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Why do some trees stay green while others lose their leaves?
12/1/2016

Did you ever wonder why some trees and shrubs stay green all year? Or, conversely, why other trees shed their leaves before winter?

You might think deciduous trees lose their leaves because they’re trying to avoid freezing weather. But they’re actually coping with the drought conditions of winter.

According to Dr. Emile DeVito, New Jersey Conservation Foundation’s staff biologist, the best clue comes from tropical forests with extended dry seasons. When the rainy season ends, soil moisture drops to near zero. Broad leaves typically evaporate lots of water, so even in the warm tropics deciduous trees shed their leaves to prevent tissue death due to drying out.

New Jersey’s deciduous forests do the same thing: becoming leafless during winter for self-protection!

Even during wet winters in New Jersey, water becomes nearly unavailable to tree roots when the soil freezes. Our deciduous oak, maple and birch trees shed their leaves each fall to prepare for drought. 

Evergreens, on the other hand, are more efficient at conserving moisture. Evergreen needles are basically rolled leaves, similar to hollow tubes with all their evaporative pores tucked away on the inner wall.  “Like penne pasta in a sieve, they resist evaporation by trapping moisture inside,” said Emile.

Hollies and rhododendrons, with tough waxy leaves that resist drying out, don’t need to drop their leaves like maples and dogwoods. Rhododendrons can roll their leaves into tubes to resist freeze-drying in the cold and wind!

Imagine viewing eastern North America from space on an early February day 600 years ago. Back then, evergreen forests of spruce and fir stretched from northern Canada south to the giant hemlock/white pine forests of the Great Lakes and northern New England.

You would see a shift to broadleaf deciduous forests in southern New England and the Highlands of New Jersey, since forests had yet to be cleared for agriculture. Deciduous forests were king from the Atlantic to the Mississippi River, from the south-facing slopes of the Catskills and Berkshires to the low elevations of the Great Smokies. But moving toward coastal New Jersey’s Pine Barrens and the pine and live oak forests of the Deep South, evergreens would re-emerge as the dominant forest types on the landscape, all the way into the tropics.

This winter view of eastern North America in the early 1400s would have been green in the far north and Deep South, with a wide, leafless brown “waistband” from Pittsburgh and Boston south to Little Rock and Chattanooga.

Deep, permanent winter snow cover, though occasional, is not common in this brown, leafless winter waistband!

Without a reliable deep snow cover every winter, soil in the “waistband” region can freeze down to the root zone. For a drought-intolerant species like sugar maple, when the soil freezes it might as well be the Sahara desert.

In the far north, the snow cover is deep and insulating. Snow comes early, before the entire root zone has frozen. Spruce and fir high in the Adirondacks, or white pines and American hemlocks in cold pockets in the New Jersey Highlands, can still make sugar when the sun warms their needles and the temperatures rise into the 40s in mid-winter.

But a short distance to the south, where prolonged snow cover is uncommon, the roots of deciduous oaks, maples and birches can freeze solid. A rhododendron shrub is more likely to have frozen roots and experience "winter burn" at Morven in Princeton than in Tillman’s Ravine Natural Area in northern New Jersey!

Deciduous forests are adaptations to winter drought. Falling leaves are an "evolutionary bet" that deep snow will not be available to protect roots from a deep freeze.

As climate change impacts the predictability of snow cover in northern New Jersey and southern New England, deciduous species could gain an edge over evergreens and expand their range northward. But as the climate fluctuates, alien species are probably more likely to invade the competitive cracks that open up in native species habitats. Natural plant communities are tuned in to long-term climate patterns over the last few thousand years and rapid changes could easily allow aggressive takeovers by alien species and a loss of our forest diversity.

So hold on to your hats when the cold wind blows this winter, and don’t let your toes freeze!

For information on preserving New Jersey’s forests and natural resources, visit the New Jersey Conservation Foundation website at www.njconservation.org or contact me at info@njconservation.org.

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