"Spring and Fall" takes place during the fall in a forest that the speaker calls "Goldengrove." We can't imagine a more perfect setting for a poem that's all about the eventual cycle (and ending) of life.
Really, you can learn a lot just by keeping your eye on the trees throughout the year. They bud in spring, flourish in summer, shed leaves in fall, and are bare and stark in winter. In that way, a tree (or a whole forest-y collection of the suckers) is a great, ready-made metaphor for the stages that all life must take, including poor little Margaret's. The falling leaves, piled around her ankles, put the speaker in mind of the coming winter—the "death" of the trees—and so provide a great jumping off point for considering the eventual decline that all lives must endure.
We think it's interesting, though, that Hopkins—a Jesuit priest—never mentions anything about an afterlife in this poem. This is a poem about the inevitability of death, and the way that affects how we live our life. To get back to the setting, though, we all know that trees, after they "die" in winter, are "reborn" in spring. The setting could have easily provided Hopkins's speaker with a chance to make such an observation to little Margaret, but he doesn't. Why not, do you think? Perhaps the setting is more of a reflection of the speaker's doubts, or fears of death.