You have to trigger it than, first make your "lumber mine" a building, than give your wisps a dummy target spell and make a dummy tree which have little or no model, and make a trigger
event
unit-A unit cast a spell
Condition
if spell being cast = <your dummy spell here>
action
Create <your dummy tree> at <the dummy building>
Order <casting unit> to right click/havest <last create unit>
than a trigger when wisp leave the tree
event
a unit ordered to blah blah (add all the orders one by one)
condition
unit tpye = wisp
action
pick all tree is area of the <triggering unit>
Btw, there might be leaks and bugs, I didn't had we open when writing this XD