Trees can be used as sand, and in some cases, they can also be used as water. But the role of trees in Feng Shui is to block evil spirits and collect Qi. There is no yin or yang. There are many factions of Feng Shui, such as Sanyuan, Sanhe, Tianxing, Xuankong Feixing, Bazhai, Jinsuo, etc. There is no Yin tree or Yang tree.
Trees are used to block evil spirits and collect qi. They can also be used as sand, but they are not used to separate yin and yang.