Is it possible to limit Trees/Lumber harvesting to 1 worker at a time?

As title says, I cant find anything under gameplay constants or in the trees object editor that could alter this?

Unlikely. You can't stop 2 people from hitting 1 unit, so its the same with trees.

You could work out something like.

'Tree is attacked'
'Make tree invulnerable for x seconds' where x = cooldown of harvesting unit attack

I sorta had some idea thingy, the wisps can only take up one tree, but the tree doesn't die, if you could find some way to change that, maybe it would work.

Do you mean 1 harvester per player or per tree? I assumed you meant player. So you would have to have only one type of unit that can harvest lumber and only allow the player to build that unit once, or add that ability dynamically to only one unit at a time.