Black Ghost Trees

So I grew a couple of birch trees in front of my dungeon (level -170), but when I chopped them down I was startled to see a ghostly 2-dimensional black image of the trees still standing. They remain, even after re-logging and even re-starting the game. These ghosts are real.

I can't interact with them in any way, and my axe passes right through them, as you would expect with any self-respecting ghost.
I re-planted two more in the same spots, and when they grew to maturity, the ghosts could still be seen within the new trees.

When I chopped this second generation down, it was not too surprising to see a second set of ghosts intermingled with the first. How many ghosts can you fit on the head of a pin?

The whole concept seems almost appropriate for a dungeon area, but I doubt this was intended.

Update -

Not far from the same dungeon as mentioned in the original post, I planted a small tree farm, again using birch trees. To my surprise, the ghosts were already visible before cutting down any trees. As the cave breeze sets them swaying, the black ghosts are revealed.

This time it's in a "Grass (DefaultBiome,0)" biome as opposed to the "Tundra (ColeBiome,1)" the dungeon is in. Apparently the issue is not biome-related. Hopefully it's not another Mac-related problem.

I can easily understand why growing trees this far underground might not be considered a *normal* thing, but you never know what crazy things players might do.

All things considered, I would expect this to be pretty far down on the issues-to-fix list, but it's worth a mention.

Not to keep beating a dead tree, but while I was harvesting the tree farm the server got rebooted (planned and controlled by zfoxfire). After the reboot I logged back in and discovered all of the trees I had already cut down lost their ghosts. I had already cut down the rest of them before I noticed that. Before I took a picture, I ran back the to dungeon to check on the untouched trees there. They still had their ghosts:

When I returned to the tree farm, Only one tree still had its ghost:

I have no idea what any of this means, but I'm just providing the information in case it is useful.

Semi-conclusion --

The black ghosts behave like dropped items after the tree is removed, and they disappear after the normal period of time for dropped objects. As long as the tree is still alive, the ghosts persist.

Only at underground levels. I might do some testing in single player.

Maybe red51 included this as a puzzle.

Confirmed that it also happens with Windows. No information about video cards, though.

The "ghosts" appear to me to be a part of the rendering process we don't normally see. I'm guessing red51 knows exactly what they are, but it certainly isn't high enough priority to distract him from more important issues.