stay sheltered beneath the ground. .
Indeed, at times the forest floor is coated with seedlings. Hundreds, .
even thousands of tiny maples, oaks, birches, and pines may be .
struggling to grow there, but not one in a hundred reaches maturity.
Despite these overwhelming odds, forests are the dominant vegetation .
not just of this landscape, but of any place where water is sufficiently .
abundant - without being too abundant. .
Before the evolution of the plants that make up today's forests, more .
primitive species still shaped themselves into tree-like forms--like .
giant club mosses (today appearing as runners a few inches above the .
ground), tree ferns, and cycads (link to glossary). But even during that .
era, as during our own, the limiting condition on forests was the same: .
moisture. All forested landscapes are wet enough, during at least part .
of the year, to allow trees to draw moisture to a height above the .
ground. .
Why do trees need water? As plants turn sunlight into sugars, through .
the process of photosynthesis, they use up water. Additional water.
transpires through the pores in their leaves. To compensate for that .
loss of water, the plant creates hydrostatic pressure--or suction--in its .
vascular system. (This is also known as the "phloem.") That suction .
must be strong enough to suck water out of the soil. The amount of .
force needed depends on the amount of water in the soil (a drier soil .
holds water more tightly), as well as the height to which the water .
must be lifted (a greater height requires more force, while a lesser .
height requires less force).
If the soil is too dry, not enough water will rise to a full tree height,.
and the vegetation that grows in that area will be shorter species, like .
brushy chaparral (a shrub). In the driest areas, such as deserts, few or .
no species of plants can grow. .
While tall trees can't survive in areas with soils that are dry year round, .
they can survive in areas with seasonal dryness.