German property law is relevant to the legal assessment of nudity in public due to the
German Civil Code section 903
Powers of the owner
The owner of a thing may, to the extent that a statute or third-party rights do not conflict with this, deal with the thing at his discretion and exclude others from every influence.
Basically, therefore, an owner can freely dispose, who and, if so, how someone may use his property. Thus, an owner can determine that nude walking is not allowed on his property. The owner's right of determination can only be restricted by generally applicable law or existing rights of third parties. Such laws are e.g. the law on the use of the forest in Germany, which obliges all forest owners to grant free access to the citizens of the country for the purpose of recreation.
However, the forest law obliges the owner only, to grant free access to the citizens, but does not prescribe the form of use by the forest visitor. Here, the freedom of decision therefore remains with the owner, who can dispose for his forest, that nude walking is not allowed, that the paths must not be left, that camping is only allowed in designated areas, overnight stays are not allowed at all etc.
The owner can certainly delegate the enforcement of his order to a representative, e.g. to a forester or to supervisors (rangers, forest wardens), so that in the private forest as well as in the state forest (owner is the state or the federal state) and in the municipal forest (owner is the municipality) instructions of these persons, if necessary, are to be obeyed.
A violation of the owner's disposition can always be punished only by civil means, i.e. for example by expulsion from the property (prohibition of use) or with the claim for damages (if damage has been caused, e.g. by additional expenses) or compensation for pain and suffering (if the well-being of persons has been impaired).
German Civil Code section 903 but also serves the owner (or tenant) of a restaurant with the free decision, whether the guests must be dressed in his restaurant or are allowed or must be nude in the restaurant or in the beer garden. The same is true for any homeowner, who celebrates a party in his garden: whether in the nude or clothed, is solely his free choice.