Is it possible to thin them out and if so how? Some airports have them too close to the end of the runways.One airport in particular is Nadi in Fiji. On and ils approach to one of the runways the glide slope takes you through the trees. I know in fs9 that problem did not exist at Nadi.

In the fsx.CFG file. find the "TERRAIN" section.
These entries control the autogen density for buildings and trees:
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=3500
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=4000
Those are the settings I run with. The default is, I believe, is
somwhere around those numbers. I think MAX is 4500 and 6000.
As for threes at the end of the runways, there is a download
available ( not sure if it;s here...try Avsim ) that will automatically
"clear" an area equal to 3X or 5X the runway width from the end
of all runways. ( 2 seperate downloads, one for 3X, one for 5X ).
Alternatively, and the one I would use for just a few airports,
would be to create an "Exclude" rectangle at the ends of the runways
in question. For FSX you will need to use something like SBuilderX or FSX_KML to create the autogen exclusion for trees since this requires
a shp2vector exclusion operation. You may need FSX Deluxe for the
SDK features that the above programs use.
Paul