Forest site productivity is how much a certain area of forest can grow based on its natural characteristics and how it's managed. Scientists have studied different ways to measure this, like looking at tree height and volume growth, and how thinning affects growth. They found that not all trees grow the same on different sites, leading to the idea of yield level, which shows how much a forest can grow based on its height. By using a model that combines tree number and size, we can estimate how productive a forest site is.