How to attach climbing holds / rungs to a tree trunk without harming the tree

Question

I've got a tall tree in the backyard that we use to practice belaying / abseiling (rappelling). However, the lowest part of the trunk does not have any branches until up to 20' / 6m, so we use a lean-on ladder. Which is a shame because I'd rather climb those feet and not "waste" them!

What would be the best way to attach wall climbing hand holds or any other things that could give us a challenge but without harming the tree (I don't want to put thick and long anchors inside a living trunk)?

UPDATE
So here's what I'll do. As advised by @Jory Geerts, I'll build a round-the-trunk planked miniwall. Meanwhile I've anchor-hitched 30 feet of 10mm static rope to the lowest branch and we Texas prusik it! Great practice and workout (the transfer from the rope to the tree branches is the hardest part)!

Answer 1

You can bolt the holds onto a (strong) wooden plank and use a ratchet lashing strap to attach the plank to the tree. Better yet: use two. One at the top of the plank and one at the bottom.

You will probably want to add some padding to protect the tree. You can place a rubbery "block" on the plank at the places where you put the strap. This way the thing getting pushed into the tree is something (relatively) soft. Padding between the rest of the strap and the tree isn't needed for the (fairly small) force this will put on the trunk of the tree.

I recommend removing the plank or at least loosening the strap if you're not going to use it for a while to ensure the tree can grow.

Answer 2

Whatever you use could have some impact on the tree, but your best bet is likely to be a combination of slings, either tightly round the trunk to hang handholds from, or alternatively, ropes from your lowest branches to take the weight, with slings round the trunk solely for locating the handholds.

On a slight tack, you could use a cargo net or knotted rope - they'd still give you something to climb.

Answer 3

In all my research and experience, the single thing NOT to do with a tree is to lash or strap anything around its trunk/limbs. The outer layer of tree cells is where new growth occurs and nutrients travel throughout the whole thing, so strapping or tying anything around the trunk/limbs will strangle it and do real harm. So, I always advise to attach things to a tree with a few fasteners (bolts, screws) which puncture through the outer layer into deeper wood. This significantly reduces the surface area affected by your attachment.

Take a look at any high ropes or zipline or canopy tour course, and everything (and I mean EVERYTHING) is attached to the trees via fasteners, never by being tied/strapped on. This includes the wires and ropes for the course, as well as any platforms or construction. It all comes down to bolts and giant hardware fasteners, if the people know what they're doing at all.