First, "over-logging" is a term I have never used. From a technical perspective, there are situations where clear-cuts are appropriate and others where the practice would be an outrage.
Part of the problem with clear cuts is that the logs are dragged over the ground surface, taking the ground cover along with them. That has to increase erosion.
This has nothing at all to do with which or how many trees are felled, but instead how they are "yarded" out of the forest. For example, helicopter yarding belies that assertion as does "high lead" cable yarding where the logs are carried in the air from a suspended cable. A rubber-tired loader moves less dirt than a crawler. Both can be used to improve conditions on the forest floor. Sometimes the cat can stir up the dirt to accelerate the return of native plants. If it isn't used carefully, it can bring weeds or accelerate erosion as you suggest. It all depends upon circumstances and methods.
If logging is done to remove individual trees and not ground cover, I don't see the problem.
Most often (as in the case of many National Forests) forest stands are so thick that there isn't any groundcover because they were shaded out or destroyed by the acumulation of acidic duff.
You would have to understand the type of situation Sean refers to in the book. I'll post an excerpt to make it clear:
From what I have seen, the environmentalists have it dead wrong. They demand no logging on steep slopes because the soils would be disturbed, which might cause a little sluffing that they call erosion. This may be true, but the real question is, how much erosion is caused by thinning compared to the alternative? If we don’t log those slopes, we’ll get trees, large and heavy enough to apply sufficient load to the slope to break loose, just like that tree 250 years ago on my place. If it’s winter, that falling tree could start a chain reaction in a saturated alluvium. It’s called a landslide. Landslides like that are all over these mountains. They choke with weeds, weep silt for years, and the mud can again become unstable slopes when they saturate while still full of rotting logs. By contrast, a large redwood stump cut to the ground line with a small tree on it makes a living retaining wall.Here is a photo of such a slope:The biggest risk of sedimentation in streams is if we DON’T thin the stands. If the forest burns too hot in a cataclysmic crown fire, the trees WILL die to a greater degree than if it had been clearcut. It will be no mosaic burn; the disturbed area will be huge. There will be no surface plants to slow the water. There will be no duff to filter the soils. When it rains, the suspended solids will act like abrasive slurry to cut the soil and destabilize slopes. There will be 0% canopy for nearby streams, but then they will likely be so full of mud it won’t matter to the fish.
On the other hand, if the cluster that grows from the old stump is thinned, and the weaker trees are removed, those that remain will sprout new branches into the gaps on the side that needs the weight. They will thicken and straighten. The bark will continue to thicken to protect the trees from future fires. They will be more capable of forcing roots around their perimeter.
You will note that there is little groundcover where we yarded the logs. It did cause a little sluffing where there was only duff. That material was captured and retained where it will do some good. Most important, note the pocket created by the tree that fell some 250 years ago. Consider the difference: Hundreds of yards that came down with the tree or perhaps two to three yards of compost that was used to reshape the drainage so that it reduced downcutting by the adjacent stream.
Logging is a tool. The impact depends upon how you do it.