A gnarled bristlecone pine tree called Methuselah growing on a mountain just north of Death Valley, Calif., is believed to be more than 4,600 years old. By contrast, the life expectancy of a tree downtown in a large city today is 10 to 15 years and in the suburbs 30 to 35 years.

Not so many years ago our forests seemed never-ending. Gypsy moth caterpillars came along and defoliated millions and millions of acres of trees, and there was no known way to stop them. Defoliation made the tree more susceptible to Armilleria mellea, a root-rot fungus, which kills trees weakened by loss of their leaves. Now there is hope that a newly developed insecticide, Gypchek, may provide control of the caterpillars.

Many acres of forest land have been taken to provide space suitable for building homes and growing food for an increasing population; now we are beginning to realize that too little forest remains to meet today's needs for wood products, clean water, wildlife and recreation.

The U.S. Forest Service has launched a program which it hopes will reverse the trend. This year, its 75th anniversary, it is urging everyone to plant a tree to celebrate it, to observe all kinds of special occasions . . . births and birthdays, weddings, new homes, new friends, anniversaries, even just a sunny day.

If you have no place to plant a tree, perhaps you can select a tree growing nearby and help keep it healthy. Maybe you can water it during prolonged dry weather.

Wherever trees grow, their health and development are influenced by the water supply. Viewed broadly, growth and survival of trees and other plants depend more on availability of water than on any other factor.

Under ideal conditions, trees absorb and give off tremendous amounts of water vapor. For example, willows and cottonwoods along stream banks may transpire as much as 40 inches of water in one growing season. It is not uncommon for individual trees to transpire as much as a barrel of water a day.

Air pollution has been increasing at an alarming rate in densely populated areas of the world. Trees and shrubs filter out many air pollutants, in fact, use some of them as raw materials for photosynthesis.

Noise pollution is an annoyance that was unknown in our agrarian past. Increased and effective planting of trees and shrubs can be an inexpensive and long-lasting solution.

They can trap and absorb noises, a typical example being plantings along a heavily traveled road. Research has shown that dense thicket plantings can reduce noise emissions by as much as 20 percent. Even grass on the lawn has some effect.

But it has been shown that trees can help in the problem of energy waste.

A recent test conducted with two identical houses in South Dakota showed that fuel consumption in one of them was reduced 40 percent by an evergreen windbreak of trees.A comparable test in the East found a 10 percent reduction after a windbreak was planted.

The difference between sheltered and unsheltered residences in the windswept plains is more dramatic than in the more sheltered eastern states, but the eastern experience is more significant because there are thousand of houses in the East for every one on the Great Plains, according to the Forest Service.

The location of the windbreak is the key to its effectiveness. Most cold winds throughout the nation come from the north and west, so shelter plantings should be located on those sides with an extension on the eastern side wherever space permits. The south side should be left open to permit sunlight to enter.

A stand of 20-foot evergreen trees, such as cedar or juniper, located 80 to 120 feet from the north side of the house should do the job.

Trees could make a big difference in the selling price of your home. Studies by the Forest Service showed that the presence of trees around the house may enhance the value of the property by as much as 20 percent, with an average increase of 5 to 10 percent.