Shades of Green: Earth's Forests -- Seasons

Different seasons bring huge changes to the forest. Not only are there changes in temperature, but also the amount of sunlight and precipitation. In the boreal forests, the entire growing season is squished into a few short weeks of warmer weather in the summer. Other temperate forests experience a longer growing season, but temperatures drop low enough in the winter that plants become dormant and animals hibernate.

Of course, seasons are not limited to the temperate zone. But the tropical forest's seasons are based mostly on precipitation, and there are only two of them: wet and dry.

Top: New leaves growing on a deciduous tree in the spring. Above: The footprint of a deer in winter snow. Right top and bottom: One tree in late fall, with no leaves remaining; another in mid-fall with leaves almost ready to drop Photos by Maya Walters

Throughout most tropical forests, some rain falls at all times of the year. There is often a definite dry season which usually lasts three to four months, although in extreme conditions it can last anywhere between two months and as many as ten. The severity of the dry season depends on location. In some places, the tropical trees lose their leaves during extended droughts, and grow new ones when the rains begin. Since tropical forests extend for great distances in some areas, one part of the forest may be experiencing its dry season while the rainy season progresses in a different area of the same forest. When the wet season begins on one side of the equator, the dry season usually begins on the other.

Tropical wet and dry seasons bring significant changes for the forest animals and plants. Since the rainy season brings dense clouds, plant structural growth is often greater at other times of the year when there is more sunlight. Trees also flower more often in the dry season when pollinating insects aren't stopped by heavy rains. Seeds that sprout at the beginning of the rainy season have a better chance of survival because there will be an adequate water supply for their first critical months.

The coral tree (Erythrina saundwicensis) is widespread in the tropics. It is deciduous, and flowers during the season when it has no leaves. Photo courtesy Al Walters.

Insects are far more abundant during the wet season. This is also the time when most trees are growing the largest numbers of fresh leaves, so there is plenty of food around for these insects. Similarly, most birds nest at the beginning of the wet season, so there will be the most insects available just after the eggs hatch.

Spring, summer, fall, winter -- the seasons of the temperate zone are perhaps more familiar to many people than the dry and wet seasons of the tropical forests. Temperate forests change even more noticeably with the time of year. Snow-covered bare branches and frigid temperatures make a northern forest in winter look quite different than during the few short weeks of summer, when plant and animal activity are at their peak.

Below left and right: The same Douglas maple tree in early spring, with leaf buds, and in late spring, with leaves about half of their eventual size. Photos by Maya Walters.

While most flowers bloom during the spring, plant growth peaks during the summer. Insects are most abundant in this season, as are the insect-eaters, including many species of songbirds which migrate to the northern forests from their warmer winter habitats. In most cases, these birds come north to breed, so they are not only catching insects for themselves but also their newly hatched chicks. Some birds that do not eat insects at other times of the year still feed heavily on them during this season, and in many cases, both predator and prey have evolved defenses such as camouflage.

Fruit ripens mostly in the fall, just as migrating birds are beginning to fly south to sub-tropical and tropical forests for the winter. The broad-leaved trees in all northern temperate forests are deciduous, and every fall they shed their leaves. Many tree species such as maples, birches, and aspens put on impressive displays of color as their leaves turn from green to red before falling to the ground -- a sight which attracts many tourists every year.

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