Why don't evergreen trees go dormant?

Question

I know that deciduous trees go dormant in the winter to conserve energy or water, but why do evergreens not have to go dormant in the winter.

Answer 1

They do go dormant just without the drama of complete canopy loss. Temperate climate plants, even evergreens, need a dormant or rest period that is activated by decreasing period of day length & cooler temperatures. Trees enter dormancy when the whole tree carbon balance shifts. They can no longer make enough food from photosynthesis to support active growth in all parts. Also if the area gets freezing temperatures they have to protect themselves from ice crystals rupturing their cells. When a plant cell freezes, it does what a frozen water pipe does: it bursts.
For growth to continue there must be more daylight to generate photosynthate (carbohydrate) than the plant requires to support its basic metabolism (growth below ground and in the stems or woody parts plus reserves for spring bud break). With less light and a decrease in temperature to between 5 & 10 C (40-50 F) the tree struggles to support foliage. So there is a break-even point between available light energy and temperature that predicts actual canopy shut down but the plant continues to live and respiration continues to support this minimal function.
Evergreens shut down for winter dormancy but mostly do not shed their leaves (needles). Instead they set up to continue these most basic metabolic functions and yet prevent possible damage from periodic freezing; the plants super-cool. Water in the cells is chemically maintained in a liquid state below 0° C (32F) but above the homogeneous nucleation point at -38.1° C. (-37F) So these plants avoid cold damage by not freezing. This is like adding antifreeze to the car's radiator water. However if the temperature goes below -38.1° C they will freeze. So this first antifreeze method is only good for zones 3 or higher.
They go through further steps in colder areas to prevent freeze damage. however all evergreens are at risk of desiccating. Trees in a dormant condition, even without leaves, lose water. They lose water through lenticels on twigs, branches, roots, and stems so those retaining leaves (needles) when dormant suffer greater water loss.

Answer 2

When Do Trees Go Dormant