I got up to the rank 20 mission before I stopped playing. I traveled all over the Black Shroud and I've never seed bark nor leaf of another treant. I've seen Lyndwurms, Marlboros, I've even seen Pyrodrakes outside Ul'dah (all those things can one shot me, and most have) but no treants.
Q: How many SE employees does it take to change a lightbulb?
A: None, it's working as intended.