If your trees are objects then you not only have to change the fill type but also the fill background pen to be transparent and the fill hatch pen to the colour you want.
I have seen some objects where you can't always control the background or hatch pens even though you can change the fill type - just bad scripting.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11