Here is a good view of a stand that has almost fully transitioned from a black oak forest to a Douglas-fir stand. Encroachment began in this stand in the 1960s and finally the last of the taller oaks are reaching the end of their lives. Over then next ten or so years the oak stems will loose all of their limbs and begin to feel the pull of gravity as their root masses die. Many species of insects and fungus will invade these trees and speed up the fall. Once on the ground, the boles will persist for decades as the forest recyclers slowly and consistently do their work.
Meanwhile the Douglas-fir canopy will close in around the space previously occupied by the oaks and the little remaining areas of filtered light on the forest floor will become dark. This will also effect the shrub and ground cover component of the forest, severely limiting what can grow there in the absence of light.
Its interesting how the full encroachment cycle changes the bio-diversity of stand over time. It would seem that during the last phases of transition, like in the photo, the diversity is highest. This is in part due to the vertical structure offered by the overstory conifer, the decaying oak trees in the mid canopy, and the collection of downed wood on the forest floor. There are also pockets of shrubs that still receive light where the fir canopy has not closed in. Oak snags make potential cavities for numerous birds and mammals. The fallen trees become home to hosts of insects that in turn are feasted on by many of the locals.
While I generally speak of this encroachment as a negative, there is certainly a positive, or at least productive element to this process from a forest health prospective. In a truly “natural” setting, there would be recruitment of oak woodlands somewhere near-by while this process was occurring, creating the balance of habitats over the landscape. However in our fire starved environment, Douglas-fir encroachment is occurring much more frequently than the rate oak stands are developing. In fact, very little oak stands are developing at all in Humboldt County. This is why, I believe, that there is an urgency to prevent encroachment into woodlands.