(336) 



and consistently higher than that under the hardwoods throughout 

 the entire season. So far as evaporation is concerned — and it is a 

 pretty fair index of moisture — the hemlock type is drier than the 

 mixed oaks. This is contrary to all expectations, and its bearing 

 on our conception of developmental trends in vegetation will be 

 discussed more fully below. 



The Ithaca hemlock and hardwoods, we see from Table I, had 

 exactly the same average daily evaporation for the season, so that 

 they neither corroborate nor contradict the New York results. 

 But the hemlock at Ithaca was consistently a little higher than 

 the hardwoods for an uninterrupted period extending from June 

 2 to August 24, or the longest and most important part of the 

 growing season. Furthermore, in describing the stations we 

 hinted that this particular piece of hardwoods may possibly be 

 merely a stage in a successional series leading to the hemlock 

 type or to a stand with a considerable proportion of hemlock in 

 mixture. 



The explanation of the higher evaporation under hemlock 

 as compared with hardwoods is probably to be found in the 

 practically bare needle-covered forest floor of the hemlock 

 which, in spite of its shadiness, is rather dry. Under the oak 

 type, the shrubby and herbaceous vegetation may lower the rate 

 of evaporation by checking the circulation of air, and also by 

 raising the relative humidity through the moisture they give off 

 as transpiration. Measurements of the relative humidity would 

 be needed to determine this point. 



The hardwood forest and transition between hardwoods and 

 hemlock have nearly the same rate of evaporation, the curves 

 almost coinciding throughout (See Figure i). At the end of the 

 season the transition averaged only .1 c.c. per day higher than the 

 hardwoods. This amount is too small to warrant consideration: 

 but is in agreement with the higher rate for hemlock. 



The average daily evaporation for all stations, given in Table I, 

 shows that, aside from Cranberry Lake, the evaporation under 

 hemlock forests is almost the same, even when the forests are 

 hundreds of miles apart. The markedly lower evaporation at 

 Cranberry Lake is to be expected because of the northerly 

 location of the station. The difference between the evaporation 

 under the New York hemlock and the New Haven ridge top 

 station is only .2 c.c. per day. The fact that New Haven is a 



