Mipmapping levels for model

Hi, I noticed that for some reason one of the models looks very ugly when mipmapping is enabled in the Pipeline Tool.

Please look at these trees on this screen:

As you can see, the quality is very bad. In comparison, the same scene but different perspective when mipmapping was not used at all:

The highest mipmapping level kicks in for his model when the camera is stupidly close.

Do you have any idea how to fix this issue?