I can't claim expertise as a forest botanist for yos, but my impression is that manzanita is a first regrower after fires, so has a tendency to fill in spaces. Because it grows fast, it fills up blank spaces.
My understanding is that this is why it is important for there to be fairly frequent fires, to clear this brush. One, so that there is bare soil for tree seeds to find and sprout, two, so that there is not the build-up of dead manzanita, which is heavily laden with oil and burns intensely (even explosively)-these build ups can burn down trees that are fire-resistant like sequoias, and three, so that the trees that require fire to open their cones, have a chance to reproduce. I think sequoias require fire.
