Tree radical on the left — 杜 originally named the birch-leaf pear tree (杜梨), so 木 is properly semantic. Today 杜 is mostly a surname (杜甫 the Tang poet) and the verb 'to shut/block', but the wood radical preserves its botanical origin.
Phonetic — 土 tǔ, 杜 dù: tone and consonant shifts but the same syllable family (compare 吐 tǔ, 肚 dù, 妒 dù). Also faintly semantic since trees grow from earth, but the dominant function here is phonetic.
