While wood is made up of the same carbohydrates called cellulose, wood is held together by a complex molecule called lignin. This makes wood tough and hard to digest as part of the cellulose remains undigested. Some bacteria however have found the right enzyme to break it down and termites have such gut bacteria. That makes them (along with some beetles) the only animals capable of eating wood. Now what if large animals got this ability?
Now try and picture what a wood eating animal would look like. Large stout jaws with long teeth to bite and chew wood. Thick skin to protect them from woodchips. Multiple stomachs to process and ferment the plant matter. This calorie intake would make them very large and powerful, capable of unrooting trees and fending off predators like hippos do with their terrifying bites. Their size would make them energy efficient and very slow creatures due to their metabolism slowing down thanks to the square-cube-law. Their young would be incapable of chewing wood right away so the parents would take care of them for years until they are developed enough. This might lead to herding behaviors like elephants or hippos but they might also be solitary. Birds would be useful to get rid of parasites.
How would such animals affect the ecosystem? Would it upset the balance completely? How would the ecosystem adapt?