When Kitsap County purchased its Heritage Parks they had been planted in Douglas fir for use as lumber. The trees were spaced close together to ensure sufficient survival. The timber company would later thin the trees for optimum harvest. While the County isn't concerned with optimizing the timber that can be obtained, it does need to provide room and light under the trees in order to benefit wildlife and to provide room for native plants and trees to grow and create a more natural forest. The process of doing so is called "restorative thinning" and you may have seen this happening in the Heritage Parks.
The process might be described as the reverse of landscaping a new home, where small plants are spaces widely apart so that they are in their proper relationship when mature. During restorative thinning that process is reversed, smaller and less healthy trees are removed, leaving space for the remaining trees individual trees. Shade-tolerant tree species (e.g., western red cedar, western hemlock) will be planted when between the remaining fir trees, to enhance species diversity and vertical canopy structure. In some locations clumps of old and young tree are left without any thinning ("skips") to create a two-tiered forest canopy. In other areas small forest openings ("gaps") are created to provide a diverse habitat for wildlife. The density, spacing, and average size of the trees that are left are based on site-specific considerations.
A difference between a tree farm and a natural forest can be clearly seen in pictures taken from about the same spot off of Millie's Trail off Big Valley Road. The lower section of the trail is on private property and contains a mixture of cedar, maple, alders. The upper part is a tree farm on timber company property.
A tree farm
A more natural forest
Waiting for the fir trees to die off naturally so other species can prosper would result in an unhealthy forest for several hundred years. When you see restoration work being done in the park, relax, the forest is being thinned for a more productive environment.