Thanks Aceton, love your work with Alaska and I am really inspired to start making airports..
Are there any ways to improve the land, i.e to place more trees, add fileds, grass etc.?
Not for the moment, but you can use some hacks involving the road tool. The transitional value not only define an area where the road terrain and the global terrain will blend but also removes trees.
To create fields, I use a very large "original material" road. It does not change the terrain texture so it's only visible when there are trees around. You can also use some of the runway markings on this fake field, to get an interesting effect.
The most important if you want a natural look and avoid most of the bugs related to roads is to use the lead-in and lead-out profile for the first and the last segment of these fields. It will make it blend more seamlessly with the terrain.
Finally, you can toy with the forest parameters in the athmospheric tools tab. It works only globaly but can be interesting to test it
Edit: And of course, good luck with your project
!