Trunkline Magazine (Louisville Zoo) December 2018 - Page 11

GARDEN TALK TREES AND WINTER DORMANCY By Matt Lahm, Assistant Curator of Education Cold weather is here with all its barren trees and evergreens. Have you ever wondered why some trees and shrubs lose their leaves in winter and others don’t? You’re actually observing an adaptation for winter survival! Those trees and shrubs that do lose leaves are called deciduous, meaning their leaves fall off or shed seasonally or at a certain stage of development in the life cy- cle. Trees like pine trees and spruces are called conifers or “evergreens” because while they lose needles oc- casionally, typically they don’t go all at once. Did you know that “de- ciduous” isn’t exclusively a term for plants? Some animals experience a deciduous adaptation including the male white-tailed deer: their antlers drop off every year around late January and then regrow starting in the spring! Beginning in the fall, the criti- cal elements plants need start to change: the daylight is shorter, the weather is colder and drier and water begins to freeze. Plants can sense these environmental changes! These are also the signals for the deciduous trees and shrubs to stop producing food/sugars via photo- synthesis, stop growing and begin the process of going into dormancy for the winter. Like those animals that hibernate for the winter, dor- mancy is a complex series of physi- cal and physiological processes. This means they are not dead but rather they overwinter in a resting phase with essential life processes continu- ing at a minimal rate. So, what’s going on inside of these deciduous trees and shrubs? One of the greatest misconceptions is that the shorter days are a signal to plants to enter dormancy. It’s ac- tually the increase in the length of the nighttime that’s critical, even in the spring and summer! This "pho- toperiodism" is instrumental in the dormancy process. A longer period of nighttime triggers the production of a chemical in the tree’s cells for the tree to enter a state of dorman- cy, while shorter periods of night- time produce chemicals that signal the tree to grow. When it comes to dormancy, tim- ing is everything. If a plant enters dormancy too soon, it loses the opportunity to photosynthesize, grow and store more energy before settling in for the season. However, if it waits too long, it risks frost damage and desiccation. Once the process of dormancy begins, the leaves on these trees begin to turn color and drop off, hence the term “fall.” This festival of color brings autumn to life before our evergreen trees take center stage during the winter season. As you celebrate the end of the year, take a mo- ment to appreciate the wonder and beauty of the trees around you. Louisville Zoo Trunkline • Winter 2018 • 11