How Trees Absorb Water:
Trees supply leaves with water because of a decrease in hydrostatic (water) pressure in upper, leaf-bearing parts called crowns or canopies. The pressure difference "lifts" the water to the leaves. Ninety percent of tree water is eventually dispersed from leaf stomata, through evaporation, into the atmosphere. That beneficial loss of water from plants is called transpiration.
Amounts of Water Trees Use:
Evaporation of water from the upper parts of trees, especially leaves but also stems, flowers and roots can add to a trees water loss. Certain tree species are more efficient in managing their rate of water loss and are normally found naturally on drier sites.
Volumes of Water Trees Use:
Depending on the efficiency of the plant or tree species, it may take as little as 200 pounds (24 gallons) of water to 1,000 pounds (120 gallons) to make a pound of dry matter. A single acre of forest land, during the course of a growing season can add 4 tons of biomass but uses 4,000 tons of water to do so.
Osmosis and Hydrostatic Pressure:
Water tends to move to regions of negative hydrostatic pressure gradients. Water uptake by plant root osmosis creates a more negative hydrostatic pressure potential near the root surface. Tree roots sense water (less negative water potential) and growth is directed towards water (hydrotropism).
Transpiration Runs the Show:
Transpiration cools trees. It also causes that massive flow of mineral nutrients and water from roots to shoots which is caused by a decrease in hydrostatic (water) pressure. This loss of pressure is caused by water evaporating from the stomata into the atmosphere.