Answer: Cassia (Senna) leptophylla, the Gold Medallion tree, is partially deciduous in your region. It naturally loses some, but not all of its leaves in the winter time. As long as the tree is putting out new growth, there's nothing to worry about. It does like regular moisture (which it seems to be getting) but it will also adapt to slightly drier soils. The important thing is that the soil drains well - it hates wet feet. I think the tree is doing just what it should be doing at this time of year.
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