Why don't evergreens freeze?

[welby]

My son asked me this, and instead of making up a good answer that he would beleive (he caught me off guard) I decided to admit I don’t know and ask here.

In the winter, trees drop leaves for a number of reason, chief among them to avoid freezeing during very cold weather. However, evergreen trees don’t do this. Why, then, don’t the trees or pine needles freeze and kill the tree?

[Ca3799]

The sap of some conifers has an antifreeze like substance in it. Also the gas exchange is conducted throught the very long ‘leaves’ (as opposed to flat leaves) which retains the trees moisture. A conifer in a very cold climate is living in conditions that can be considered desert-like in regards to available water.

[belladonna]

During the winter photosynthesis stops in firs and spruces of the high forests. The water in the ground is frozen, and sometimes the water inside tree cells freezes! The trees combat the severe threat of freezing and winter water loss by developing tough cell walls, antifreeze-like sap, and a waxy coating on the needles. The tough cell wall prevents splitting when ice crystals form inside the cells; the waxy coating reduces water loss by evaporation. Cessation of photosynthesis also retards water loss. In the winter the fir trees are alive and green, but not growing.

http://home.earthlink.net/~swier/SubalpineFir.html