I came across a really interesting article yesterday about some hadrosaur coprolites from the Upper Cretaceous Two Medicine Formation that contained woody materials (Chin 2007). This is direct evidence that at least some hadrosaurs ate wood. Coprolites at this locality regularly contain wood indicating that conifer wood was regularly ingested. Wood contains lignin which cannot be digested by vertebrate herbivores so there is no nutritional value on its own. Thus, in order for any animal to intentionally ingest wood it must have a very good reason of doing so as processing wood is such an effort, both in mechanical digestion (chewing) and chemical digestion. The absence of little twigs from the coprolites pretty much rules out accidental ingestion while foraging leaves.
On the other hand, the author found signs of fungal decomposition in the wood material. So apparently, the hadrosaur was eating fungus-infected, or in other words, rotting wood. Fungus de-lignifies wood and makes cellulose available for other organisms to consume. 'From an energetic standpoint, it is evident that partially degraded wood would provide a nourishing resource augmented by fungal tissues, associated microbial symbionts, and invertebrates. Decaying wood would require less energy to process and could be easily comminuted' (Chin 2007).
Modern megaherbivores consume fugus-decayed wood when high-quality browse is sparse. However, these Two Medicine coprolites were preserved during the rainy season. The author suggests that the high proportion of fungal degraded wood in the hadrosaur diet may have been because of a lack of vast grasslands. Modern large herds are largely supported by the fast regeneration of grass after consumption by a herd of herbivores. In a grass-poor Cretaceous world, large herbivorous dinosaurs obviously had to eat other sources of food, e.g. ferns, angiosperms, conifers AND decaying wood.
Chin, K. 2007. The paleobiological implications of herbivorous dinosaur coprolites from the upper cretaceous Two Medicine formation of Montana: Why eat wood? Palaios 22(5):554-566.