Why do some reputedly hardy trees look happy and healthy through the coldest weeks of winter, only to suddenly drop their leaves and die as soon as the weather warms up in spring? Why do my rhododendrons appear to be on the brink of death on frosty mornings?
These and similar ques ons are regularly asked by tree owners and gardeners at field days and elsewhere. To understand how to answer them, it is necessary to consider some of the largely invisible ways in which trees prepare themselves for winter.
Humans can simply don more, and warmer,
clothes, turn up the dial on the heat exchanger, or chuck more logs on the wood burner when the weather turns cold. Domes c animals can be covered. Birds can migrate seasonally to warmer climates. But trees can’t escape. They can’t walk or talk (although there is some evidence, reported in a recent newsle er, that they can communicate with each other in a limited way). So they have to be subtle about preparing for winter.
It is neither a simple nor a quick process. Trees begin their an freeze prepara ons even before the end of summer when they ini ate internal and invisible changes at the cellular level that allow them to survive normal winters. Key parts of these processes in both conifers and broadleafed hardwood trees are aimed at countering freezing and preven ng water loss during the cold season.
All trees have a measurable “killing temperature”, at which ice crystals form within their cell structures, with fatal consequences.
Killing temperatures vary between different species and even between popula ons of the same species They set the clima c limits for the different tree species. Part of the an freeze process involves changes in cell
membranes so that they become more elas c. This lets water move out of the cells and into the spaces between cells, where it can freeze without fatal consequences.
A second unseen but important part of the prepara ons for winter is the conversion of starches in the sap to sugars, which serve as a kind of an freeze compound. The sugars become concentrated in the cellular fluid found within a cell, lowering the freezing point of the contents. Because the membrane is more elas c during the cold season, it bends rather than bursts when ice crystals form between the cells. In addi on, when the water surrounding the cells freezes, it releases small amounts of heat energy which, although ny, helps to prevent the internal cellular fluid from freezing.
Deciduous trees begin their prepara ons for winter in early autumn when they respond to the shortening days and lengthening nights by changing the produc on rates of a number of chemicals and hormones. Among the most important changes is an increase in abscisic acid, which slows the produc on of protein and RNA, two key factors in growth, and increases the permeability of cell membranes, an important part of the adapta on to winter cold.
The visible part of this process is the consequent chemical breakdown of green chlorophyll molecules in the foliage, which reveals the yellow and orange pigmenta on of carotenes and xanthophylls. The scarlet colours are enhanced by hard frosts affec ng residual sugars and anthocyanins. This is why we get those vivid autumn leaf colours.
At the same me special ssues develop and form what are called abscission layers between leaf stems and twigs. Cells along this line expand at different rates, and enzymes degrade ssue. As a result, a physical line of weakness develops and rapidly forms scar ssue at the a achment point that prevents water loss when gravity and wind cause leaves to fall.
Not all trees lose their leaves at the same me. Some species, notably beech and oaks, retain brown dead leaves well into the winter. This appears to play li le part in protec ng the dormant buds from cold. Things happen in nature by adapta on rather than accident, but it is not clear what evolu onary advantage beech and oak trees gain from this habit.
In temperate forest at high la tudes, all broadleafed trees lose their leaves. Conifers also drop leaves in winter, but with a few excep ons (including the larches) do not shed them all. Most conifers retain their needles for two to three years before shedding them. Although conifers require resources to produce new needles each year, they gain a large measure of economy by using a set of needles for more than one year.
Conifer needles have a thick, waxy coa ng that significantly reduces water loss. In some species, it does this so efficiently that in extreme circumstances the tree can be effec vely dead of drought before damage to the needles becomes visible. Needles also have much ghter stomatal closure. Stomata are the pores that allow air and water to pass in and out of the needle. Lastly, ssues in the needles undergo a process of adapta on to cold, similar to that of other living ssues in trees.
