Dormancy of trees involves many complex layers of activity, ranging from metabolic to physical processes. By the first day of Autumn, when the day and night are of equal length, perennial plants are getting ready for winter. The prolonged photoperiod (length of night/lack of sunlight), as well as cooler nightly temperatures, are some of the most significant indicators that plants should prepare for dormancy —#WinterisComing. Interestingly, urban trees are known to keep their leaves longer than trees in rural areas due to the radiance of city street lights.
Pathway to Dormancy
Physiological processes divide this hibernation into three intuitive stages 1) Predormancy 2) Winter rest 3) Post dormancy. During pre-dormancy, trees can still resume growth and respond to stimuli but begin making the hormones needed for hibernation. To enter true dormancy, plants must decide what to do with their interstitial water and nutrients. Some plants will maintain water in their cells in a liquid state by increasing the number of solutes (minerals and hormones). The process of producing chemicals that lower the freezing point of water is called supercooling. The other option is that trees will excavate the cellular water into a central location where it can freeze without damaging the cells, referred to as intra-cellular dehydration. This unclaimed water is typically stored between cells within the root system. Intra-cellular dehydration and supercooling and intracellular dehydration prevent frost cracking and fatal dehydration within trees.
For trees to survive the brutal winter winds and temperature, they must summon all nutrients to the base of the tree for seasonal storage. Energy and nutrients must be used for branch preservation instead of leaf production. The base of leaves, fruits, and needles are coated with abscission cells to inhibit new growth. According to FSU a thin layer of cells, typically responsible for the transport of water, grow across the stem at the base of the leaf allowing the tree to separate from its terminal appendages. The abscission cells create a waxy substance that inhibits the migration of nutrients. The leaves are then unable to produce chlorophyll and therefore allowing the appearance of our beloved autumn pigments. As leaves fall and the abscission layer disappears, a protective scar grows to prevent the infiltration of pets, frost, and disease.
Chilling units influence the extent of dormancy. Chilling units are the hours spent above freezing during the winter season. The chilling requirement (number of required chilling units) is dependent on the geographic region of the grove. Long chilling requirements allow trees to remain dormant through the long stretches when the temperatures cycle above and below freezing.
Warmer environmental conditions trigger post dormancy. An idyllic slow rise in temperature allows trees to transport water back into their shoots without the immediate expectation of growth. Once the plants start to grow, they lose the ability to readjust to colder temperatures. There is usually a slow progression of development when the plant begins to grow as the temperatures slowly rise. Growth first becomes apparent when buds swell, and then green tissue emerges from the bud. However, woody plants begin growing long before the human eye notices the buds swelling. So just because your yard tree may be brown and dormant- its cells are still working hard to maintain sustainable internal conditions.