What gives the foliage of deciduous trees the signal to let go?
Why do conifers turn partly yellow and brown, in preparation for shedding?
And as winter approaches, what gardener does not wonder how dormant buds and other tender-looking parts of plants survive intact?
Putting some botany into our horticulture can help improve results in the garden. But best of all, it deepens our appreciation of how plants live their hard-working lives.
Batten Down the Hatches: Dormancy
Dormancy is a “virtual metabolic standstill,” wrote botanist Brian Capon. It’s an ecological adaptation for living in a cold environment, to survive the cold.
Herbaceous plants have two choices: They can complete their life cycles and leave only their seeds behind for the following year (annuals), or their aboveground portions can die back, leaving the roots and storage organs like rhizomes, bulbs and corms to carry on when favorable conditions resume (biennials and perennials).
But woody plants can’t completely tuck in like that. Even those that drop their leaves as part of their overall defense have parts that remain exposed. Those include organs as small and seemingly vulnerable as the buds of next year’s leaves and flowers, or the growing tips of twigs and branches where elongation will resume again come spring.
In preparation, the undeveloped flowers, leaves or shoots may become encased in overlapping bud scales every autumn. Some species may also coat the covered buds in a thick resin to protect them from the cold and wind.
It’s not just the buds that benefit from the waterproof sealant. Some insects do, too. Honey bees, for instance, mix the resin they scrape from bud scales and other plant parts with their saliva to produce propolis, which they use as a glue to seal cracks in their hives.
Coloring Up, and Letting Go
Shorter days and cooler weather trigger the breakdown of chlorophyll, the predominant pigment in most leaves. What gets unmasked are accessory pigments, including yellow and orange carotenoids that were there all along. Although hidden during the growing season, they were helping with photosynthesis.
Then—no matter the color, but all too soon for our liking—the foliage on most deciduous trees takes flight. The big event’s timing is determined by changing chemistry in the tiny abscission zone, a narrow band of cells at the base of each leaf stalk, where it attaches to the stem or branch.
None of this would happen without the plant hormones. Which hormone is at work in leaf drop? Ethylene—better known for its role in ripening fruits—is the catalyst. (Fruit and flowers, with their own specialized abscission zones and timing, are likewise influenced by ethylene on when to drop.)
It starts to break down the cell membranes and form this zone where the leaf eventually can just fall, sealing itself off and leaving a scar on woody plants. A thin cork layer forms to prevent water loss and fungal invasions.
Yellow and Brown Inner Conifer Needles
The often narrow foliage of conifers is winter-adapted: less vulnerable to the effects of ice, snow and wind than broader leaves, and coated in a waxy substance that guards against the elements.
In early fall, when the inner foliage of many conifers turns yellow and brown, people might wonder what’s wrong with the trees. As part of their life cycle, conifers undergo leaf drop, too. It’s sequential, though, not an annual process like that of deciduous trees, and not to be confused with discolored foliage throughout the tree or at the branch tips at other times, which may indicate disease or injury.
Each year, the oldest foliage fades and prepares to fall. How long each needle holds on before that is particular to the species, ranging from two years to four or more.
Admittedly, the process can look alarming. There’s no need to panic, though. Nothing’s wrong, provided you know a little about how to read the tree leaves.
Condensed from New York Times article by Margaret Roach, “How to Read the Tree Leaves,” 11/16/2022. See also Brian Capon’s Botany for Gardeners: An Introduction to the Science of Plants (1990).
For more cozy winter reading, visit our Facebook page (Buck Lake Native Plant Garden).