BULLETIN 12.33, U. S. DEPARTMENT OF AGRICULTURE. 



is known that in these very open situations the reproduction is 

 never abundant and often is not typical of the climax forest. In 

 the Pikes Peak region there is a strong tendency for limber pine to 

 invade 1 every open site; in the Wagon Wheel Gap region bristlecone 

 pine is the pioneer. 



Although, then, it may be seriously questioned whether the evapora- 

 tion stress is the controlling or limiting condition in many instances, 

 -till there can be little doubt that this measurement, sensitive as it is 

 to every atmospheric condition, gives an extremely convenient and 

 valuable index to site conditions, and particularly as it reflects in a 

 simple term the general temperature and sunlight conditions. 



The evaporation measure seems particularly useful in expressing 

 the broader differences between regions. Thus, only a confused 

 picture of the Nebraska sandhills is obtained by determining that the 

 temperatures are higher, the humidity higher, the wind movement 

 lower, and the sunshine (this is said for nlustrative purposes only) 

 less than at the control station. Orientation is, however, immediately 

 possible when it is found that all these differences are algebraically 

 summed up in less evaporation in Nebraska. When this result is 

 considered in connection with each of the individual factors, the 

 phenomena of tree growth become very clear. 



On the other hand, the high evaporation rates which are found at 

 i he Wagon Wheel Gap stations, in comparison with the control 

 station, demand an explanation. If the evaporation rate has any 

 direct hearing on the choice of species, how is it that unity evapora- 

 tion gives essentially the same result in the one locality (compare 

 Stations F-9 and W-Al) as three units of growing-season evaporation 

 in the other \ It should be noted that even the highest rate recorded 

 in these data is probably not excessive for established vegetation; 

 with a reasonable water supply, the highest rate may, possibly, rep- 

 resent the most favorable and not the least favorable growing 

 conditions. As the high rate in the Wagon Wheel Gap locality 

 is induced primarily by a dry atmosphere, which represents a normal 

 condition, it seems probable that that region is not, in the long run, 

 sub j eel to any more extreme maxima than the Pikes Peak region, 

 where sunshine, humidity, and temperatures suffer greater fluctua- 

 tion < from year to year. This can be certainly determined only by 

 much Longer study. 



It seems, therefore, that, if evaporation rates bear directly on the 

 composition of forest type-, this can be determined in only two ways. 

 It may he determined by a more intensive study of evaporation 

 rates ;i- they affect young seedlings, in close correlation with the 

 determination of the moisture supplies of those seedlings. This 

 point- bo the -nine situation that has been reached in the study of air 

 temperatures and -oil temperatures, namely that the conditions 

 which must be critical in the early establishment of forest trees are 

 i ltd, perhaps short-term, conditions, and are not covered by 

 tiir e genera] comparisons. 



On the other band, if evaporation is measured as in these beets 

 hut through :i Longer term of years, it is probable that extreme 

 conditions in the various forest types, particularly for the more 

 widely separated localities, will be brought to light, which, although 

 not Decessaril) constituting criteria of the resistance of the species, 

 w ill give a better conception <>f the relative Btressea which each species 