Broadleafed trees and conifers also have different plumbing. Broadleafed trees lose most of their ability to circulate water a er the first freeze and need to regrow the necessary ssues in the spring. Conifers be er accommodate the transfer of water in winter. They have special “check valves” that can allow resump on of water movement should condi ons be just right, if unseasonable. The conifer cell walls are stronger than those of hardwoods and be er able to cope with ice expansion.
Bark-spli ng is poten ally another winter problem. Bark is excellent insula on but thin-barked trees exposed to either direct or reflected sunlight can register rela vely high temperatures just below the bark although the air around them is below freezing. This makes the tree vulnerable to changes in temperature. As the sun sets, or if a shadow is cast on the tree, sub-bark temperatures can drop too fast for the ssues to react. When growth
resumes in the spring, the damaged ssues dry out and then crack. In severe cases an apparently healthy tree will collapse and die.
Frost-cracking can some mes be prevented by shading the trunk with a winter covering of sacking or some other solar barrier.
Conifers have higher leaf densi es than hardwoods, which means that the extra weight of accumulated snow would damage stems and break branches if they had not evolved alterna ve growth and branching pa erns to counter the problem. Where hardwood trees usually have mul ple leaders or main stems (indeterminant growth), o en with narrow branch angles, conifers tend to have single leaders (determinant growth) and wider branch angles that enable them to shed snow without bending as much. Conifers also usually have longer wood fibres that make their stems more flexible.
Broadleafed hardwoods, on the other hand, whether they are deciduous or evergreen, are o en more vulnerable to snow damage at lower al tudes because lowland snow in New Zealand is usually classified as heavy and wet and is apt to accumulate and s ck even on bare branches. This is par cularly no ceable in species such as silver birch which are adapted to deep snow in their natural habitats but vulnerable to breakage in New Zealand, probably because the snow in their natural habitats is more powdery and less likely to accumulate in the crown.
Some evergreen species from lower la tudes, including rhododendrons and buddlejas, are vulnerable to snow damage but have evolved strategies to cope with frosty mornings. One is the lame duck syndrome. Typically, the leaves of plants that adopt this technique curl up or wilt when the temperature drops below zero, but return to normal as soon as the day warms up.
Other factors, including the amount of summer or autumn ripening the growth receives, and the frequency of damaging early-autumn frosts, or spring frosts a er growth resumes, can also affect hardiness. This applies mainly to plants from stable con nental climates grown in changeable mari me climates such as that of New Zealand. An example is the silk tree, Albizia julibrissen, which was being promoted for hor cultural shelter in the North Island in the 1960s and 1970s and is a common ornamental tree in Christchurch gardens. Known as silk tree in New Zealand and mimosa in the United States, Albizia is na ve to the Asian con nent and is an a rac ve small to medium-sized garden tree with very fragrant flowers. In the United States it is rated as hardy in Zone 5b, and is widely grown. It seeds freely in Massachuse s (zone 6, -23 deg) and has been declared an invasive plant pest in some states, including Florida and Tennessee. This hardiness ra ng means it should be tolerant of winter frosts down to about -25 degrees. On parts of the Canterbury Plains much lesser frosts (of about -10 degrees) are usually fatal to it.
Numerous plants of con nental origin suffer a similar fate here. The wonder tree (Idesia polycarpa) is another example. In Rotorua this is a familiar sight and is even used as a street tree. Its big bunches of sealing wax- red berries hanging from bare branches are a charming sight in winter. Sadly, in most of the South Island away from a narrow coastal strip sheltered from salt-laden winds wonder tree is a non-starter. A na ve of northern Japan, the Korean Peninsula, and nearby parts of China, wonder tree is rated as hardy to US Zone 6 and should be hardy everywhere in the South Island except the coldest parts of Central Otago and the Mackenzie Country. Again, the problem is not extreme winter cold but our unstable oceanic climate with its frequent unseasonable cold snaps.