Considering that "There is another sky" speaks of a worldly place (Amherst, Dickinson's home) and an otherworldly place, heaven, the main poetic device is metaphor. The speaker describes a worldly place with natural imagery but with otherworldly qualities. "Another sky" directly means a sky that is not the sky of the Earth. This "other" sky would have to be in another place. Presumably, this would be heaven because only a perfect sky could be "Ever serene and fair." The same could be said for "another sunshine." So, the speaker describes this "other" place but uses natural things to do so. In this other place, the leaves are always green, there is no frost, and flowers never fade. This perfect forest (and/or garden) is a metaphor for heaven.
But we can turn this metaphor around. As it becomes clear that Dickinson is describing a heaven-like place, we could also say that she is using hyperbole to describe a worldly place such as Amherst. In other words, in order to convince her brother, Austin, to come to Amherst, she exaggerates (hyperbole) its qualities (to heaven-like levels) in order to evoke some nostalgia and entice him to come to Amherst.
The adjective "whose" can be the possessive form of "who" or "which." That is to say that "whose" can mean a "thing's" or a "person's." So, the little forest "Whose leaf is ever green" can refer to heaven's (God's) place or the forest's place. Thus, the metaphor works both ways: a description of heaven as a metaphor for Amherst, and a description of Amherst as a metaphor for heaven. Therefore, you could say that the imagery used describes either heaven or the natural world. It depends on how you read the poem. Is it a poem describing the landscape of Amherst as a metaphor for heaven? Or, is the poem describing heaven as a metaphor for Amherst?