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Mount Desert Island, Maine. This does not, of course, in any- 

 way detract from the importance of the moisture relations. 



Solar Radiation 

 As already explained in the section on instruments, the differ- 

 ence between the readings of the black and white atmometers 

 does not give a wholly satisfactory measure of the light conditions 

 under the forest canopy. When the values were plotted, the 

 curves, except of course for the open station, crossed and recrossed 

 in apparently hopeless confusion. Yet when the records were 

 averaged for the whole season, the rate per day seems to cor- 

 respond in a general way with the density of the forest canopy. 

 For the New York stations the hardwoods are a little higher, 

 and the canopy is slightly less dense, as shown by the richer under- 

 vegetation. But all the New York forest stations are very much 

 alike in respect to crown cover, and the solar radiation values run 

 close together. The New Haven ridge top gives about the same 

 value as the New York stations, though we would have expected 

 it to be a little higher since the stand seems to be a little more open. 

 The north slope shows a much lower solar radiation value than 

 the ridge top, which agrees with its denser shade. The lower 

 value for the Ithaca hemlock as compared with the hardwoods 

 agrees with the denser canopy. The Cranberry Lake station was 

 in a very shady spot, and shows a correspondingly low value. 



Position of the Hemlock Type in the Successional 



Series 



The records show that the hardwoods, so far as evaporation is 

 concerned, are moister than a forest of pure hemlock. It is true 

 that records in other hardwood stands might show a higher rate 

 of evaporation, but this could not be determined one way or the 

 other without a considerable number of additional stations. For 

 the present we will have to take the results we have, recognizing 

 that they are tentative, although supported by indications from 

 another study. There has been a common tendency to consider 

 moisture as the criterion of the climax forest, the climax represen- 

 ting the highest degree of moisture. Nichols, however, considers 

 that the climax is not necessarily any more moist than certain 

 other stages, and that exactly the opposite may be true. "Eco- 

 logical advance," or in common parlance the stage of progress, 



