Beeches are singularly attractive and rare trees as they grow only in fertile soil.
In the United States beech trees were mostly destroyed by settlers who cut them down to clear farmland. Thus the poem suggests the rare beauty and character of the friend through this simple allusion. It would seem that the friend is both genuinely true to her convictions yet simultaneously capable of being flexible. Instead the friend is young enough, and perhaps inexperienced enough, to be able to change poetic modes on command. Indeed, the friend would appear to be in the autumn of life, suggesting late middle age.