Looks very nice indeed. I think they will be quite fragile but they are lovely.
I don't know if they will be fragile. They appear to be made of clumps of string glued together, with each clump separated into "branches" and the trunk covered with "pasted paper" (I think that is "papier encolle"). This would be easily strengthened by using paper glued with PVA and then coated with a texturing mixture. So long as the flock is stuck on with a good quality glue and then sprayed with some sort of fixer, like diluted PVA for example, the trees should be sturdy enough for gaming use.
It's worth experimenting with, though. At the moment I use either wire wool or those acrylic pan scrub things (used in washing dishes) for the "branches", which I tease out and then have to spray black before flocking. Looking at the string trees, they wouldn't need much work, just a trim with sharp scissors and no spraying. Use pre-coloured texturing for the trunks and it seems to me it would be a very quick and easy process.
Ideal for large forests, jungle etc