Why don't evergreens change color and drop their leaves every fall?

phys.org | 5/27/2019 | Staff
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It's autumn in the Northern Hemisphere—otherwise known as leaf-peeping season. Now is when people head outside to soak up the annual display of orange, red and yellow foliage painted across the landscape.

But mixed among those bright, colorful patches are some trees that stay steadfastly green. Why do evergreen conifers sit out this blazing seasonal spectacle?

Challenges - Problem - Winter - Trees - Way

Like so many other challenges, the problem of winter can be solved by trees in more than one way.

As temperatures begin to dip, broad-leafed temperate trees—think maples and oaks—withdraw the green chlorophyll from their leaves. That's the pigment that absorbs sunlight to power photosynthesis. Trees store the hard-won minerals, chiefly nitrogen, they've invested in chlorophyll in their wood for reuse in a future growing season. Yellows and oranges and reds are left fleetingly visible before the leaves drop for winter.

Foliage - Year-round - Strategy - Winter - Stresses

Evergreen conifers—cone-bearing trees—retain their foliage year-round and have a different strategy for withstanding winter's stresses.

Staying evergreen is not about continuing to conduct photosynthesis throughout the winter. Cold temperatures affect conifers' metabolism just as they do any other organism's. In fact, on cold wintry days, evergreen conifers perform no more photosynthesis than their leafless neighbors.

Way - Benefit - Evergreenness - Construction - Costs

The best way to understand the benefit of evergreenness is by considering the construction costs of leaves. Needles are really just modified leaves, after all. How do trees balance the energy it takes to grow a leaf with the energy that leaf produces via photosynthesis? In other words, how long do the leaves take to repay their construction costs and offer the tree a return on its investment?

Deciduous trees must recoup their investment in their leafy canopy in only a single growing season. In contrast, evergreen conifers, by hanging onto their needles, grant those needles multiple growing seasons to contribute to their tree's balance sheets. That's the real benefit to staying green.

Evergreens - Leaf

Evergreens' greater leaf...
(Excerpt) Read more at: phys.org
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