One of Nature's cleverer accomplishments is to suit a region's vegetation to the range of climatic conditions likely to occur. It is no coincidence that the luxurious plant-life of the tropical rain forest is so different from the coarse grass of the cold, northern tundra. The vegetation of each climatic zone is that which is best suited to local climatological conditions.
In general, the stature and complexity of vegetation increases with the level of precipitation. Where water is plentiful, it is dense and lush; where moisture is in short supply, plant-life is sparse and stunted. A continuous spectrum can be identified, which runs from the arid desert-with no growth at all-to equatorial regions where the abundance of rain allows the tallest and most luxuriant of trees to thrive in the evergreen rain-forests.
In between, rainfall less than about 200 mm per year is sufficient to support only grassland and shrub, while the temperate forests of northern Europe thrive in regions where annual rainfall is more than 1000 mm.
The other important influence on plant-life is the lowest temperature likely to be experienced during the winter. Broad-leaved evergreen trees thrive in the torrid atmosphere of the tropical and sub-tropical belts. In regions where the temperature tends to fall below about -3oC, the broad-leaved trees lose their leaves in wintertime- a familiar phenomenon in this country-extracting the nutrients from them for storage during the winter.
Where temperatures drop very low, broad-leaved trees give way to pines, firs and spruces._ Many trees and shrubs in cold climates provide themselves with a thick, craggy bark, whose insulation protects them from the worst effects of falling temperatures. Others add a kind of anti-freeze to their hydraulic systems that lower the freezing point of their internal fluid.
And yet more exploit the fact that water finds it difficult to lapse into the solid state unless it contains specks of impurities on which the ice can grow; some plants maintain their liquid water "supercooled" well below the nominal freezing point by insuring that their internal plumbing is kept scrupulously clean.
Another way of coping with severe cold is to avoid the problem altogether. Annual plants just fade away and leave survival to their seeds.
The seeds themselves are dehydrated, so there is no water to expand on freezing and cause damage, and they can survive for months, years, or even centuries, until charged with moisture and triggered into growth by rising temperatures.