IN THE ITALIAN Renaissance galleries of the Metropolitan Museum of Art there is a huge religious picture by a minor master from Verona, Girolamo dai Libri, memorable for the magnificent tree that occupies much of the upper half of the painting. In his "Lives of the Painters," Giorgio Vasari praised this tree "that truly seems to be a living one," and told how trustworthy citizens of the town had many times seen swallows mistake Girolamo's tree for the real thing.

The picture was fresh in my mind when I walked into Roger Laux Nelson's exhibition at the B.R. Kornblatt Gallery, so there was no trouble at all deciding that Nelson, a contemporary artist from New York by way of Minnesota, belongs to the rare company of painters passionate about trees, of which Girolamo is the honorary chairman. Nelson is a landscape painter whose pictures often feature trees in starring roles, isolated like solo dancers upon the sunlit stages of rolling midwestern fields.

Nelson's large oil paintings are meticulously figured out in preliminary sketches; he's particularly interested in pattern (the intricate shadows cast by a tree upon a roadway) and rhythm (the flowing repetitions of the planted fields). The main adjustments from sketch to finished painting seem to be intensifications of color--the key to Nelson's passion. The technique is quite painstaking; the color is near romance.

In "Johnson's Road/Entrance," for instance, the piece that sets off the whole cluster of trees in the middle of the picture is an astonishing aqua-green bush; in "Fall Shadow" the burning yellow of the principal tree still takes a second place in intensity to a bursting red bush on the picture's outer edge; in "Ninety Degrees in the Shade" a larger tree with a deep-blue undershadow does a slow dance with a smaller fruit tree whose shadow is a wonderful ruby red. These colors are not totally non-naturalistic, but they are simple and stark and a bit outlandish: Girolamo probably would not have approved, although I can't help but think he would admire Nelson's feelings and respond to the simple monumentality of these compositions. It is not so much description of nature that Nelson aims at as an intensified sensation of it.

The exhibition continues through Jan. 5 at 406 Seventh St. NW. Olshonsky Gallery

A spirited (though not exactly Christmaslike) group exhibition of color photographs enlivens the walls of the Olshonsky Gallery. The five artists represented are Billy Burns, Joseph Gordon, Moki, Alyson Pilzer and Paul Tillinghast. All but Tillinghast, who is from New York, live in the District. The work shares certain New Wave attitudes (theatricality, self-consciousness, stylization) and certain subjects reappear (violence, sexuality, self-examination), but beyond this the range is wide and the wit sharp.

The show also is notable for the me'lange of color techniques. Moki's mise en sce nes involving a young blond woman in black dress at the Corcoran grave site, at once elegant and hilarious, are technically straightforward. Gordon's studio photographs are technically abstract and basic (and sexually kinky). Pilzer's mimicking, feminist commentaries are combinations of collage elements and hand-tinted photos, while Burns' portraits, almost straightforward, are layered with sheets of Plexiglas. Tillinghast's little incantatory images combine just about every technical possibility. The show continues through Jan. 8 at 443 Seventh St. NW. Middendorf/Lane

Leon Berkowitz's new pastel drawings, on view at Middendorf/Lane Downtown, show the mystical color master in an explicit mood -- for him. More or less tangible forms emerge from the mists of color -- orbs, ovals, vertical shapes -- so that one gets a feeling of beginning from many of these works, rather than of his more characteristic continuum. The texture of the works, touches of color on heavy paper, is very beautiful.

The group show that takes up the rest of this wide open space consists of a variety of small things by a dozen artists. John Magiotto's furtive photographs and Chris Gardner's tiny black-painted wire constructions seem especially fresh. Both shows continue through Jan. 15.