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The factor of light is of importance in plant ecology. Within the habitat of given plant association or community the degree of light available will depend in large part upon position of the plant. Tree crowns of the upper layer receive maximum light, but correspondingly reduce the amount available to lower layers. In extreme cases forest trees so effectively cut off light that the forest floor is almost free of shrubs and herbaceous plants. In certain deciduous forests of middle latitudes, the period of early spring, before the trees are in leaf, is one of high light intensity at ground level, permitting certain herbaceous plants to go through a rapid growth cycle. In summer these plants will largely disappear as the leaf canopy is completed. Other herbaceous plants in the same habitats require shade and do not appear until later in the summer.

Treated on a global basis, the factor of light available for plant growth is varied by latitude. Duration of daylight in summer increases rapidly with higher latitude and reaches its maximum poleward of the arctic and Antarctic circles, where the sun may be above the horizon for 24 hours. Thus, although the growing season for plants is greatly shortened at high latitudes by frost, the rate of plant growth in the short frost-free summer is greatly accelerated by the prolonged daylight. But in still higher, sub arctic latitudes plant growth is greatly slowed by the low heat budget, despite perpetual summer daylight.

In middle latitudes, where vegetation is of a deciduous type, the annual rhythm of increasing and decreasing periods of daylight determines the timing of budding, flowering, fruiting, leaf-shedding, and other vegetation activities. As to the importance of light intensity itself, it is generally believed that even on overcast days there is sufficient light to permit plants to carry out photosynthesis at their maximum rates and that direct sunlight constitutes a considerable excess of light.

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