Well the area I lived in (and the photos are from) is considered SW Oregon. And the firs there are by far the most aggressive/invasive of tree species. Alders are by far the fastest to re-establish themselves, but they are rapidly overgrown by the firs and cedars. After a road clearing, alders came in by the thousands. Lots of invasive species as well; blackberry, brooms, and hawthorne were the biggest headache to remove.
It was impossible for us to discern what was 'natural' diversity. It was one of the few 100+ acre places around there with a mixed stand of trees. The mixed stand on that property was deliberately planted, created by logging, or by fires by the Indians. The topography varied, and there were hot areas that favored madrones, draws that favored red cedars, flat flood plain creek areas that favored ash and alders and oaks, and meadows that had oaks but were reverting to firs. The rest was fir, mainly Dougs and grands. There are some huge first growth stumps and logs that are still there. Mostly Doug fir. My own survey of several 80-160 acre old growth BLM tracts near there were mainly a mix of conifers, mostly Doug fir, some cedars and grand fir, and a few alders, madrones, and bigleaf maples. No hemlocks or pines are in that area now.
Going back in time, the area was logged and replanted or farmed from about 1880 on. Before that the Indians burned almost every fall and lived in the area for several hundreds of years, or more. David Douglas made a good survey of that area in about 1860, and noted a thickly forested area that was near impossible to travel though due to the tangle of vine maples and steep terrain. However, that was after the Indians had been there for many centruies previous. Looking farther back requires fossil records. Coastal redwoods and other trees now extinct were common there in fossil records, and Ponderosa pines were far far more common toward the coast.
In the end, we decided to clear out the firs under the large oaks to preserve the anceint Indian burning features of the property. That area is about as far north as Califirnia black oaks grow, and they are critical trees for habitat of several species of wildlife. We also cleared the firs to make the oaks more fire-proof. A fire through there with the firs as shown in the photos would have killed the oaks. We set aside about 10 acers of those oaks and meadows in the forest Rx plan. The locals thought that we were nuts; everyone said that we should plant Doug firs, or let the firs crown out those old 'trash trees'. It would be very interesting to see if the area were left alone for 500 years what would happen over time. Random fires, and invasive species, and all the rest. At best it is a war zone between plant species. I think that in the end, the firs would win hands down though.