Dormancy in trees: surviving the winter

We’ve had our first frost this year! Many trees are already showing their fall colors. Soon we will see many trees drop their foliage.

Most plants cannot function when the temperature approaches freezing. If water in tissues freezes cells will rupture, causing frost damage. If ice develops in vascular tissue it will block water transport, drying out tissue and resulting in frost drought damage.

Trees are responding to the lower temperatures and shorter days (actually they are responding to the longer nights, as experiments have shown) by “shutting down,” going into a state of dormancy. In this state, the tree reduces or foregoes photosynthesis, respiration and growth. Chlorophyll is lost from the leaves so that yellow and orange carotenoid and xanthophyll pigments are visible.  Red and purple anthocyanins are synthesized in late summer as the chlorophyll breaks down, resulting in even richer fall colors. Roots continue to grow as long as there are nutrients and moisture, halting growth only when soil temperatures become too low.

Plants that do overwinter have to prepare for it; if you take a tree in the middle of summer and expose it to freezing temperatures it will suffer irreversible damage. The same tree can tolerate freezing temperatures without being damaged if the tree has been allowed to harden. Trees harden in response to shorter days and lower temperatures. Sugar concentration increases in hardened trees, resulting in an increase in cell-sap concentration.

Most trees still respire in the winter, but at much lower rates. Dropping foliage that would normally be respiring intensely helps deciduous trees reduce water loss. Buds that formed in the summer are protected by bud scales, also to reduce water loss. Twigs form a layer of cork that slows down water losses as well.

Tree species with broad ranges extending from north to south often have idiosyncratic dormancy responses according to their latitude. Red maple trees, Acer rubrum, from Massachusetts develop winter dormancy in response to short day lengths and lower temperatures, but trees from Florida do not enter dormancy, even when exposed to shorter day lengths and lower temperatures.

Withholding water or nitrogen and other mineral nutrients can also cause trees to enter dormancy. In fact, fertilizing trees, particularly with nitrogen, in the late summer or fall can have negative consequences by stimulating growth and delaying the onset of dormancy.

Once in a state of dormancy, the trees can withstand a certain level of stressful conditions. Very low temperatures can still damage a tree. If a tree trunk freezes solid the ice inside will expand, producing pressure on the wood. Cracks may form as the wood ruptures, which are not fatal to the tree, but can increase susceptibility to diseases and insect pests.

It is not the temperatures themselves that are damaging, but the formation of ice crystals, which expands and ruptures cells or blocks vascular transport. By eliminating the formation of ice crystals trees can survive sub-freezing temperatures. Most trees do this by removing ice-nucleating particles. Although we commonly think of ice forming at 0°C (32°F), it needs an ice nucleus to start the formation of crystals. Without ice-nucleating particles, water can remain in a liquid state at -37°C (-34.6°F). Trees can survive even lower temperatures by maintaining dissolved solutes (increased cell-sap concentrations) that lower the freezing point of the water; salt water freezes at a lower temperature than fresh water. (In an aside, it seems very odd that Daniel Gabriel Fahrenheit would invent a scale where water froze at 32°… in fact he consciously chose to calibrate his thermometer scale to zero for the temperature at which a salt water mixture would freeze!)

Removing ice-nucleating particles and maintaining dissolved solutes in the sap are perfectly fine for most of the southeast where temperatures rarely, if ever, get close to -37°C, but in parts of the world where winter temperatures consistently get that low, trees rely on another method to survive; intracellular dehydration. In these trees water freezes in the extracellular spaces where it does not damage the cells, and pulls water out of living cells so that they become dehydrated. Thus, ice forms but does not damage living cells.

Coming out of dormancy requires exposure to chilling (temperatures between 0 and 10°C (32 and 50°F) for a particular number of hours, typically between 500 and 2,000 hours; the temperature and duration will depend on the tree species and its provenance. If this chill requirement is not met, growth will not occur, even if the temperature is high and the days are long.

This maple tree is getting ready for winter. Chlorophyll in the leaves has broken down revealing the yellow carotenoid pigments that were already there and the red anthocyanins which are now being synthesized prior to leaf-drop.

 

Species that normally go into dormancy may grow continuously if temperatures stay high and day length remains long. However, most species that normally undergo dormancy will enter dormancy eventually, even if the cues to do so are lacking.

One hypothesis explaining how trees enter dormancy is that abscisic acid builds up in the fall, inhibiting growth. Cold temperatures then break down the abscisic acid, removing its inhibitory effects. As the soil warms, growth promotors, including gibberellin and cytokinins are manufactured, signaling for growth to resume.

If the tree resumes growth too early or a late frost occurs, that new growth will be damaged and lost. The tree will not die, but carbon resources (sugars) will be used to produce extra growth, leaving the tree more susceptible to diseases and insect pests.

Dormancy is a great time to prune many trees. Although I personally avoid pruning if it is not necessary, pruning while the tree is dormant causes less damage to the tree. Also, transmission of diseases is less likely during the winter and there is less chance that a wood-boring insect will investigate a cut made in the winter.