RELATION BETWEEN EVAPORATION AND PLANT SUCCESSION I7I 
run in immediate proximity to a young healthy seedling of maple, 
pine, or white cedar and represents the conditions successfully met by 
those seedlings. Where virgin hardwood forest is cleared during a 
winter, the vegetation in the following spring consists of such forest 
species as can withstand the new conditions. This includes the seed- 
lings of Acer saccharum. Weeds appear later in the season, but not 
in great abundance during the first year. During this time maple 
seedlings have little or no protection from the full sun, yet large 
Fig. 9. View in a Tliuja bog, showing atmometer No. 33 in the center of the 
background where the shade is so dense that no green ground vegetation is present. 
August 12, 1916. 
numbers of them survive. Is a downward change in evaporation a 
necessary prerequisite to succession or is the evaporation changed as a 
result of succession? If the former is the case, since Acer saccharum 
seedlings are normal to the floor of the climax vegetation where the 
rate of evaporation is very low, it might be logical to suppose that 
maple seedlings will not be found except where the rate of evaporation 
is much less than that over bare ground. If the latter is the case, 
maple seedlings will be found growing wherever the soil is suitable, 
regardless of the rate of evaporation of the habitat and regardless of 
any change that their development may subsequently have upon the 
habitat. 
