A general comment:
In Washington, trespass is a misdemeanor - so LE can't arrest anyone unless they see it happening (under the misdemeanor presence law). If the reporting party can somehow prove it happened, a prosecutor may issue an arrest warrant. (maybe....)
But if the land isn't controlled access (no fences, no gates, no signs) then technically it isn't trespassing. It may be a civil land use dispute. Those things are difficult, protracted and LE tends to stay out of them. We had neighbors up the street who DELIBERATELY PAVED A PARKING SPACE ONTO someone else's land. This stuff happens all the time, and usually it is a civil matter.
Even inside a building (controlled access) LE usually waits there to see the owner/agent ask a person to leave, and then and only then will arrest someone only if they refuse to leave.