forrest parameter in conf.ini Opengl section is for the autogenerated trees which appear on the *_field.tga textures and their placement is influenced by the *.tree 2 bit tga files
(irrc 0 is for no trees, 1 is trees without shadow, 2 is for normal trees with shadows, 3 is High Quality tree - my favourite)
the texture used by the palms is C:\Program Files\Ubisoft\IL-2 Sturmovik 1946\MODS\***\3do\Flora\Palms\palm1o.tga
I made a mod for you which reduces the texture size of the palm trees:
http://www.megaupload.com/?d=LIIP8APE