Why don’t conifers change color?

(The Conversation) – It’s fall in the northern hemisphere – otherwise known as leaf season. Now is where people come out to soak up the annual display of orange, red and yellow foliage painted across the landscape.

But among these bright and colorful patches are trees that remain decidedly green. Why are evergreen conifers absent from this flamboyant seasonal spectacle?

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

When temperatures begin to drop, temperate broadleaf trees — think maples and oaks — strip green chlorophyll from their leaves. It is the pigment that absorbs sunlight to fuel photosynthesis. Trees store hard-earned minerals, primarily nitrogen, that they have invested in chlorophyll in their wood for reuse in a future growing season. Yellows, oranges and reds are left fleetingly visible before the leaves fall for the winter.

Evergreen conifers – cone trees – retain their foliage all year round and have a different strategy for resisting winter stresses.

ROI in sheets

Staying evergreen is not about continuing to perform photosynthesis throughout the winter. Cold temperatures affect the metabolism of conifers like any other organism. In fact, on cold winter days, evergreen conifers do no more photosynthesis than their leafless neighbors.

The best way to understand the benefits of persistence is to consider the costs of building the leaves. Needles are really just modified leaves, after all. How do trees balance the energy needed to grow a leaf with the energy the leaf produces through photosynthesis? In other words, how long does it take for the leaves to repay their construction costs and provide the tree with a return on its investment?

Deciduous trees must recoup their investment in their hardwood canopy in a single growing season. In contrast, evergreen conifers, by clinging to their needles, allow those needles several growing seasons to contribute to their tree’s balance sheet. That’s the real benefit of staying green.

The greater leaf longevity of conifers means they can survive in environments that simply don’t work for their deciduous cousins. At higher latitudes and altitudes, shorter, cooler growing seasons can limit photosynthetic activity. Drought can still interfere with photosynthesis. Under these harsher conditions, a year may not be long enough for a leaf to produce enough energy to pay back its growing costs to the tree.

This may explain why evergreen conifers dominate mountain peaks and boreal forests that stretch across high latitudes in Alaska, Canada, and northern Europe. Deciduous hardwoods are largely abandoning these habitats – conditions mean they cannot balance their books when it comes to leaf investments and leaf photosynthetic return in a single season.

The longevity of evergreen needles varies greatly and corresponds to the degree of stress of the growing season. Some common temperate trees in southern New England, such as white pine, only retain their needles for two growing seasons. Any white pine needle overwinters only once, meeting at least the definition of an evergreen tree.

Some conifers, such as larch, don’t even make it, shedding their entire crown of needles each fall in a bright golden display that can be a highlight of the fall foliage splendor they are found in.

In contrast, bristlecone pines, inhabitants of the high elevations of the arid southwest, cling to individual needles for nearly 50 years. It can take almost as long for bristlecone pine needles to gain a photosynthetic return on the investment in their construction, given the growing season constraints they face.

Adapt to cope with winter stress

Overwintering is deeply stressful for trees.

Sub-zero temperatures pose a risk of cell frost in evergreen needles – which would be deadly. To avoid freezing, evergreen conifers accumulate high concentrations of dissolved substances called cryoprotectants that lower the freezing point of water in their cells and protect key cell structures, without interfering with metabolism.

Cold, snow and blowing snow, along with demands for longevity, cause evergreen conifers to invest their energy in needle resistance. Conifer needles vary in tenacity; for example, relatively short-lived white pine needles are more delicate. The fibrous materials that make the needles more durable further strengthen the investment of conifers, extending the period needed to get a return on the construction costs of the needles.

Heavy snow loads can lead to broken branches, a predominant risk of persistence. The thin, often drooping needles of conifers catch less snow than the broad leaves of deciduous trees. Indeed, when deciduous trees lose branches due to snowstorms, it is usually during thunderstorms on the edges of the snow season – in autumn or spring – and not in the middle of winter. , when the crowns are leafless. If you’ve ever wondered why deciduous trees take so long in the spring to shed leaves, causing them to lose prime growing days, keep in mind that trees don’t want to risk the damage that could result from an abnormal spring. storm.

The architecture of the branches of conifers is also adapted to the evacuation of snow. Evergreen branches typically extend outward and downward from the trunk: think of a Christmas tree. Additionally, the branches of conifers are generally more flexible than their deciduous tree counterparts. Heavy snow collection weighs evergreen branches down until they reach an angle where it breaks loose.

No matter the species, in mid-latitudes, where snow flies in winter and growing seasons are generally mild and favorable, trees need coping strategies. Some recreate a wreath of leaves every spring. Conifers equip their needles and branches with the characteristics needed to survive winter and thus live to see another spring – and, for some, many springs afterward.


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