Hey Jenny,
Since the lines are well defined in each image, and you are using lightmass with your foliage the issue could be that you are exceeding the maximum texture resolution per landscape component.
Lighting Foliage Instances
I found this bit of information at the bottom of our Foliage Instanced Meshes documentation which provides some other useful information in regards to the foliage system.
Foliage Instanced Meshes
Would you also mind telling me how you are applying your foliage to your landscape? i.e. procedurally generated using the (Foliage Type), Procedurally painted using the landscape material (Landscape Grass Type), or simply hand painted using the Foliage mode?
Let me know if you have further questions.
Thank you,