No, by the "final stage", I mean the stage in which you find the trees "naturally". Currently, the trees that are planted only grow to the stage right before that.
And, the trees currently have 6 stages each: the tiny stage in which they begin when planted, 3 intermediate stages, the "finished" stage for planted trees, and then the stage in which they are found naturally (and into which they will, eventually, be able to grow, if given a lot of time). So that means that each planted fir will, currently, give rise to four tiles of forest terrain.