The Relationship of Evergreenness, Crown Architecture, and Leaf Size

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Abstract
Most trees in northern-temperate or boreal forests are either neeedle-leaved and evergreen or broad-leaved and deciduous. Needle-leaved deciduous trees or broad-leaved evergreens exist but are much less common. It is suggested that evergreenness and needle leaves go together because small, needle-shaped leaves can readily be displayed in a patter that disperses the incoming light over a larger number of leaves, which increases totla photosyntheis by reducing the energy wastage that occurs when light energy falls on leaves that are already light-saturated. This strategy is most effective for trees that retain their leaves for several years and develop a deep canopy with many layers of leaves, in which light passing through the first few layers is captured by leaves deeper in the canopy. Since low temperatures reduced photosynthesis more in light-saturated leaves than in unsaturated leaves, mecahnisms that minimized saturation are especially important for trees that may carry out substantial phosphosynthesis at low temperatures. For deciduous trees, however, broad leaves displayed more or less perpendicular to the sun are more advantageous, since they intercept more light with fewer layers of leaves, reducing the biomass of leaves that needs to be produced each year.