The earth’s natural vegetative cover is a direct result of climate, with trees representing the most significant plant form. Climate is the chief determinate of tree growth; collectively, trees play a key role in terms of global climate. Extensive areas of tropical rainforest, such as in the Brazilian Amazon, have evolved in response to conditions of high average annual temperatures, above 64°F, and abundant rainfall every month. The Amazon Forest, in turn, has a major world impact on the atmosphere through the absorption and storage of large quantities of carbon dioxide and the release of substantial quantities of oxygen. The Amazon Forest has been called the “lungs of the planet.”
At the local level, cultivated trees can modify conditions to create a slightly cooler or warmer microclimate. Air temperatures in the shade of a tree are always a few degrees cooler than in direct sun. The cooling effect is a direct result of transpiration from tree leaf surfaces which takes up latent heat from the atmosphere and cools the surrounding air.
Consider an oasis in the Saharan Desert in Africa. The desert climate is hot and dry; rainfall is sporadic and unpredictable; ground surfaces are bare sand or rock with few plants or animals. But if a subsurface water source exists and is exploited by people, that same parched landscape can be transformed into a productive multilayered garden of date palms and other fruit trees, casting cooling shade for an understory of annual food crops, along with creating favorable conditions for livestock rearing and human habitation.
In any climatic regions, strategic landscaping with trees and shrubs around houses, apartment buildings and commercial structures can minimize the negative impact of sun and wind and create desirable microenvironments, even in a densely populated urban area. Trees and shrubs planted toward the prevailing wind, reduce its velocity, reducing the negative effects on structures in terms of cooling or heating. Depending upon the local conditions, deciduous or evergreen trees may be selected to fulfill this role. Trees also have a role in the earth’s carbon cycle by extracting carbon dioxide from the atmosphere and storing the carbon in the tree where it remains until the tree dies and decomposes. The increase of carbon dioxide in the atmosphere has been identified as the leading greenhouse gas contributing to the steadily increasing global temperatures. This is another important way trees affect climate and ultimately weather patterns.