'tree-related' is a modern English compound formed from 'tree' and 'related'. 'tree' originates from Old English 'trēow' meaning 'tree', and 'related' comes from the verb 'relate' (from Latin 'relatus' via Old French), where 'relate' carried the sense 'to bring back' / 'to tell'.

'tree' comes from Old English 'trēow' and retained the meaning 'tree' into Modern English; 'related' developed from Latin 'relatus' (past participle of 'referre') through Vulgar/Old French into Middle/Modern English as 'relate' + past participle ending, and the compound 'tree-related' was formed in Modern English by combining these elements.

Individually, 'tree' originally meant 'tree' and 'related' originally carried senses from Latin of 'brought back' or 'told'; together as the compound 'tree-related' the meaning stabilized as 'pertaining to trees' or 'pertaining to a tree-shaped structure'.