"Why do trees conceal/the splendor of their roots?"
This can be seen as a political metaphor about the economic base upon which the superstructure is built. These are Marxist terms, relevant to analyzing Neruda’s poetry because he was an active member of the Communist Party. So, this short poem can be seen as a nod to the masses who do the work to keep society alive, but whose labor is concealed by ideology.
Alternatively, this could be read a spiritual metaphor, or a reworking or reversal of the famous Dylan Thomas poem that begins “The force that through the green fuse drives the flower/Drives my green age; that blasts the roots of trees/Is my destroyer.”
Roots can also refer to ancestry, so it could also be read as a metaphor for an improper shame towards one’s family history.