The seamoss/foam/whatever trees look great. Very birch like in fact and so perfect for the highlands. Why don't you try giving the "trunk" a bit of a whitish highlight and adding some green leaves. The look fine as it is of course!!!
How durable are they?
Good idea! Might do.
The trees are tougher than I thought. Don't get me wrong, they won't survive someone slapping their hand straight down on top, but they don't just snap at the slightest bend either. I wonder if doing a coat of superglue up the trunk might make it even sturdier.

This really looks the part. But are those trees not as fragile as an erection in a cold shower?
That's got to be the best metaphor I've ever heard!

Cheers guys!