I'm probably not mistaken, dear reader, in supposing that you would be quite incapable of distinguishing between a bit of bark from, say, a walnut tree and a similar bit of bark from a plum tree. That's because you're not a donkey. These animals seem to find the bark of young plum trees considerably more tasty than that of walnut trees.
It's not as if the donkeys are consuming tree bark because they're starving. As indicated in my article entitled Learning a thing or two about horses [display], there's abundant grass in the part of the property I recently opened up to cater for the arrival of Will's horses. The simple truth is that donkeys are fond of plum tree bark in the same way that we humans are fond of plums. As for the poor plum trees (growing wild at that spot, and not particularly valuable), I'm afraid they might not survive this in-depth attack. But they will have died for a good cause: the epicurean tastes of my donkeys... and I'll have a better view of the valley.