。
The leaves on the trees have started to turn red.
Literal
Trees [possessive-の] leaves [subject-が] crimson-leaf-do began.
紅葉する ('to turn autumn colours') is a suru-verb built from 紅葉 (こうよう / もみじ — 'crimson leaves'). The same kanji are read もみじ when referring to the red maple itself. The verb captures specifically the autumn colour change — the crucial seasonal event in Japanese cultural awareness, with a vocabulary all its own.