David, with most softwoods I know the tighter the growth rings the stronger the wood. In my tree planting background I have planted similar species of spruce in Saskatchewan, British Columbia and England. Trees in the UK, and BC mature at 50+ years depending on location. Trees in Saskatchewan mature at 80+ years. That means two trees of similar diamater taken out of forests in SK and BC would be different ages.
In theory, a 80 year old tree would have more growth rings and therefore more strength. Durability, I would imagine is more genetics than anything, and stability is based on a mix of genetics and growing conditions. An example of genetics would be white spruce vs black spruce. Growing conditions could be anything from density of the forest, rainfall, elevation, sunshine, and other things like growing on the side of a hill.
Now, in much of Europe you have managed forests. Managing a forest in Europe is much more intense than it is here in Canada. Here we plant the tree a year or two after the forest is cut down. It is surveyed at least once in the first seven years after planting to make sure that the obligation to replace the trees cut on crown land is met. Sometimes if the natural competition is too great the natural regrowth will be cut back around the trees to give the seedlings a fighting chance. Then assuming there are no major epidemics like fire or beetles it is left till maturity.
In England, trees are planted tighter than they expect them to grow when they reach maturity. They are planted in rows thinned and trimmed till they reach maturity. By doing this there are fewer branches and fewer knots in the lumber.
In the mountains of Bern I suspect it is a carefully managed forest. Thinned, trimmed, and selectively cut. Interest taken with quality of wood in mind not quantity. Then there is the milling, that is a whole new topic.
N.H. Seedlings for todays monocultures are often harvested locally taking a helicopter with a basket and raking the acorns off a local tree to produce the seedlings from. This is done with the theory that the local tree already has the genetics required to thrive in the environment.