The likely mechanism controlling the interaction between fir and aspen trees is facilitative succession. This occurs to be the mechanism forwarding the secondary succession found in the example, as aspen tree density expansion allows for fir trees to be able to establish themselves within the environment. This is shown in Figure 2 as the figure shows the progression of the respective densities of the two trees in the context of the different successional stages. In Figure 2 it shows that aspen trees are able to establish themselves in meadows and in turn increase in density until aspen trees are able to grow and eventually overtake them density wise. Aspens allow for the conditions that that lead to fir population growth, thus showing the facilitative succession that is occurring. Figure 3 also shows that facilitation succession is occurring. In the experimental circumstances created by the scientists, aspen trees were thinned leading to mortality (or rate of death) for fir trees to increase drastically when compared to the control scenario. This demonstrates that without a significant aspen tree density, the mechanism of facilitation that allows for the fir to grow cannot occur. The reduced aspen tree density is unable to support fir tree growth and survival as it typically would, leading to an increased rate of mortality for fir trees, thus indicating the mechanism that controls the interactions between firs and aspens.