Clare begins by speaking to a tree as if it were a battle-weary veteran. The elm is "rifted" and "notched and scarred", and the simile that follows, "Like to a warrior's destiny!", is a little strange. A destiny can't be seen, but scars can, so the damaged bark becomes a whole life story written on the surface. The exclamation mark and the abrupt "I love" show feeling breaking into the description before the speaker has even settled down.
The next few lines set a scene of deliberate idleness. The speaker stretches out on the "shadowed sward" and listens to "the laugh of summer leaves above". That personification gives the tree a cheerful voice, which sits oddly with its wounded trunk. The elm is both battered and merry, and that mix makes it a strong figure for endurance. Then the speaker moves to its "buttressed roots", a word borrowed from architecture that makes the tree a cathedral or fortress, something built to carry weight. He leans back "in careless attitude", and the line break after "lean" makes the verse lean with him. The thoughts that follow are not careless, though. He reflects on "times, and deeds, and darings that have been", a rolling list that sounds like a half-remembered ballad. These deeds are "old castaways, now swallowed in neglect". Castaways are things thrown aside, and "swallowed" suggests they have been eaten up and left no trace.
The poem turns at line nine with "While thou art towering in thy strength of heart". The tree stands for everything the forgotten deeds are not: upright, present and alive. "Heart" here means courage, but it also can't help suggesting the heartwood at the centre of the trunk. The tree then stirs the soul "to vain imaginings", and "vain" cuts two ways. The daydreams may be futile or even a bit conceited, yet the speaker defends them by what they leave out: they are the thoughts "in which life's sordid being hath no part". "Sordid" is a harsh word for ordinary daily existence, and it suggests the speaker needs this shade as an escape. The poem admits that daydreaming is useless and then makes a case for it anyway.
The form helps hold these tensions together. It is a fourteen-line sonnet in the Shakespearean pattern, rhyming abab cdcd efef gg, in iambic pentameter, with alternate lines indented on the page. The language is deliberately old-fashioned, with "thou", "thy" and "hath", and the whole poem is an address to the tree. That gives it a slightly ceremonial tone, as if the speaker were paying respects. The sentences often spill over the line endings, which keeps the rhythm unhurried and conversational, like a mind drifting. Only in the closing couplet do the rhymes "mind" and "behind" snap shut together.
The couplet is where the poem's real subject appears. The wind sings an "eternal ditty", and the word "ditty" is telling. It is a small, humble tune, not an epic, and it is "humming of future things". Earlier the past was something that sinks and is swallowed. The future is something that burns. What the fire demands is modest: the mind longs "to leave some fragment of itself behind". The word is "fragment", not "monument" or "legacy". The speaker does not dream of being a warrior, only of leaving one piece of himself that survives. The syntax is a little tangled here, since the wind is both the singer and the song, but the muddle suits a moment when nature and the speaker's own desire blur into each other.
Once you reach the last line, the poem looks like an example of what it describes. The forgotten "deeds and darings" have vanished, and the writing of a poem is the speaker's answer to that fate, a fragment set down under a tree. Clare is best known for writing about the English countryside, and that fits the title, "The Shepherd's Tree", which names a humble figure and a shared, ordinary place. The poem suggests that grand ambition does not need a grand setting. It can start with someone lying in the grass, looking up at a scarred old elm and deciding not to be forgotten.