The conifers are gymnosperms that bear seeds in cones instead of within fruits as do the flowering plants. Familiar conifers include pines, spruces, firs, cedars, and redwoods.
A wide band of conifers girdles Eurasia and North America north of the deciduous forests and grasslands. Most of these conifers retain their leaves the year around and shed one set only after the next is in place.
Persistence of the leaves permits conifers to take immediate advantage of good weather, since they are able to begin photosynthesis without waiting to develop new leaves. This is a distinct advantage toward the poles, where the warm season is short.
These conifers also tolerate low temperatures. One reason is that they have stiff, wax-covered leaves known as “needles.” When cold winds blow through the forests, the thick layer of wax retards water loss through evaporation. Of course, the leaves do dehydrate, and they would wilt if they were not rigid.
Thus the stiffness of the needles, due to tissues with exceedingly thick cell walls, prevents damage that would otherwise be caused by wilting. The leaf structure of conifers clearly suits them to subarctic regions, where winter winds sweep across the landscape.
Conifers communities have fewer species than do deciduous forests. One reason is that the ever-present shade of the year-roung canopy restricts photosynthesis by understory plants. As a result, the ground is nearly bare except for a deep carpet of slowly decomposing needles.
Slow decomposition coupled with leaching of the soil by acids from the needle litter results in low fertility. Conifers generally have root fungi, known as mycorrhizae, which aid in accumulation of minerals. Trees without these symbiotic partners are at a disadvantage.
A boreal forest usually consists of one species of high latitudes. The major differences are in the length of the day and the intensity of sunlight.
Where mountains are at temperate latitudes, organisms experience a day length typical of the temperate zone. But because mountains stand above much of the earth’s atmosphere, the sun’s radiation is more intense here than at lower attitudes. (This results from the fact that the atmosphere absorbs part of the radiation that reaches it.) Yet mountains cool quickly at night because of loss of radiant heat through the thin atmosphere.
This and exposure to winds make high altitudes colder than those in adjacent lowlands. The cool weather of high altitudes creates environments resembling those towards the poles.
Southern Conifers: A Fire Climax
The same adaptations that fit conifers to winter dryness in cold climates suit many to hotter areas, especially where the soil is sandy and the water supply is unstable. In the southeastern United States, pines colonize abandoned fields and form major forests.
Perhaps their initial success depends on mycorrhizae, since these soils are usually poor. Left undisturbed, many such pine forests are replaced by oak, hickory, or magnolia. Apparently fire set by lightning or by humans maintains these pine forests, since they tolerate fire, whereas the broad-leafed trees do not. The pines have a thin trunk bark through which new buds sprout if existing limbs are destroyed.
A thick tuft of needles protects the tenninal bud of the southern longleaf pine and makes it extremely resistant to fire. As a matter of fact, longleaf pine seedings seldom survive in the absence of fire, for they are sensitive to crowding and shading. Only when other vegetation is burned back can the little pines get enough light to grow.
If southern pine forests are burned every few years, the fires do little damage to wildlife, because the animals can escape by running through of flying over the low fire line. In fact, occasional burnings of patches of pine forest increase the population of bob-white quail and wild turkeys.
These birds find food and shelter in the successional stages that follow those fires. We are not suggesting that fire is always beneficial even in the forests.
In contrast to the low, cool surface fires of frequently burned regions, fires in pine forests that have not burned recently blaze up high and hot. Over many years without fire, a thick blanket of needles and fallen limbs accumulates on the ground. Once this litter is on fire, it provides so much fuel that the heat is sufficient to ignite the tree-topsand produce great crown fires. Such fires destroy the plant community and much of the wildlife.