How to map very thin tree cover?

I think Krummholz (https://en.wikipedia.org/wiki/Krummholz), which I think is what you are referring to, is generally accepted as an ecological woodland habitat rather than a scrub one, despite the relative sparsity of trees. Long ago someone suggested to me Hexbinning tree locations as a way of estimating density.

Agreed. The tagging options for ground cover are a bit binary really, forcing you to choose between Natural Wood, scrub, meadow or grass, when often none really reflect what's out there, which is often scruffy grass with a small number of small bushes/trees, or scrub/grass with a few mature trees

@mikey-co I'd definitely have a look at the subtags that people have used with those top-level values.

@SomeoneElse The sub tags still aren't very useful though, as while you can describe the type of tree, you can't describe the density. For example I'm looking at some land at the moment, where the tag "Natural Wood" would imply dense forest, when in reality the trees are far more spaced apart. Too many trees to be scrub also.