In Russell Kincaid's epitaph, (this is Russell speaking from beyond the grave) he recalls the last spring season he experienced when he was alive. He describes a "forsaken orchard" in which he saw a an old apple tree which he identified with. The apple tree was old and could no longer bear fruit. Perhaps Russell identified with this tree because he was also experiencing the last days of his life.
Just to muse on the apple tree
With its ruined trunk and blasted branches,
And shoots of green whose delicate blossoms
Were sprinkled over the skeleton tangle,
Never to grow in fruit.
The apple tree is "ruined" with its "skeleton tangle" of bare, bone-like branches; and its now delicate blossoms are too delicate and/or frail to bear fruit. Russell adds that his spirit was "girded" (encircled) by "flesh half dead" and "senses numb." This could refer to the tree or Russell himself; perhaps in his old age, his senses and his body were not as responsive as they used to be. Again, this is similar to the apple tree which is old, ruined, and can no longer bear fruit.
Russell then expresses frustration that he (and the tree) lose part of their vitality before they die ("O earth that leaves us ere heaven takes us!"). Russell then dreams of being a tree in the spring, still with dreams of fully blossoming. He dreams of being that tree (essentially in his "spring" with his whole life ahead of him, or at least one more vital season), and in dreaming he considers his own death and particularly the time right before his death, when he had lost his vitality (symbolized in this line as "earth") - "O earth that leaves us ere heaven takes us!" In a sense, Russell is comparing those last days of his life as a place/time between heaven and earth (as noted in the last line). This was a time when he'd lost some of his senses, some of his vitality, and was left only to comprehend his situation in relation to a dying tree. Although it is quite melancholy, it is profound in Russell's identification with life (the tree) and the complexity of his transition from life to death; a transition that is "neither earth nor heaven."