Frage zu Grasdächern: Müssen die Bäume nach einiger Zeit gefällt werden oder lässt man sie einfach stehen?

You are supposed to remove the trees when they are small. This will eventually ruin the roof.

Yes, trees should be removed, preferably when they are way smaller than what you see in the picture. Trees that grow large will over time damage the building. Ideally the conditions on the roof are such that trees do not thrive, but it is fairly common to have to remove some threes every few years.