Upside down forest problem

when I generate a forest, depending on my c-plane, my trees all come in upside-down. What am I doing wrong?

Are you working in a Bottom view? Trees are created with its up vector following the UCS plane’s normal.
This let you create vertical gardens if you use a Front view or a cplane placed on a wall. But you can get strange behaviors in other cases, like using a cplane which normal points down.