Open the game to the screen where the boxes appear for tutorials, routes, scenarios, ...etc. Go up to the button in the top left corner labeled Main menu. Click it and choose Options. It should open to the Video Settings. Check that the Enable compatibility mode for older content is checked.
Here is the short version of the issue with bill board trees. By the rules of the content creation guide, grayscale alpha maps were restricted to just glass surfaces but some creators decided to try them on billboard trees as a way of feathering the edges and making them better looking. However, this had a serious impact on game performance. TS2009 was the first version to offer a "native mode" that did not support the use of grayscale alpha maps in non glass surfaces. But both TS2009 and TS10 offered compatibility mode to allow the items to continue to be used at the cost of game performance. TS12 removed compatibility mode entirely. So bill board trees that don't use a black and white alpha map are going to render in odd and unpredictable ways. In various newer versions, N3V appears to have tried to change the way these trees are rendered by default but in most cases the results doesn't look good.
Interestingly, TRS19 saw the return of a setting in the launcher labelled Compatibility Mode but anytime I have changed it the program becomes unstable so it may or may not be effecting assets that use grayscale alpha maps. Maybe it has more to do with scripting issues.
