Why Don’t Evergreen Trees Lose Leaves and Change Color?

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Coastal Redwood, El Dorado Hills (9/7/17) John Poimiroo

Actually, they do.  It just doesn’t happen all at once.

Evergreen trees have both broad leafs and needles. Madrone, magnolia and photinia are examples of broadleaved evergreens, while pine, fir cedar, spruce, redwood have needled leaves.

Evergreen needles can last anywhere from a year to 20 years, but eventually they are replaced by new leaves. When that happens, the old needles turn color and fall, but not all together, and not as dramatically as deciduous trees (e.g., maple, oak, dogwood, alder, birch).

The reason needles are green is that they are full of chlorophyll which photosynthesizes sunlight into food for the tree and reflects green light waves, making the needles look green.

Needles, just like deciduous leaves, contain carotenoid and anthocyanin pigments. You just don’t see them until the green chlorophyll stops being produced. Once that happens, hidden carotenoids (yellow, orange and brown) emerge, as is seen in the above photograph. Additionally, red, blue and purple Anthocyanins – produced in autumn from the combination of bright light and and excess sugars in the leaf cells – also emerge once the chlorophyll subsides. Yes, even evergreen leaves change color… eventually.

Evergreen trees tend to carry needles in snowy regions. A waxy coating on needles along with their narrow shape, allows them to hold water better, keeps water from freezing inside the needle (which would otherwise destroy the leaf), prevents snow from weighing down and breaking evergreen branches, and sustains the production (though slowed) of chlorophyll through winter. Whereas, broadleaved deciduous trees would be damaged if they kept producing chlorophyll and didn’t drop their leaves.

Evergreen trees do lose their leaves and the leaves do change color. It just isn’t as spectacular.

 

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