When expressing degree and followed by a plural, it is predominantly construed like an adverb, meaning that the following phrase is not necessarily dative, but takes its construction from its syntactic function. Thus: Bis zu zwei Drittel der Insel sind bewaldet(“Up to two thirds of the island are made up of forest”), where zwei Drittel is treated as the subject, rather than Bis zu zwei Dritteln der Insel ist bewaldet, where the prepositional phrase as a whole serves as subject.
When followed by a singular, the dative construction is usually maintained: Es fiel bis zu einem Meter Schnee(“There fell up to a metre of snow”), but at least informally bis zu ein Meter is also possible.
