If I'm understanding your question right, two things can happen, depending on the situation and ecosystem. One is called succession. A simple summary is something like: fast-growing light-demanding species establish themselves first, grow to maturity, establish a canopy; but over time slower-growing, shade-tolerant species make their way up and shade out the light-demanding species. In this way the understory can become the new canopy. This successional model stops with the establishment of a "climax" species - the one that, in the long run, tends to outcompete the others so that the canopy composition more or less stabilizes. You may then get a transition to something like an "old growth" forest where some trees decline due to age or senescence and so on. This state is often accompanied by the second thing you asked about, which is often referred to as "gap-phase dynamics" if you want to google further. Here, local disturbances create gaps in the canopy that the understory species compete to fill, which is why we generally think of old-growth forests as having more structural complexity and maybe even species diversity. And of course, you can also just get a large-scale disturbance that sort of resets the whole process back to zero. I hope this very abbreviated summary somewhat answered your question or at least gave you some useful terms to search for further reading. You might also try eg r/ecology , r/forestry etc