While "A White Heron" is written in a mostly realistic fashion, it takes a sharp turn for the magical when Sylvia starts climbing the tall oak tree, which she uses to help her reach the top of the even taller pine. We get hints of this when Mrs. Tilley describes how "the sunshine always seemed strangely yellow and hot" (1.23) around that spot—indicating this location as special—but her brief warning doesn't prepare Sylvia for the wonders she experiences.
As she climbs, that simple pine tree transforms (in Sylvia's head, at least) into "a great main-mast to the voyaging earth" (2.6). Sylvia can feel this magical energy passing into herself, making her believe that she "could go flying away among the clouds" (2.7). Sylvia, in other words, is tapped into her own power and potential; she feels filled with possibility. If the hunter has tempted her away from her true course—being connected to nature—then here she gets back on track, reconnecting with herself and loving it.
It is worth noting that, just like the gun, trees can be understood as phallic symbols. They jut into the sky, after all. Importantly, though, whereas the gun is connected to another person—and a male person, at that—the trees are something Sylvia's alone with. So the oak and pine can also be read as symbols of sexuality, but instead of being wrapped up in violence and masculinity, here they are all about Sylvia and her own power and pleasure.