Shades of Green: Earth's Forests -- Forest Life

Climate is weather, rainfall, cloud cover, temperature, growing season, daylight. Photo courtesy Naomi Woods.

Climate affects forests and determines where they will grow and what species will inhabit them. Forests simply can't live where there isn't enough rainfall or where the temperatures are too low: these areas are taken over by desert and alpine tundra. Climate is the reason that fir trees don't grow in the Amazon, and why jaguars don't live in the boreal forests. The earth's climate is always changing, but in the past it has always changed slowly enough to allow forests to adapt. And just as climate affects the forest, the forest to a certain extent affects the climate.

Climatic diversity helps create diversity in the forest. Because the average annual temperature, precipitation, and the length of the growing season vary so greatly depending on location, the types of plants and animals also vary. Species adapt over thousands of years for the climate in their particular area, and the variety in local climates creates diversity among species.

However, the more days that are too dry or too cold for plant growth, or the more "extreme" the climate, the less diversity there is in any particular forest. Any single area where there are great fluctuations in temperature and rainfall will have limited diversity. This is simply because a limited number of species can cope with such conditions. Tropical rainforests, which enjoy a year-long growing season and relatively little temperature change during the year support many more plants and animals than the boreal forests, where temperatures drop well below freezing for much of the year.

Left and above: A northern temperate forest is covered in snow for about four months each year, while a tropical rainforest has a year-long growing season. Photos by Maya Walters.

While the temperature remains constant throughout the year in tropical forests, rainfall varies dramatically, depending both on time of year and location. There is usually a pronounced rainy season, often accompanied by extensive flooding, and a dry season, during which many trees shed their leaves if too little water is available. While the seasons in a tropical forest are based on rainfall and not temperature, they still have a definite impact on the plants and animals in the region.

Left: A tropical dry forest looks quite different from a rainforest. It is lower, with more underbrush and fewer types of plants. Photo by Maya Walters.

Mt. Waialeale, Hawaii, receives the most rainfall on earth, averaging 11,981 mm (472 inches) every year! During a dry season in the rainforest, less than 10 mm (3.9 inches) of precipitation fall during a month.

"Microclimates" are created within forests. A valley bottom collects water and cold air, mountain habitats will be drier on one slope than the other because of the rainshadow effect, and south-facing slopes in northern temperate zones are warmer on average than those facing north. These microclimates affect only relatively small areas within one type of forest.

A valley creates a microclimate, collecting water and cool air. Photo courtesy Al Walters.

The forest affects the overall climate, plus the microclimates. The tree canopy softens the fall of precipitation hitting the forest floor, protecting smaller plants from being crushed under the weight of a heavy snowfall. Water vapor evaporates from trees' leaves, contributing to moisture levels in the area. Shade from the tree cover helps keep snow from melting too quickly and causing spring floods. Forests keep soil temperatures cooler and create shelter from wind. Without this impact the forest has on the climate, many species could not persist.

The huge amounts of carbon dioxide (C0₂) being released into the atmosphere by human activities is seriously affecting the global climate. Forests could be extremely important in helping to reduce carbon dioxide levels, because trees trap carbon from the air and retain it in their trunks, leaves, and roots. Trees and other plants capture the carbon during photosynthesis and use it for growth. But if rapid climate change kills forests, trees will die and release their carbon just when carbon levels are already rising.

Vehicles are one of the major sources of carbon dioxide. Large amounts of C0₂ are also released when cleared forest is burned. Photo credit Corel Photo Clipart CD.

Because forests trap carbon and release oxygen, they have been a major factor in the development of the global climate. At one time there was virtually no free oxygen in the atmosphere; now, several billion years later, it makes up 21% of the atmosphere and we couldn't live without it. This change is thanks to plants -- including trees, and especially those in the tropical rainforests. One hectare of rainforest is almost twice as productive in terms of trapping carbon as one hectare of boreal forest.

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