Shades of Green: Earth's Forests -- Forests Through Time

Time is crucial in thinking about forests. Compared to trees, humans have very short life spans, and because forests evolve over long periods of time, humans have difficulty understanding them.

Even though forests may look solid and permanent, they are always changing, due to everything from wildfires and storms to pest and disease cycles, climate cycles, and even continental drift. In recent history humans have caused increasingly rapid changes in forests.

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As landforms and climate change over time, forests change with them. During the last ice age, for example, there was far less available land and the climate was cooler. Forests retreated to ice-free areas, and world forest cover was therefore greatly reduced.

Tree ferns have existed for millions of years, and are one of the oldest types of plants. Photo by Maya Walters.

Amber, the hardened sap from ancient trees, can give us a window into Earth's distant past. Insects, plant material, and even small animals became trapped in the sweet, fluid tree sap millions of years ago and were perfectly preserved. Amber is found all over the world. One hundred million years ago, New Jersey was warm and swampy, covered with cedars and tropical trees. The discovery of amber from these ancient forests can tell us about the trees that lived then, and the insect species that were trapped in their resin.

During several periods when the climate in Africa was moister than it is now, the Sahara desert was almost entirely covered by scrub vegetation. Also during those times, many areas that are now savanna were dense tropical forests.

This information about past climates and forests has been discovered with the help of data from fossilized pollen, which can indicate what types of plants existed in what regions. Grains of pollen can form "microfossils" which can help us find out about the plants they originated from millions of years ago.

Changing landforms and the movement of continents over millions of years change the habitats available to forests. For example, 70 million years ago, what are now North America's Rocky Mountains had not yet been formed, and that area was covered by a vast sea. This was followed by intense volcanic activity and uplifting of mountains. New land was created, though at the same time, the climate was cooling and becoming more arid.

Of course, forests can and do change on much faster time scales as well. Many disturbances can greatly change the composition of a forest extremely quickly, including fires, floods, pollutants, "alien" species and pests, volcanic eruptions, humans.

The hala tree grows to about 35 feet (10 meters). These trees are over 250 million years old, and one of the world's most ancient plants. Photo by Maya Walters.

The way in which cleared areas are re-colonized by plants is called ecological succession. Different plants grow in different places, but the patterns of ecological succession are similar all over the world, in both temperate and tropical forests. First the tough, annual grasses move in, followed by shrubs, and finally after several years, trees. The intermediate stages of vegetation are necessary for a new forest to grow, and if an area is disturbed too often, the soil loses too many nutrients and instead of returning to forest, the area remains a grassland.

Tree ferns and cycads, some of the most ancient plants on earth. Photos by Maya Walters.

Geography as well as climate, plays an important role in determining what species will be found in different areas. Animals and plants evolved separately in North and South America, but then came into contact over the land bridge which is now Central America. Certain types of plants, such as the several thousand species of bromeliads, thrive in the Americas but are completely absent from other continents.

No communities of plants and animals are stable. Many factors are constantly disrupting these ecosystems -- weather, predation, food supply, and, above all, humans. Conditions are favorable to different species at different times. Ecosystems are constantly changing, and after every change, it is impossible to re-create the ecosystem that existed before. That is one of the problems humans face when trying to "manage" forests. As hard as they might try, people can't mold the forest into something that has been, or keep the forest at the state it is now.

Disturbances to forest ecosystems happen all the time; some create short-term havoc, others take several years to begin showing their effects. Some disturbance is necessary to create the diversity of habitats. For example, small, frequent fires in certain areas make it easier for pines to survive where oaks would otherwise grow. When fires happen regularly, species adapt, and some trees even depend on fire to release their seeds.

If forests change naturally, why worry about the disruptions humans cause? Because humans are fast. If the climate changes naturally, it takes a while. Forests have time to migrate, some species die out, new species develop. But humans are dramatically changing ecosystems all over the world at a rate at which animals and plants just can't keep up.

Seasons bring major changes to forests, but at predictable times. Plants and animals have adapted to take advantage of food sources and habitat available at different times of the year. Those that inhabit the temperate zone have to deal with the freezing temperatures of winter and the very different conditions of the other three seasons, while the tropical forests experience only two distinct seasons.

Weather can change forests in less dramatic ways as well. Temporary cold spells and droughts can seriously affect certain species, though don't usually change the entire composition of the forest. Other disturbances, such as pests, may also be confined to one or two types of trees, creating small gaps in the forest. Diseases can also be responsible for far more serious changes in forests. The effect of any given disturbance depends on its frequency, predictability, and magnitude.

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[humans & forests] [temperate regions] [tropical regions] [prehistoric forests & amber] [ecological succession] [seasons]

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