Hi,
There is Enable Horizontal Mirror option under Transform tab - Mirroring. Please check attached screenshot.
But as it's calculated on random base probably in your case you should use another workaround. Mirror please the source Custom object and add it to your Geometry list inside Forest object. Now while placing in Tree Editor - Custom Edit mode you can choose the Model - original or mirrored. Hope this helps you. Attaching screenshot showing also this workflow.
Kind regards,