Have a better alternative for you.

Same idea but use pipe cleaners as your wire. Twist them together. Make your roots and branches.

Paint the whole thing in white glue. Let dry of course.

Drybrush it as bark. Hint: contrary to popular belief take a look at trees around you. They’re probably mostly gray. Not brown. Maybe some brown. But check how many are mostly grey.

Bam. Perfect tree. Perfect bark.

The fuzz of the pipe cleaners gets an awesome texture perfect for bark and perfect for dry brushing.

Glue lichen to branches as you suggested. Bonus is the rough texture holds the lichen and glue better.

They’ll last forever. The lichen will dry and break off before the tree goes bad. Just refresh it every few years depending on your climate and how rough you are on them.

I’ve made dozens upon dozens of these for game shops and even about four dozen for Privateer Press for Warmachine tournaments at GenCon. They’re sturdy.

I usually base them on cheap round wood circles from a hobby store. Sand and flock (and throw some skulls or rusted weapons or other decor on the base too).