The circulation of water in the lower atmosphere is influenced by transpiration, the process in which plants give off moisture from leaf surfaces. About one-half of the rain which falls on a forest is returned to the atmosphere through transpiration. This has important consequences for the formation of clouds and precipitation. The heat fluxes which accompany water condensation and evaporation also affect atmospheric turbulence, which we notice as storms. Forests simultaneously influence and are influenced by the chemical and physical climate which surrounds them.
Forest decline is a significant worldwide problem.
Forests in Germany and eastern Europe have suffered much damage. In the United States, red spruce have declined in the northeast and the ponderosa pine have been damaged in California. Acid rain is declared by some to be the culprit, but the scientific evidence is not conclusive. Other factors relating to atmospheric pollutants or climate may be involved. Deforestation, especially in the tropics, is also an important concern.