紅葉 as もみじ and こうよう: maple objects and autumn color
Read 紅葉 as もみじ for a maple tree or leaf and こうよう for autumn foliage or trees beginning to change color. Compare the named objects and the changing action.
もみじ for a tree and a drifting leaf
紅葉 read もみじ names a maple tree or maple leaf. 「庭の紅葉が秋になって赤く色づいた。」 means “The maple tree in the garden turned red in autumn.” The tree in the garden is the object that turned red.
「川面に落ちた紅葉がゆっくり流れていく。」 means “A fallen maple leaf drifts slowly down the river.” Here 紅葉 is a fallen leaf drifting on the river, a different object from the garden tree.
こうよう for foliage and changing color
紅葉 read こうよう means autumn leaves or changing color. 「山の紅葉が夕日に照らされて輝いた。」 means “The autumn leaves on the mountain glowed in the setting sun.” The sentence describes the mountain's autumn foliage in the setting sun.
「気温が下がると木々が紅葉し始める。」 means “When the temperature drops, the trees begin to change color.” 紅葉し始める describes the beginning of a color change when the temperature drops. It does not report that every leaf has fallen.
Reading the object and action alongside the spelling
The もみじ examples name a tree and a leaf; the こうよう examples describe autumn foliage and trees beginning to change color. The shared spelling 紅葉 does not merge the two readings.
These examples establish their own readings and meanings, without deciding every possible Japanese context. In particular, distinguish the beginning in 紅葉し始める from the leaf named in 川面に落ちた紅葉.
How are 紅葉 naming a tree or leaf and 紅葉 in 木々が紅葉し始める read in these examples?
Show the answer and why
The tree and leaf examples use もみじ, while the trees beginning to change color use こうよう. The latter describes the beginning of change, not all the leaves having fallen. Keep the readings and meanings distinct despite the shared spelling.