The theme of a poem is the underlying message of the poem, or the poem's big idea. Two possible themes come to mind for Gieve Patel's "On Killing a Tree."
One possible theme has to do with the resilience of nature. The tree in this poem can sustain a lot of injury and hardship and still survive; if the reader of the poem views the tree as a symbol representing nature as a whole, humanity, or even an individual person, then a discussion of this theme of resilience can have a positive tone. After all, resilience is an admirable quality, and one that deserves celebration.
Another possible theme has to do with the vulnerability of nature, especially as the title of the poem is "On Killing a Tree." The tree in this poem is strong and resilient, but only to a point. Like any living thing, the tree has vulnerabilities, and if these vulnerabilities are exposed by violence, the tree will die. If the tree symbolizes anything else that is alive, then a discussion of this theme must contain a warning to the reader to protect vulnerabilities, as exposure can lead to death.