Basically, you should be able to right click a tree and use the "edit appearance" menu to set a certain tree variant.
Some of the trees in the nature store have differently-shaped variants which change when the tree is stashed and placed down or when the condo is reloaded. However, the issue is that a lot of the variants are way different in shape, such as these two palm tree variants:
Of course, this causes issues if you want a specific shape of tree (such as for size reasons), since you have to keep placing and stashing the tree until you get the shape you want, and you have to redo this every time you load into your condo. Being able to manually choose what shape you want your tree to be would fix both of these issues.