Answer: Michelia champaca is an evergreen, but it isn't unusual for older leaves on evergreens to die in the autumn and fall from the tree or shrub. This process allows for new foliage to develop in the spring. Since the leaves falling are those closest to the trunk, I'd guess they are the older leaves. Healthy new growth is an encouraging sign, so if the tree looks otherwise healthy, the falling leaves are probably nothing to worry about. Michelia champaca will grow in full sun to partial afternoon shade. They like moist, well-draining soil.
Since your tree is still in the nursery pot, the roots may be suffering from heat stress, especially if the pot is in full sunshine for most of the day. Until you can plant your tree, try to shade the container from the sun's rays.
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