Ontogenetic changes in leaf phenology of Ulmus davidiana var. japonica, a deciduous broad-leaved tree.

To determine how plants control leaf phenology to maximize annual carbon gain, I examined ontogenetic changes in leaf phenology of Japanese elm, Ulmus davidiana var. japonica Nakai plants of different ages growing in contrasting light environments. Leaf emergence occurred earlier in 1- and 2-year-old seedlings than in current-year seedlings. Although leaf emergence was not affected by light conditions at the sites, it was influenced by plant height. The delay in leaf emergence increased with increasing plant height. These traits indicate that seedlings that received the least light during the summer intercepted light for a long period during the spring; however, the advantage of earlier leaf emergence decreased with increasing plant height. At each site, 1-year-old seedlings had a longer duration of leaf emergence than adults, because of a longer period of favorable light conditions even in the forest understory. Duration of leaf emergence, leaf duration and leaf longevity were usually longer in sun than in shade for both seedlings and adults; however, flexibility in the response to light was greater in seedlings than in adults. The plastic response in leaf phenology during the juvenile stages may contribute to the optimization of light acquisition in habitats with differing light conditions, thereby enhancing seedling survival.