Leaf Survival and Evolution in Betulaceae

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Abstract
Studies on the changes in leaf number of 12 species of Betulaceae were carried out by repeated observations during several years. The inner structure of the winter buds of these species was also examined. The periods of leaf emergence were long in Alnus and Betula, intermediate in Corylus and short in Ostrya and Carpinus. A heavy summer leaf fall is characteristic only of the Gymnothyrsus subgenus of Alnus. One lamina and two stipules is the basic unit constituting the winter buds. Only the buds of Gymnothyrsus are composed of several of these units. In the Alnaster subgenus of Alnus, Betula and Corylus, there are one or two scales that seem to have originated from the two stipules remaining after reduction of the lamina. Eight and 24 scales of similar origin were found in Ostrya and Carpinus respectively. A common ancestral species having the following primitive characters is proposed: shoot constituted only of the equal-sized units, period of leaf emergence long, and leaf fall usually occurs. Alnus (Gymnothyrsus) is assumed to be the most primitive type as it has many characters similar to those of the proposed ancestral species. Carpinus is assumed to be the most advanced group. The leaf survivorship curve is assumed to have changed from the primitive bell-shaped to an advanced trapezoid concurrent with an increase in the number of bud scales.

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