I think that you would use a mix of Portland and sand mixed to a slurry, then cover a metal structure of rebar and lathe. Not an easy process.

Mike, what do you think about using a thicker mixture such as "ShapeCrete" but a homemade version of it, forming it around the lathe and rebar? Any suggestions on making a diy ShapeCrete as I think the product would work, it will require a few buckets for each tree which could get pricey!

I have used ShapeCrete to make bowls and it is great stuff! I am sure it would work for you project, but it could get pricey. Photo of a bowl using SC My Blog -

Also you may check out ( seach this forum ) how to make fake rocks, the concept is similar. Make a few small projects then advance to the tree.

Perhaps PVC pipe roughly shaped how you want it. Then wrapped in chicken wire to hold the slurry. Texture like a tree truck and I think you could get a reasonable facsimile.

I found something that may be useful.

Link