The American landscape has changed dramatically since 19th-century painters such as Thomas Cole and Asher B. Durand set out to glosify its seemingly endless beauty and fastness.

So has American art.

"I am interested in time and change rather than illusion," says Alan Sonfist, the 32-year-old American artist whose show "Alan Sonfist Trees"opened last week at the National Colection of Fine Arts.

Creating the illusion of nature in paint was the chief expressive device available to the 19th-century artist.

But, if today the means are different, the impluse to make art about the landscape and nature has not changed, though current efforts tend to express concern about its disappearance.

It is the interaction of the artist with nature which is described in all of Sonfist's art, but rather than creating the illusion of leaves, trees and rocks in paint, he uses real ones, organizing them so as to make us aware of natural phenomena that might otherwise go unnoticed.

For example, in earlier days, he left dampened canvases out of doors, when brought them inside so that the mold could be observed as it grew and spread over the surface. In the show here there are works from his Leaves Frozen in Time" series, in which canvases were left on the forest floor for a week in the fall, and the fallen leaves later affixed by applying a layer of wax.

In another series called "Leaves Collected in Time," Sonfist explains, "I set up collecting baskets throughout the forest and used the leaves that had fallen into them during the day to make collages." Elsewhere, a green leaf was placed on green paper, allowing the viewer to observe the change in color which subsequently took place.

Sonfist's vocabulary is based largely on a whole new visual vocabulary that evolved in the '60s, and included not only Earth Art, but Body and Conceptual Art. There are several pairs of photographs wherein his hands mimic the shape of a drying leaf. The conceptual component is of major importance, and is often expressed in legends hung next to his work.

Sonfist's relationship to nature is an extraordinarily intimate one, and began when he was a child in the South Bronx, and lived near a hemlock forest, one of the few remaining virgin forests in New York. There, he took refuge from the surrounding street life, and made endless drawings of trees. One of them, showing Sonfist actually within the trunk of the tree, is included in this exhibition.

He has often returned to that forest and to others, making rubbings of tree bark, or taking photographs and gathering sticks, stones and leaves from the site. He subsequently assembled these into collages, along with the very personal recollections and feelings they aroused in him.

Sonfist subsequently went off to Illinois to study agriculture. "I had never thought of art as a viable way of making a hiring," he says, "but agriculture turned out to be a study in big business."He soon quit, studied architecture briefly, and ended up getting a degree in art from Ohio State, where he received his first encouragement. He later took a master's degree in art at Hunter College.

It was after returning to New York that he began to work on his largest realized project to date - the re-creation of a pecolonial forest in a small plot in Greenwich Village, the first spring planting of which took place last April. "I didn't know how to go about showing my art, so I wrote letters to all the museum directors in New York. The Museum of Modern Art referred me to the sculpture department, which referred me to the architecture department, which did nothing." (Sonfist has subsequently willed his body to MOMA.)

"But Tom Hoving, then director of the Metropolitan Museum, responded positively and called me into his office," says Sonfist. "He was interested in creating a precolonial forest inside the projected amarican using the museum, and turned the project over to his deputy, Ted Rousseau. When Rousseau died, the project was dropped.

In 1974, Sonfist went to his own local planning board in Greenwich Village, which decided upon a site and approved the $100,000 project in 1977. He was helped by a $15,000 grant from the National Endowment for the Arts' "Art in Public Places" program, matched by several New York banks, including Chase Manhattan. With the help of the Ney York Horticultural Society, an ecologist and an historian, the first trees have now been planted.

"The 'Time Landscape' will ultimately recreate a sculptural environment representing three stages of the forest as it would have been observed during the colonization of the land," he explains. Sonfist is currently involved with planning a Brooklyn waterside project that would bring people into contact with the ecology of the water.

Though he is paid no fee for public projects, sonfist does not get a commission for the privately executed ones. He is considering a possibel "natural house" for the Maryland collector, with the house and environment "totally integrated," and a rooftop "historical forest" in Manhattan. His chief source of income is currently from lecture fees. A lecture at the NCFA before the show closes on Jan. 7 has been discussed, but not date has been set.

"It is not my goal to be a staying artist," says Sonfist, who continues to have problems with those who suggest that his "Time Landscape" is not art but landscape architecture. "Landscape architects aim to redesign the land; my aim is to restore it to its natural state to remind people that we cannot separate ourselves from the environment," says the unshaven (naturally) artist.

And as to the larger question of whether his other work is art, Sonfist replies that since the dave painters, whatever their specific goals, artists were trying to create a bridge between themselves and the natural world that surrounded them.

"The churches, too, used art to create a dialogue between people and higher spiritual forces. That is really all I am trying to do - to make people more aware of their relationship to the natural forces which surround them, and of which they are a part. It is only fairly recently that art has been used chiefly as interior decoration, or as something to buy and sell in galleries."

If Son fists exhibition might be said to begin with his childhood drawing of himself within a tree, it may be said to end in his "Box of Dreams," a square plywood box hanging on the NCFA wall, and beside it the following explanation: "In the box are objects that have been important to me from childhood to the present. The secret of opening the box is to have dialogue with me."

"I think Asher B. Durand was trying to communicate this same sense of the value of nature, and he often expressed his frustration at not being pressed his frustration at not being able to communicate it more directly. He was stuck with a 19th-century visual vocabulary. But I am absolutely convinced that if he was working today, he'd be doing what I'm doing now," says Sonfist.