Day Length and Flowering Plants
Most trees change with the seasons. Deciduous trees lose their leaves as winter approaches (or, in the seasonal tropics, as the dry season approaches) and enter a state of dormancy. This is not a simple shutting down. Dormancy takes weeks of preparation. Before trees shed their leaves, they withdraw much of the nutrients within them, including the protein of chlorophyll, leaving some of the other pigments behind to provide at least some of the glorious autumn colors; and to conserve water, they stop up the vessel ends that service the leaves.
How do trees sense seasonal change? There are many clues to season, including temperature and rainfall. But shifts in temperature and rain are capricious; they are not reliable signals. Sometimes a winter may be warm – but frost is never far away. Some autumns and springs are freezing, some balmy. The one invariable, at any particular latitude on any particular date, is the length of the day. So at least in high latitudes, where day length varies enormously from season to season, plants in general take this as their primary guide to action – while allowing themselves to be fine-tuned by other cues, including temperature. So temperate trees invariably produce their leaves and/or flowers in the spring, following the rigid pattern of solar astronomy, but they adjust their exact date of blossoming to the local weather. This phenomenon – judging time of year by length of day – is called photoperiodism. Most of the basic research on photoperiodism has been done on crop plants. But trees and crop plants work in the same way.
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