In "The Pastoral Art of Robert Frost," John Lynen wrote:
"Frost sees in nature a symbol of man's relation to the world. Though he writes about a forest or a wildflower, his real subject is humanity."
Connect this quotation to some of his poems.
I'm inclined to agree with wordprof on this one. Sometimes a nature poem is exactly that: a poem about nature and its beauty, its simplicity, its complexity, its grandeur, etc. I've always liked Frost for that reason. He writes poems about nature in a way that is accessible to readers at all different levels. Consider, for example, "Fireflies in the Garden." The poem is about fireflies, and Frost compares them to stars at night. It's a simple and straightforward comparison. At no point do I consider humanity's place in the world when I read this poem. I'm sure John Lynen would disagree. Good for him. Differences of opinion are what make discussing literature fun (although Captain Beatty from Fahrenheit 451 would claim otherwise).
However, since your question asks for a poem that supports Lynen's quote, I'll take a crack at it. In the poem "Nothing Gold Can Stay," Frost opens by saying that "Nature's first green is gold." He then walks the reader through how those first greens do not stay around forever and closes with "nothing gold can stay." S.E. Hinton must have thought that poem was highly representative of humanity, because she chose to reference it in "The Outsiders" as Johnny Cade is dying. He tells Ponyboy "Stay gold, Ponyboy, stay gold." That book focuses on the loss of innocence of many of its main characters. The Greasers recognize that Ponyboy has the best chance of maintaining his innocence and not turning "hard" like the rest of them. The innocence is golden, and the hardening is the loss of the golden innocence. Nothing gold can stay. People have to grow up. I'm sure Lynen would agree.
In some respects, the opposite is true: Frost’s primary goal is to draw sketches of New England life—the mending of stone walls, the walking paths in the pastures, fields, woods, the horse-drawn carriages, etc. That his observations touch on the Jungian universal symbols by which humans understand each other is perfectly predictable; it is scholars and critics who attach these symbols to the poems. In other words, Frost is in no way a symbolist. He is best compared to a visual artist who captures moments in time that represent the whole scene; he is a sketcher of the world around him. Probably the strangest thing about his output is that his efforts never culminated in a longer full-length work.