I find it strange that you can improve a forest by chopping down healthy trees,
Spacing and pruning is pretty basic forestry, with significant forest health benefits.
The forest on Grouse is second growth near the trail as I understand it.
Second growth forest often loses trees as it ages. As trees mature, they compete with each other for light and water. This competition is more intense, the closer together they are.
Old growth tends to have big trees far apart. Second growth has smaller trees closer together.
So, although they might be healthy now, they could still get outcompeted by neighbours over time as the forest ages towards climax seral stage.
Therefore, if you have a tight cluster of three healthy trees now, and you thin it by cutting down (hopefully the smallest) one of the three, not only do the remaining two grow more vigorously afterwards because they are competing less, but you essentially hasten what was already going to occur - number 3 was probably going to be outcompeted anyways and would die on its own in future.