The evolutionary purpose of all plants and animals is to ensure their species reproduce themselves and survive. The main strategy of trees to overcome the environmental hazard of freezing winter temperatures is to go dormant; they stop growing until the weather turns warm in the spring. Trees prepare themselves in many other ways to endure drops in temperature that would otherwise freeze their sap and split their bark, limbs and trunks.
Tree dormancy is like hibernation. Each species has a "kill temperature," when interior ice kills cells. In response to the onset of winter, plants increase a substance called abscisic acid; plant cells experience an increase of soluble fats; the proteins in their cells break apart; and the cell membranes become more permeable. As temperature drops, the water on the exterior of twigs freezes first; although it seems counterintuitive, this freezing releases several degrees of heat. This helps keep the fluids in the twigs from reaching the killing temperature.
Shorter days and cooler nights of late summer trigger a response from abscisic acid and plant hormones in the broad leaves of deciduous trees: They prepare to go dormant. They begin to lose their leaves, and as the leaves die they lose the green color of the chlorophyll they used to create nutrients through photosynthesis in the spring and summer. Lacking chlorophyll, the leaves turn shades of yellow, gold, red and brown before they drop.
Conifers and trees that produce needles appear not to lose them. In fact, they do shed them, but only once every two or three years. As needles drop, they are replaced, so it appears that they never lose any. Needles contain sugars and resins that help keep them from freezing. Their dark green also absorbs more heat, helping keep the interior from freezing. A waxy layer covering the needles called the cutin reduces the movement of cold air around, them creating a layer of slightly warmer air, a form of insulation.
Leaves lose water from their leaves through their stomata, or pores, in a process called transpiration. Trees have to cope with a loss of water in the winter. Deciduous trees take care of that problem by shedding their leaves in the fall. Photosynthesis in evergreen needles during the winter requires water in a season when water may be scarce. The stomata or pores of needles close tightly, preventing the loss of water through transpiration.
Sources of Water
Trees need water in the winter, although they need less than when they were growing in the spring and summer. Even dormant trees have water in the sapwood of their trunks and branches. A winter warming can make this water available. Snow acts as an insulation that can make water available in soil that would otherwise be frozen. Trees can also absorb water from the bottom layers of snow.
In order for tree vessels called xylem to transport water, temperatures have to be at or above freezing. The xylem are composed of tube-like cells called tracheids. Water is cohesive; the tracheids "pull" thin columns of water up through the tree. As ice forms, it expels gases that can form gaps in these columns. Each spring, broadleaf trees grow new xylem cells to regenerate the transportation of water. Conifers form a kind of valve between each long tracheid cell, and a form of float called a torus seals the end of each cell. When the ice melts, the column of water is restored.