Shades of Green: Earth's Forests -- Coniferous Forests

Dry coniferous forests are typically quite open, with large spaces between the trees where sunlight is not blocked from reaching the ground and grasses and shrubs grow. Photo by Maya Walters.

Most temperate forests consist of broad-leaved trees. However, these trees require a lot of water because they lose large amounts of moisture through their leaves. In some areas the soil is just too dry and sandy for broad-leaved trees to grow. In these places, conifers take over. Like the conifers in the northern boreal forests, very little water evaporates away through their needles, so they require far less moisture than broadleaf trees.

The conifers in these temperate forests are mostly pines. They have different shapes than the conifers of the boreal forests, because it is unnecessary to grow tall and pyramidal to shed snow in an area that stays warm all winter. Pines do also grow in areas that are moist enough for oaks and other broadleaf trees. What keeps the broad-leaved trees from dominating these areas? Fire.

The thick, layered bark of a young Ponderosa Pine tree. Photo by Maya Walters.

Pine trees depend on fire in order to grow in areas which would otherwise be taken over by oaks. Pines have thick bark which does not burn easily, protecting the wood underneath. The buds on their stems are surrounded by thick clumps of needles which do burn, but at a low temperature which does not damage the bud. The broad-leaved oak trees don't have these defenses, and will be killed even in a small fire. This means that in areas with frequent, relatively small fires, the oaks never have time to grow large enough to take over from the pines. Some pines even require fire to release the seeds in their cones.
North America's Jack pine forests are particularly dependent on fire -- without it, the jack pines would be shaded out and replaced by firs, spruce, and certain hardwoods. The seedlings of these other species are, however, killed in the fires that occur every hundred years or so (not long at all in the life-span of a tree). The Jack pines are completely shade intolerant, and could not survive if these other trees grew too tall.

Pines are actually partially responsible for making areas prone to fire. Their dry, fallen needles build up on the forest floor and burn easily. However, because the fires are relatively frequent, they are also relatively small because there is never too much available fuel left after the previous fire.

The needles and seed cones of a pine tree. Photo by Maya Walters.

When people stop these small natural fires, the dry needles continue to build up and eventually an enormous fire breaks out, which burns for long enough to kill the broad-leaved trees as well as the fire-tolerant pines. In places that used to burn every 20 years or so, humans have now prevented fires for many years, and the oaks are beginning to take over. Pines are not shade tolerant and, without fire, the oaks manage to grow large enough to shade them out.

The dry pine forests are ideal habitat for reptiles. Various lizard species are common, as well as snakes which are not found in more dense, moist broad-leaved forests. As in many other forest types, squirrels and chipmunks can be found running about energetically. Here they feed on the acorn crop produced by oaks growing among the pines. This food source also attracts birds including acorn woodpeckers. Another species of woodpecker in the southern pine/oak forest takes advantage of the sap-producing pine trees to surround its nesting holes with a thick, sticky layer to keep predators out.

For most animals, the fast-moving, small fires common in these forests are not a real threat. Birds can fly away from them, and mammals can usually escape or take shelter underground for the short while it takes for the fire to pass. The huge fires bound to happen after years of fire suppression, however, are much more dangerous, not only for the animals living in the forest, but the humans living around the forest.

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