You don't give much context, but predominant plant or predominant species will fit quite widely.

Eg:
The predominant plant covering the surface of Love’s Marsh was the water lily. The large amount of lily pads might be due to the amount of the nutrients in the water, the shallowness of the marsh, and the ability for these plants to benefit from the sun.

http://www.fhgradeschool.com/carp/2000plants.html

This seems to compare with the two Swedish references.