External Map Editor

K, so the short explanation:

Spoiler

1. Go to view-> turn on "Pattern mask"
This will show you what is part of the pattern and what not (Reverse pattern in this case):

2. Plant some trees and whatever you want on the map. Textures, etc...
3. Go to the pattern mask options and use 'draw' to paint the red mask. Everything you paint red won't be part of the pattern. If you make many patterns, use mask size and paint the map red, then use clear to only include what you want, its a bit faster this way.

4. Draw a rectangle around the part you want in the pattern (yellow line) in ss above.
5. Click create pattern, give it a name.

6. After pressing ok, a new option box will open. Most options are good by default, except one : PatternmaskReverseStand. Make sure this is ticked on or else, if these patterns are used for random map generation (needs to be defined) you might see these patterns spawn in water, which is obviously not desired. Once in a while this option box will dissapear before you can actually make changes , if it happens, you can change the option in the file it saves to (data\pattern\pattern.lib)

7. Click on the Save Patterns (this might take a bit as it's resaving all the patterns, which is quite a bit)