Ah, there's another thing that's commonly overlooked. Diseases such as
Laminated Root Rot weaken the trees from the roots, and the likes of
Brown Top Rot makes trees prone to breakage in the wind. Note that both of these diseases are endemic to forests of a specific age class, that is, 50-100 years old. That's the exact age of the majority of timberlands harvested in the Pacific Northwest today.
A simultaneous strength and weakness of clear-cutting as a silvicultural treatment in 2nd-growth stands is that it captures the vigor of the young forest before the diseases have a chance to do major damage. Contrarily, it captures the entire genetic diversity of the stand before Natural Selection has a chance to produce disease-resistant individuals through trial and error. A longer rotation (say, 70 years) would only show increased losses to disease and to stem exclusion.
A well-planned thin program, however, can capture both mortality and survival, and repeated entries fine-tune the desired stand structure so that a given stand can be visited again and again. The most important thing to get away from is the idea that there's some kind of hurry -- that's Wall Street thought, and those guys don't work in the woods with us. Trees have their own pace about things.
Finally, I want to address the issue of "Clock-Resetting" events like fire or volcanism and their role in forests. Yes, these things do happen. In some places they happen relatively often, and the ecology adapts to accomodate it. In other places, such as the outer Pacific coast of the Northwest, they happen very infrequently, and the ecologies are poorly adapted to such events.
The time between major disturbance events is called a "Return Interval". Eastside Ponderosa stands have a fire return interval on the order of 5-10 years. Coastal Spruce and Hemlock stands? More like 1000. It takes centuries for the organic material in the top and middle layers of soil to grow thick and rich enough to support the kind of plant and animal diversity we take for granted. For every major disturbance event, it takes time to recover.
The most extreme examle locally is, of course, Mt St. Helens. Go walk around the Hummocks between Cold Creek and Johnson Ridge and have a look at the soil. It's thin, sterile, and fragile. That's because it's only been 30 years since it was nothing but a fine glass dust. Now walk around in the big timber near Staircase. Your soil will be under a couple of feet of leaf litter, branch debris, moss, lichen, and will be soft, moist, aerated, full of worms and bugs and fungus.
That's what disturbance destroys. It's not just the soil itself -- it's everything that makes the soil. Those things, in turn, are what makes a forest, not just trees. I almost don't care about trees at all, except as they affect the larger system that is the Forest. That's why I'm so adamant about limiting disturbance. I don't want to save any special tree or something -- I just want to make sure that there are forests for later so fifty generations from now there'll still be raw materials to make buttwipe out of.
