The most important new recording keyed to Aaron Copland's 90th birthday, which will be celebrated Wednesday, is his opera "The Tender Land" (Virgin VCD 791113-2, two CDs with libretto). Commissioned in 1953 by Rodgers and Hammerstein, this is a gentle, homespun celebration of simple country living, imbued with American folk idioms, that may make you think of "Appalachian Spring" with voices. It has been relatively neglected, probably because its plot (about a young woman coming of age and conflict between generations in a farm family) falls short of the violent passions usually associated with opera. But it does have suspense, tension, well-defined characters and a melodic charm reminiscent of Broadway.

Lack of a good, complete recording has long been the most significant gap in the Copland discography; that need is now handsomely filled by this recording. Elizabeth Comeaux and Dan Dresen give appealing performances as the young lovers, and Philip Brunelle conducts impressively.

There has been no shortage of good recordings of Copland's Symphony No. 3, his most extended and impressive piece of absolute music. Some of its popularity may be based on its use of his well-known "Fanfare for the Common Man," which serves as the introduction to the last movement and supplies motifs heard throughout the symphony. It is brilliantly and sensitively played on RCA 60149-2-RC by the St. Louis Symphony, Leonard Slatkin conducting. Also on the disc is "Music for a Great City," a symphony-like suite Copland made from his score for "Something Wild." In movements with such titles as "Skyline" and "Subway Jam," this music reflects the urban landscapes that are the most memorable part of the film.

Two discs with more familiar Copland material also deserve mention. "Music for Martha Graham" (Koch 3-7019-2 H1), with Andrew Schenck conducting the Atlantic Sinfonietta, is a first-class interpretation of the original, complete score of "Appalachian Spring" in the version for 13 instruments that was premiered at the Library of Congress in 1944. Schenck, a specialist in the music of Samuel Barber, also conducts Barber's powerful "Cave of the Heart" (a k a "Medea").

For Nimbus (NI 5246), William Boughton conducts the English Symphony in outstanding performances of familiar Copland (the orchestral suite from "Appalachian Spring," four dance episodes from "Rodeo," the Fanfare and "Quiet City") as well as the unfamiliar and exquisite Nonet for Strings. Among its other virtues, this disc demonstrates emphatically that English musicians can play Copland as well as Americans. American Composers Even more impressive is the demonstration (on Teldec 244 924-2) of how well Germans (at least some young Germans) can play Charles Ives even in his most inscrutably ethnic moods. The Trio Fontenay gives a lively, polished and very down-homey performance of his Piano Trio, a work bursting its seams with the idioms of American folk music, hymns, square dance tunes, bits of ragtime and practical jokes. The same disc has the Opus 8 Piano Trio of Brahms, a work this group performs predictably well.

The association of Ives and Brahms seems strange at first, but one hears echoes of Brahms in occasional segments of the Ives Symphony No. 2 as performed by Leonard Bernstein and the New York Philharmonic on DG 429 220-2. The tradition in which the rambunctious American was trained was essentially Brahmsian, and it shows through sometimes when he is not working hard to shock and amaze his audience.

Ives was one of the composers Bernstein did best, and now this disc stands as a monument to both of them. Besides the symphony, it has two of Ives's key orchestral works: "Central Park in the Dark" and "The Unanswered Question," as well as the less familiar "Hallowe'en," "Tone Roads No. 1," "Hymn: Largo Cantabile" and "The Gong on the Hook and Ladder or Firemen's Parade on Main Street." The music's quality is variable, but Bernstein makes it all sound interesting.

Virgil Thomson's two great film scores, "The River" and "The Plow That Broke the Plains," ventured into Copland's territory (symphonic treatment of American popular or folk motifs) before Copland and with brilliant results. Excellent performances can be heard on ESS.A.Y Records (CD 1005), with Richard Kapp conducting the Philharmonia Virtuosi.

Even more interesting (because the material is less familiar) is a collection of Thomson's "Portraits and Self-Portraits" performed by seven excellent New England chamber musicians on Northeastern Records (NR 240-CD). Thomson began dashing off musical portraits of his friends, made in one sitting with the subject present, like snapshots or an artist's quick sketch, in 1928 and continued until his death, completing 147 portraits.

Some of these little impromptus are under one minute in length and many are under two minutes, though one is a 15-minute flute concerto. They have a marvelous freshness, spontaneity and sharply defined character. More than 30 are performed on this disc, most recorded for the first time and a few played in new arrangements. The disc includes two fine youthful works, the Piano Sonata No. 2 (1929) and the violin sonata (1930) because Thomson, years later, recognized them as self-portraits. A fascinating disc, superbly played and annotated.

Now Playing The Hanover Band, which plays at the Wolf Trap Barns on Wednesday night, is celebrating its 10th anniversary but sounds about 20 times that old. Recognized as one of the world's finest historic-instrument orchestras, under the leadership of conductor Roy Goodman, it has recorded much of the repertoire from Mozart and Haydn to Berlioz with dazzling results. Its high standards are maintained in a stylishly adept, bright and energetic recording of the complete Schubert Symphonies (Nimbus NI 5270/3, four CDs available separately). This is a total delight; you can see Schubert maturing and styles changing from the 18th-century mannerisms of the first works to the full-blown romanticism of the great No. 9.

Discoveries Familiar melodies are becoming rare as the Marco Polo label marches onward in its intrepid exploration of the complete orchestral music of Johann Strauss Jr. In the latest dispatch (Volumes 13, 14 and 15), ardent Strauss fans might recognize the "Demolirer" (demolition-men) Polka, and many people will recall the "Champagne" Polka, with its percussion reminiscent of popping corks, but the top-20 Strauss favorites simply aren't numerous enough to spread through a collection that will probably go beyond 50 volumes. No matter; the level of quality remains consistent, and the performances (by Czechoslovak orchestras conducted by Alfred Walter in Volumes 13 and 14, Johannes Wildner in Volume 15) are bright and bouncy if less opulent than the Vienna Philharmonic.