In answer to Aaron, I found this case, which is directly on point in Wisconsin.

A person may acquire title to land owned by a state political subdivision by taking actual possession of the land and either protecting it by a substantial enclosure or cultivating or improving it in the usual manner of an owner, for a period of forty years. Sections 330.09 and 330.10, Stats., 1947; 5 Petropoulos v. City of West Allis, 148 Wis. 2d 762, 767, 436 N.W.2d 880, 882 (Ct. App. 1989) (because adverse possession statutes are prospective in nature, the limitation period in effect at the time possession is first taken applies). A person claiming adverse possession must show [*10] that the disputed property was used for the requisite period of time in an "open, notorious, visible, exclusive, hostile and continuous" manner that would apprise a reasonably diligent landowner and the public that the possessor claimed the land as his or her own. Pierz v. Gorski, 88 Wis. 2d 131, 137, 276 N.W.2d 352, 355 (Ct. App. 1979). An adverse claimant may "tack" or add his or her time of possession to that of prior adverse possessors with whom he or she is in privity in order to establish continuous possession for the requisite statutory period. Perpignani, 139 Wis. 2d at 724-25, 408 N.W.2d at 13. An enclosure may be either artificial, such as a fence, or natural, such as a line of trees or bushes. Illinois Steel Co. v. Bilot, 109 Wis. 430, 441, 85 N.W. 402, 406 (1901) (following remand). Furthermore, planting trees and maintaining the land around them may constitute possession of land by usual improvement. Otto v. Cornell, 119 Wis. 2d 4, 8, 349 N.W.2d 703, 706 (Ct. App. 1984).

So that means I have to prove 40 years. I have witnesses going back to 1956 that will testify to the existence of the fence that far back, but the City is saying the fence couldn't have existed before they granted the encroachment document, making it only 39 years before they tore it down.