248 



DRY AND MESOPHYTIC FOREST COMMUNITIES 



of moisture of the atmosphere, decreased light, decreased temperature 

 maximum in summer. The poisoning of the soil by root excretions 

 and the modification of conditions on the ground brought about by a 



JUNE 



JULY AUGUST 



SEPTEMBER 



OCTOBER 



30 



t 



25 



\ 



20 



15 



Cottonwood dune 

 Pine dune __.__._._ 

 Oak dune .-..------ 



Beech-maple forest _.._.. 



FIG. 251. Mean daily evaporation rates (c.c. per day) in the ground stratum of 

 four of the animal communities (after Fuller). 



given set of trees are believed to prevent the germination of seeds of 

 most of such trees, and at the same time to prepare the way for those of 



