This answer is based upon research rather than first hand knowledge, take it for what it is worth.
There appears to be a civil remedy with the closest thing in German law to trespass to land/chattel found under property law at §§ 861-64 BGB, sections outlining remedies for property right interference by so-called disturbers. In particular BGB § 862 appears to allow for both money damages and prospective injunctive relief to prevent future violations.
I've looked around quite a bit and have not located any statute other than § 123 of the Germany Criminal Code that applies (incidentally, it is a section of the German Criminal Code for which private criminal prosecutions are authorized), although the law prohibiting poaching could prevent hunting on private lands without permission. And, the translation of that section as burglary, rather than trespassing, is really a poor translation choice because in American and English legal language, burglary is trespassing with an intent to commit some other crime, while a violation of § 123 does not require such a showing. I wonder, however, if "enclosed" may be a term of art that could include a sign that says "stay out".
It could be that trespassing in open spaces might have been encompassed within the former section 361 of the German Criminal Code, which criminalized vagrancy and begging as misdemeanors under threat of a fine of up to 500 German Marks or up to six weeks’ imprisonment. It was abolished in 1974.
Some conduct that would be barred by criminal trespassing laws in the U.S. would be barred by the Ordnungswidrigkeitengesetz (Regulatory Offenses Act) such as § 113 (failure to disperse from an unlawful gathering), § 114 (trespassing on military property), § 117 (noise violations) and § 118 (public nuisance), or a local police bylaw which would be offenses roughly comparable to municipal ordinance violations in the U.S. and might ban, for example, sleeping in a public park.
But, none of those offenses correspond to simple trespassing in Anglo-American law and it appears that the default rule is that entering onto an open meadow or forest or unenclosed space that is owned by another, by itself, is not a crime in Germany. Given that many Germanic language speaking countries and other countries in the region recognize a "freedom to roam", this may well be an intended result. This source claims that Germany also recognizes this right. According to the New York Times (April 24, 2016):
Germany allows walking through privately owned forests, unused meadows
and fallow fields.
A law review article by Brian Sawers, entitled "The Right To Exclude From Unimproved Land" (2012) examined the issue on a comparative basis.
The have been incidents in which Germans visiting the U.S. and entering onto private property have been shot and killed for conduct that would not be criminal, or would be only the most petty of offenses, in their homeland, under "stand your ground" type laws.