THE BASIS FOR VARIATION IN LEAF LONGEVITY OF PLANTS

Any theory of leaf phenology must predict leaf longevity, leaf habit, leaf expansion and its timing among other variables. These phenologica l traits may be important keys to understand the response of trees to climatic change. Here I concentrate on and review two of these critica l phenological traits, leaf longevity and leaf habit. Theories of leaf longevity were re-evaluated and leaf longevity is concluded to be opt imized to maximize plant carbon gain. From this perspective, three poi nts are predicted. Leaf longevity is short when the photosynthetic rat e of the leaf is high, when the photosynthetic rate decreases rapidly through time, or when the construction cost of the leaf is small. Thes e predictions are well supported by empirical as well as experimental results on various plant species. The theory, which is extended to sea sonal environments, is general and applicable to seasonal as well as a seasonal environments. The theory simulated the bimodal geographic dis tribution of evergreenness.