ORIGIN OF DECIDUOUS AND EVERGREEN HABITS IN TEMPERATE FORESTS

Abstract

Deciduous hardwood forests in temperate parts of the Northern Hemisphere, brilliantly colored in autumn and leafless in winter, provide a striking contrast with the somber broadleafed forests that extend to tree line in the temperate Southern Hemisphere and remain green through the year. These northern and southern temperate forests are also very dissimilar in composition, for the genera differ in each area and most of them belong to divergent families. The fossil record suggests that the morphologic differences between the plants that contribute to these forests are the result of their independent evolution on opposite sides of the tropical rainforest belt during Cretaceous and later times. Presumably the deciduous and evergreen habits displayed by woody plants in boreal and austral regions reflect fundamental differences between the environments in which they evolved. However, the factors that may account for the presence of a predominantly deciduous habit for woody angiosperms in the temperate forests of one region as opposed to typically evergreen in the other, are obscure, as is the evolution of the deciduous habit itself in northern temperate hardwood forests. In searching for clues to the factors that may have been responsible for the origin of these contrasting habits, it is desirable to turn first to evidence provided by plants living in environments where gradual gradients can now be observed between the evergreen and the deciduous habit.