A shaft or branch, from chopping down bushes

[Shmeric 581]

Chop any kind of tree, and you get at least one log (or scrap, if it's only a sprout).

Chop down a bush, and you get nothing.

 

Let those pesky bushes  "drop" a shaft, or a couple branches.

 

[Muse 2,216]

Some reward for every action wherever possible. +1

[Gawain 622]

You can actually get these wood types thru archaeological

[zethreal 582]

As Gawain said, if you take time to do some archaeology ( investigate tiles and identify fragments ) you can get tenons, branches and shafts of all of the wood in game, shrubs included ( lingonberry, grape, thorn, etc ). 

This not only gives you access to the wood, but gives value to that skill until there are more settlements that have disbanded to be discovered. It also makes those woods "rare" and at least a bit more valuable.

[Shmeric 581]

Wasn't as much aiming to get those "rare" woods, just weird that bushes give nothing, while even a tree sprout drops a woodscrap.