The plank system is the floor for the story above. The grooves the planks set into are in the walls. Each room has its own floor system. The boards on the edges are cut with a taper and the middle board is cut with the opposite taper, this middle board is inserted through a hole in the wall and wedges the planks tightly together.

Yes the 'Unterzug does appear to be smaller than it should be. But keep in mind, it laps or dovetails into the wall logs at its full height.

As to the question of trees,

Swiss forests are incredibly well managed. Unlike most of Europe, these resources were never depleted -and at the same time have managed to stay surprisingly wild (you don't have the orderly groomed forests that you might see in Germany, for example) Very large trees are still to be found here, I don't know how many spruce trees I saw with diameters in excess of 3 feet and even larger, it was quite impressive really. Here are some pictures of the forest situation:











So changes in building process and the switch to half timber cannot be attributed to a shortage of large timber. It is probably more due to the typically Bernese obsession with efficiency, even where efficiency isn't important.

The types of trees is fairly universal across the canton, even from the mountains to the lowlands. The primary building material is Fir and Spruce, which is by far the most common timber. The biggest difference is what they use for sills, lowlanders use oak because they have it. In the mountains oak cannot grow, so they use larch.

But I do think the matter of space and movement affected the changes. In the lowlands it is easier to build an expansive timber frame, and you build larger structures because your operations tend to be more spread out. In the mountains, space is important. It's also a matter of weather. The lowland timber frame would have a hard time surviving mountain weather -think 15 feet -that's right, FEET, of snow. It's kind of hard to explain, but it really does just make a lot of sense to build a log structure in the rugged mountains.