Leaf phenology as an optimal strategy for carbon gain in plants1

Since leaves are essentially energy-gaining organs, the arrangement of leaves in time (leaf phenology) and in space (canopy architecture) in both seasonal and nonseasonal environments can be viewed as a central element in plant strategies for carbon gain. Interrelationships among leaf longevity, leaf habit, and leaf-emergence pattern together with shoot architecture affect plant productivity. Leaf longevity is shown to maximize carbon gain through three parameters: leaf photosynthetic rate, the decrease in photosynthetic rate with leaf age, and the initial construction costs of the leaf. This theoretical approach has been extended to seasonal environments and effectively simulated the geographical pattern of leaf habits. To avoid self-shading, plants adopt two alternative modes of leaf emergence. One is successive leaf emergence, in which plants expand one leaf at a time on a shoot; this unshaded leaf utilizes full sunlight and is only replaced by a second leaf when its photosynthetic ability declines. Plants with successive leaf emergence attain high production and have straight shoots with multilayered canopy architecture. The alternative is simultaneous leaf emergence on shoots inclined to minimize self-shading through a monolayered canopy architecture. By the inclination of the shoot, each leaf on the shoot can receive sufficient light. Plants with simultaneous leaf emergence utilize the entire growing period effectively. Taken together and in the context of shoot and canopy architecture these interrelationships among leaf longevity, habit, and emergence pattern provide the basis for a synthetic theory of leaf phenology.