Animal cells can tolerate frozen temperatures for short periods of time because they are constantly burning sugar to produce energy in the form of heat. Plants, in contrast, make sugar, taking in energy in the form of light. If the sun is not strong enough to keep the air above freezing, then the tree is not kept above freezing either. The Earth’s rotation is such that the North Pole tilts away from the sun for part of each year, reducing the amount of heat that is supplied to the high latitudes, and this is what causes winter in the Northern Hemisphere.
