Placing the Time of Leaf Emergence in an Evolutionary Context.

The American Naturalist originally promised its readers that through “our monthly calendar of the periodic returns of animals, farmers and gardeners will be warned of the attacks of insects injurious to crops” (Introductory Comments 1867, p. 3). In spite of this promise, however, periodic returns of life cycle events, that is, phenology, did not appear in the journal’s pages until the 1970s (Leck 1970). Papers on flowering and fruiting phenology in the following years focused on competition for pollinators or dispersers as forces structuring plant communities (e.g., Howe and Estabrook 1977; Cole 1981), a paradigm dating from an 1895 paper by Charles Robertson that Nickolas Waser has highlighted in this series (Waser 2015). Here, I want to highlight the paper that first shone the light of evolution on the greatly varying times when different species restart photosynthesis after a leafless period during the winter. Even within single forests, leaf emergence in spring varies over several weeks among coexisting native trees. Why? In 1984, The American Naturalist published Martin Lechowicz’s study that “put forward an evolutionary and ecological interpretation of interspecific variations in leaf phenology” (Lechowicz 1984, p. 824). Why was Lechowicz thinking about the evolution of leaf-out strategies? He had received his graduate training in physiological ecology and community ecology at the University of Wisconsin in the early 1970s and, prior to that 1984 paper, had never worked on phenology. In fact, Lechowicz started thinking about tree flushing because of gypsy moths, which reached Montreal in 1976, the same year that he took up a position atMcGill University. As he told me by e-mail, the moth outbreak provided funding for research on tree defenses against insect herbivores.