This is bark. It is the tree's protective outer layer and a world of moss, lichen, and insects.
This particular bark is also a farming operation for a bird called a yellow-bellied sapsucker. The uniformly constructed and arranged holes are wells for the bird to harvest sap (and the occasional sap loving bug). Some of the holes are shallow, to collect sap as it flows up the tree in the spring and some are deeper, for later in the year when sap is flowing down to the roots.
Like an elusive forest graffiti artist the sap sucker leaves these tags behind for everyone to marvel at but few see the artist himself. The work is rendered with fine craftsmanship and there is no doubt in my mind this is functional art.
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Pencil drawing (and lots of erasing). I drew this one pretty large because I really wanted to get at and understand the detail. Working larger takes more time but its rewarding when you come across things like sap stains on bark, intricate edges of crustose lichen, or discovering that some of the unique shapes on these trees are created from the alternating forces of the tree trying to heal/scab and the sap sucker keeping the holes open.