Daylight Doesn't Matter: Native Trees Need Cold

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Northern native trees’ early awakening in spring may depend on chilling out during a cold winter. Whereas invading plants from the south have less need for a long winter’s nap.

Biologists recently observed that trees from further north, including syrup-dribbling sugar maples, sprouted earlier if they suffered through long cold periods. The effect of chilling exceeded that of increasing day length. The complete lack of a frigid time greatly delayed the budding of new leaves by native trees.

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However, more southerly species, such as walnut and locust trees, responded to increasing day length, regardless of whether or not they had rested through a cold snap. As winters warm up, these species have been moving further north and taking turf from chill-cherishing oak and beech.

“Contrary to previous assumptions, the increasing length of the day in spring plays no big role in the timing of budding,” said lead author Julia Laube of Germany’s Technische Universitaet Muenchen in a press release. “An ample ‘cold sleep’ is what plants need in order to wake up on time in the spring.”

Climate change may give an advantage to invaders from the south. Since the newcomers get an early start in the spring, they have more time to overgrow native trees. However, nature could take revenge on these southern species. A sudden cold spell can destroy the tender buds of trees that sprang for spring too soon.

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To conduct the experiment, Lauge’s team placed cuttings from thirty-six tree and shrub species in growth chambers to test the plants’ reactions to changing light and temperature. Growth chambers resemble refrigerators or walk-in freezers. The enclosed chambers allow scientists to alter and control important aspects of plants’ growing conditions, such as humidity, temperature changes and day length.

This research was published in Global Change Biology.

IMAGE: A winter forest in Germany (Böhringer Friedrich, Wikimedia Commons)

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