When Mormon pioneers entered Salt Lake Valley in 1847 and started to build a city in the desert, they planted a lot of trees. Greenery helps make a city a pleasant place to live. It even improves the air.

That's a thought to remember as Utahns observe Arbor Day this Friday an occasion set aside to encourage tree planting. Unfortunately, Salt Lake City is losing ground in this regard.The city has about 47,000 trees on parking strips and other city property. Each year, the city's urban forestry program plants 400 new ones. But each year, 800 old trees die. Some 23,000 trees, about half the existing supply, are in poor to dying condition. At this rate, the city theoretically could become a desert again, at least as far as trees are concerned.

Trees are more healthy and last longer if they are trimmed on a regular basis, but Salt Lake City's urban forester can't get around to that job very often because of lack of manpower. On a national basis, the lifespan of a tree has been calculated at 32 years. At the present pace, it would take the city forester 33 years between trimmings.

Clearly, the city budget needs to devote at least little more money to urban forestry, but in the current budget squeeze that may be difficult.

There are other options.

Schools are trying to help by collecting old newspapers for recycling. Dumpsters are located on school grounds. This program raises about $2,000 a year, but more involvement by citizens could boost this to, say, $20,000.

Private business can help by sponsoring a few trees each year. It takes about $48 to buy, plant, and care for a new tree. There are plenty of places available. City officials estimated that 17,000 sites need trees.

That many trees can't be planted all at once, but Arbor Day is a good day for the public to at least help the city get started.