The Warden's Thorn Strike has a range of Melee 2, it's keywords are Primal and Weapon. A polearm has Reach letting you attack 2 squares away.
Does a polearm's range stack with Warden's Thorn Strike for a range of 3?
They do not stack unfortunately. You need one or the other to state that "Your melee reach increases by 1"
Examples would be:
The Rules Compendium p100 reads, in the Range entry:
So, reach weapons only add to reach when using powers with a range of Melee weapon