There are farmers who do keep portions of their acreage in maple trees and who tap them every year. Some have permanent lines attached for collecting the sap at centralized tanks. I guess you could call that a crop.
Here is an interesting article on making maple syrup in North Carolina. I think these folks have permanent lines for collecting sap.
http://waterfallfarmnc.com/
I suppose but I always associated the word “crop” with “cut down”... maybe I should blame MS Paint for that.