John Wayne Farber
Winter is finally here. Our trees have dropped their leaves and snow is falling. While winter sets in we begin to bundle up and seek shelter indoors during inclement weather, but what about the trees. Trees don't have the advantage of pulling root and taking shelter indoors on a cold winter night. How do they survive? Why don't they freeze?
Cold weather preparation begins long before winter arrives. The first day of Fall is also called the Autumnal Equinox, this is a special time of year when the daylength shortens and our nights become longer. One trigger for trees to get ready for winter is the photoperiod, or how much time they encounter light. More accurately how much dark they encounter. When trees begin to receive longer periods of dark they begin to biochemically react and prepare for winter. Temperature is also a factor. Shorter days means less solar radiation (heat) we receive and the temp begins to drop. Trees react to reduced ambient and soil temperatures. Water and nutrient availability affects tree dormancy. Having less water and nutrients available in the soil is a natural sign of fall and trees react by preparing to become dormant. Exceptional years of water and nutrient (mainly nitrogen) availability late into the season may delay dormancy. This is why during real wet falls it takes longer for your trees to drop their leaves.
Now that the trees know winter is coming they need to act. Trees main prompt is the formation and accumulation of the plant regulating hormone, abscisic acid (ABA). ABA controls all kind of dormancy functions essential to tree survival. Leave begin to package up and move reusable elements back into the stem and roots, for use next spring. A special layer between the leaf and branch form, called the abscission layer, letting the leave drop while protecting the branch. Buds harden and roots grow to prepare for harsh conditions. All growth and activities are slowed to compensate for the reduced available energy.
Trees are not dead or inactive in winter. They are still creating energy from photosynthesis, dividing cells, respiring, and metabolizing, but just at a much slower rate. Green leaves are the main energy producers for trees because of the high concentrations of chlorophyll. Chlorophyll is the component in leaves that change light, water, and carbon dioxide and turn it into usable energy and oxygen. Chlorophyll is green in color and the reason we see leaves in the summer as green. When all the leaves are gone the tree is not without any ability to produce energy. Just below the bark is a layer of chlorophyll that continues to absorb sun and create energy for the tree even in winter. With this reduced amount of energy the tree is able to maintain its life support systems and survive winter.
Another neat feature is tree's ability to not die in freezing weather. As we all know water plus freezing temperature equals ice. The formation of sharp ice crystals within a cell will shred the cellular membrane and damage the cell. Fortunately trees have a couple of features they are able to use to combat cold weather. Supercooling, is when water stays liquid below 0 degrees celsius. By adding dissolved solutes trees can resist ice formation down to around -40 degrees celsius. This is the same idea behind salting your driveway to remove ice. Sometime trees encounter really cold climates and have to protect themselves even further. To manage temperatures below -40 degrees celsius trees employ controlled dehydration called intracellular dehydration. Just outside the cell the plant will allow water to readily freeze, as a result a pressure gradient is formed and water is pulled out of the cell across a membrane that is too small to allow ice to pass through. The process will dehydrate the cell, but not allow it to freeze or rupture. Plants can recover from dehydration but not freezing.
After surviving winter trees will need to break dormancy and resume normal functions of annual growth; daylight, temperature, and hormones again play a role. The term chilling is given to temperatures slightly above freezing about 2-4 degrees celsius. For each tree species a certain number of hours at these temperatures dictated when the tree might break dormancy, called chilling-hours. As temps rise and the ground begins to warm new hormones are released like gibberellin and cytokinins that promote growth. Once enough growth hormones accumulate buds begin to swell and new leaves are released. Temperature released dormancy also explains why the same species of tree will remain dormant for longer in colder climates. Lets not forget about increased daylight. Temperature aside trees with shorter days and longer nights will take longer to break dormancy than trees exposed to longer days and shorter nights. This is the reason it is not suggested to put spotlights on trees at night.
Amazingly tree dormancy is an evolutionary adaptation for trees to conserve energy when resources become limited, and conditions become adverse. Trees reduces their need for water and nutrients so they can survive extreme environmental conditions. Dormancy keeps trees protected every year by quing dormancy in fall, incredible resistance to freezing in winter, and brilliant dormancy breaking blooms in spring. Surviving winter allows trees to become established in the coldest of climates and allows us a humans to appreciate their robust vertical nature all year long.