Chapter 17 Physiology of Tree Dormancy

The two main triggers of physiological dormancy are declining daylight hours and exposure to colder temperatures. This occurs naturally at the end of the growing season. Other common methods used in nurseries to slow seedling growth and help harden them for winter are decreasing the frequency of irrigation and altering fertilization regimes. Decreasing or suspending nitrogen fertilization, and maintaining or increasing potassium and phosphorus is commonly used to help condition seedlings for dormancy.

17.1 To Keep Its Leaves a Tree Needs Auxin

Auxin is made by the growing tips of branches, and stimulates them to grow longer. Green photosynthesizing leaves also produce auxin. It is not enough to affect the growth of branches, but leaf auxin IS enough to signal to the branch that a leaf is attached and needs to be supplied with water and nutrients. In response to this bit of auxin, the branch keeps open the tubes that move water, minerals, and sugar (made by photosynthesis) in and out of that leaf. Anytime a leaf is damaged or diseased, its auxin production falls. In response, the branch quickly seals off the connection at the base of the leaf with a layer of cork–like material, called an abscission layer (abscission is just the technical term for dropping of leaves, fruit, or flowers). The abscission layer is fragile and the damaged leaf breaks away. The corky scar tissue left behind prevents the branch from losing water and prevents bacterial or fungal attackers from entering.

To demonstrate this process find a tree outside that still has green leaves, and cut off just the blade (flat green portion) of several leaves; do not remove the petioles (the basal stems that connect the leaves to the branch). Within a day or two the petioles that had their green blades removed will form an abscission layer and break off, just like dead leaves do in late autumn.

Other hormones that help regulate dormancy include the drought hormone (also known as dormin or ABA (short for abscisic acid)); and ethylene.

17.2 Long Days, Short Days

Nearly all plants contain light–sensitive pigments that they use to “measure” day and night length. Another set of molecules lets them sense and “remember” the average or minimum overnight air temperatures. (The exact mechanisms differ between plants, and are not that important for this particular story.) During the long days of summer, these sensors cause leaves to make auxin. In turn, the auxin prevents the leaves from making ethylene or dormin/ABA. In autumn, broadleaf trees that are native to temperate zones sense the shorter days and cooler nights. In response they turn off auxin production, and the leaves begin to make ethylene and dormin/ABA instead. Drier days in late summer and early autumn also increase ABA production.

Ethylene has a variety of effects on a tree. First, it spreads throughout each leaf and triggers breakdown of the chlorophyll and any other useful nutrients. The leaves send these breakdown products back into the trunk and branches, where they are stored by specialized cells in the green cambium. These storage cells hold the nutrients until spring, when they are recycled into the new leaves that form from breaking buds. Ethylene also stimulates branches to make the corky abscission layer where the dying leaves will break off.

Close-up of the abscission point where leaves break from the stem. Modified from original image

Before the leaves break off completely, the rest of the tree must be prepared for winter too. During the growing season, the branches contain a large amount of water. If these branches freeze during winter, they can split and break. To prevent this, dormin/ABA closes the pores on the leaves, and reduces water movement through the main trunk and branches. As a result, the branches shrink slightly. Tightening and drying out the branches in this way helps prevent damage from freezing.

Dormin/ABA circulating around the tree also affects the outermost layer of tiny, unexpanded green leaves around the buds on the branch tips, and at the bases of older leaves. These outer leaves thicken and form bud scales; they fill with protective waxes and cellulose, and form a hard case around the remaining leaves in the bud. The bud scales will remain sealed until another hormone triggers their destruction during bud break next spring.

The final step in the process of entering dormancy is for the tree to actually shed its leaves. Once most of the useful nutrients have been transported out of leaves, ethylene production jumps dramatically. The abscission layer builds up quickly (just like after an injury), until the leaf is sealed off from the branch entirely. At this point, most species rely on the elements (wind, rain, snow) to knock off the useless leaves. However, there is one more treat in store: the autumn foliage colors.

17.3 What Makes All Those Leaf Colors?

All green plants contain several different pigments. Most have at least two different types of chlorophyll. There is so much chlorophyll that the colors of other pigments are masked. However, when the chlorophyll breaks down in autumn, the other colors become visible temporarily. Carotenoids are the second most common pigments found in leaves. This same family of pigments is what give flowers, carrots, squashes, and pumpkins their yellow to orange colors. Nearly all leaves contain some type of carotenoid pigment, but the specific type determines whether the leaf will be yellow, amber, bronze, or orange once the chlorophyll breaks down. Oaks and beeches tend to have brown leaves because they are so rich in tannic acids, another family of pigments. The most dramatic pigments are also the most unusual. Anthocyanins are not present in leaves during summer. Instead, they are made by the last few living cells in leaves, using of the last remaining sugars. If the remaining water inside the leaves is acidic then the anthocyanins will be bright red; if this remnant fluid is basic, the pigments turn purple.

17.4 How Is This Useful in Bonsai?

It is possible to alter the rate at which a tree progresses into dormancy, and to some degree control the color show. According to a flyer produced by the Georgia Forestry Commission:

“Warm weather can encourage late–season production of chlorophyll and vegetative growth and decrease fall colors…Long periods of cloudy, wet weather can produce a drab, fall coloration due to low light intensity. In contrast, the more sunshine that leaves receive, the more vivid the color. Thatʼs why shaded trees will be less colorful than those that get lots of sun during autumn changes. Trees that donʼt get enough water during the growing season may drop their leaves before the color display. Optimum conditions for fall color displays are cool (but not freezing) temperatures, mild late–season drought, and sunny days.”