This experiment is interesting, but not as enlightening as it should be. For results can vary tremendously with caliber, speed, distance, wood variety; and, essentially, whether it is a dead or a living tree (i.e. dry or still full of fluid?).

When a bullet traveling at high speed gets through a living tree, the sap that flows inside reacts “about” the same as water (not “exactly” because it is mixed with wood fiber). Anyways, this last characteristics makes the wood react very differently than when dead and dry to the impact of a high velocity bullet. Fluid in a living tree “helps” the bullet shattering much more wood fiber in a much greater area than in a piece of dry wood. To the extent that one could say that a living tree reacts to the impact almost as flesh or gelatin does. The same bullet traveling at the same high speed will barely do more as making a hole about the same diameter in a piece of dry wood, and what makes wood fiber shattering near the exit of the hole is nothing but the bullet expanding and eventually shattering itself too.
Besides, it can be much harder for a bullet to drill through wet wood than to dry wood, only because of this fluid.

My own experiments with guns and various materials, some decades ago, brought results that happened to surprise me sometimes. Thus, when once firing a 7.92 x 57 mm Mauser rifle (a WWI Gewehr 98b with a long barrel in pristine condition using WWII German Army FMJ cartridges) in a living sycamore with a trunk diameter of about 10” at a very short distance of about 30 ft., my first bullet, though doubtless powerful and with a traveling speed of about 2400 fps, failed to drill the trunk through. It remained stuck inside the tree! How come, I thought?
However, I noticed with surprise an increase of the circumference of the trunk of more than 1” where the bullet hit it, which I found equally impressive. Wood expanded under the impact indeed. And after five more 7.92 bullets only in the same area of the trunk, the 50 or 60 feet high sycamore started moaning while bending very slowly, and thus until it fell in a clatter of broken branches. Of course, twice more bullets in the same conditions would certainly not make this tree falling down if it was dead and dry. And, this time, a MG 34 or 42 with a band of 50 cartridges perhaps would be required to get the same job done.