Why do Fujian tea potted leaves turn black?

The leaves of Fujian tea potted plants will turn black in winter because of the weather and then fall off.

Fujian tea, also known as basic tree and cat tree, is an evergreen shrub of Lithocarpaceae. The leaves are small, rectangular, dark green and shiny. Small white flowers bloom in spring and summer, and the drupe is spherical, which turns green first and then red. It is one of the four bonsai trees in Lingnan and one of the five trees exported to the United States by China after its entry into WTO. It belongs to the angiospermaceae, and its main distribution area is Fujian Province.