Steven Cassidy, age 11, of Lynch, Neb., for his question:
WHAT IS THE TIMBER LINE?
An incredible variety of plant and animal life fills a forest. Scientists recently recorded nearly 10,500 different kinds of organisms in a single deciduous forest in Switzerland. The life is part of a complex system in which all the physical environment is included. Organisms interact with one another in the balance of nature.
Savannas are forest areas of widely spaced trees. In some savannas the trees grow in clumps, while in others individual trees grow throughout the area.
Growing above foothill savannas of western United States and Canada are mountain evergreen forests. As one, moves up in altitude, the climate becomes colder, wetter and windier. Forests of the lower and middle slopes are called montane forests, while those of the upper slopes are known as subalpine forests.
Lower montane forests are made up of unmixed stands of ponderosa pine in the Rockies. At the higher elevations, Douglas fir becomes dominant. Douglas fir is mixed with grand fir in the northern Rockies and with blue spruce and white fir in the southern Rockies.
Above this lie the snowy, cold subalpine forests which are dominated by spruce and subalpine fir. Lodgepole pine is also common in both the montane and subalpine zones, especially in areas that have been affected for forest fires.
The highest elevation at which trees can grow is called the timber line. Beyond this point, the climate is too severe for tree growth.
In the timber line zones of mountain forests, the trees grow in a scattered, savannalike way. This timber¬line country is dominated by bristlecone pine in the southern Rockies, by lumber pine in the central Rockies and by Lyall's larch and whitebark pine in the northern Rockies.
At subalpine elevations in the Sierra Nevada, the mountains support forests of red fir mixed with lodgepole pine and mountain hemlock. These subalpine forests then thin out into savannas of bristlecone and whitebark pine as elevations reach the timber line.
Well below the Sierra Nevada timber line in central California grow the magnificent stands of giant sequoia trees.
The sequoias are the bulkiest of all the world's trees, although they are not the tallest. Tallest honors go to California's coast redwoods which may tower more than 360 feet. The giant sequoias can measure about 100 feet around at the base.
Scientists say that today's world's forest regions are not permanent. It is possible that another ice age or other dramatic environmental changes could alter the forests.