration in drawing conclusions as any direct comparison of con- 

 ditions in various forests which it is possible to make. 



Relative evaporation rates at other stations. — Prior to 1916 the 

 efforts to measure evaporation continuously were largely unsuc- 



ul, and during 1916 satisfactory instruments were available for 

 only a few stations. In 1917 practically the same line-up was 

 -ccurcd with the latest type of instruments. After this trial the 

 number of instruments was greatly increased, and more satisfactory 

 records were secured through calibration of the instruments and 

 through their frequent shifting from one station to another. From 

 March to the cessation of observations in September, 191.S, all the 

 local stations were covered, with systematic rotation of instruments 

 among them; hence this portion of the record is very satisfactory. 

 The record was further rounded out from May, 1919, to September, 

 1920, a Bpecial effort being made to cover the winter period at all 

 stations. 



As there are no long records for any of the stations that were 

 secured with a single type of instrument, it has seemed best to ex- 

 press the evaporation at an outlying station as a percentage of that 

 for the com ml station, for each month of record, regardless of the 



• of instrument currently in use. WHen, however, the same month 



i covered during more than one year, the arithmetic means 



for 'I.'' control station and the outlying station have been similarly 



pared. The ratios for years and growing seasons are based on 

 the mean total amounts of evaporation for the respective periods. 

 In Table 23 the comparative evaporation rates, so far as any 

 proximate basis will permit, are shown. 



