As to the original question, I also say tree stump, but more accurately a cut-off sapling, and crudely cut.
Why was it cut? Well, notice the other cut-off sapling to the right in the image. The cannon sits somewhat centered between the two cut-off saplings (were you to view the scene directly in front of the muzzle, but don't do that
). At times such fall-back markers were used, one on each side of the wheel, as reference points from which to pace out the distance to a pre-determined (pre-ranged) firing line position. Those sapling stumps could be intended for that.
Why fall-back markers then? Well, being able to deploy quickly to a pre-ranged forward position is a huge advantage compared to having to range the gun from an untested forward position. From fall back markers the gun would have already been serviced and ranged in a drill, say, the previous day. (There would also be markers near the wheels at the forward firing line position to enable rolling the gun back to
aim position after each firing up there, but to avoid tripping up there they would be small holes or twigs -- hence the fall-back markers to pace from).
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As an aside but to the same point, if you've watched the modern U.S. Army (national guard) demonstrate their modern field artillery, the first thing they do is to post markers (posts) some yards from the gun, and as in the CW, these are reference points for ranging the gun. Today however the markers are a digital "zero point" in a GPS grid addressable on a CRT on the gun itself, and/or on a ruggedized laptop nearby. Such guns are able to fire well over the horizon with GPS grid-point accuracy.
Unbelievably to me, such field artillery is still fired by tugging a lanyard! (I would have thought a push button!)