Read here. In regards to the infamous Michael Mann and IPCC 'hockey-stick' chart, it was claimed that past climate temperatures could be ascertained by tree ring growth. Subsequently, it was determined that Mann had ignored other important factors in tree ring growth, including: water availability, atmospheric CO2 levels, atmospheric nitrogen levels, parasites, and sunlight.
Now, scientists have discovered a much more important influence than temperatures on tree ring growth: animal (domestic or wild) behavior, especially chewing on the trees. If browsing on the trees has a great impact, then the animals' dung may be another influence that goes unaccounted for.
Regardless of the dung issue, clearly the 'hockey-stick' reliability as a past temperature record is further weakened by this study.
"Nibbling by herbivores can have a greater impact on the width of tree rings than climate, new research has found...“We found tree ring widths were more affected by sheep than the ambient temperature at the site, although temperatures were still visible in the tree ring records. This shows that the density of herbivores affects the tree ring record, at least in places with slow-growing trees.”..."Our study highlights that other factors interact with climate to affect tree rings, and that to increase the accuracy of the tree ring record to estimate past climatic conditions, you need to take into account the history of wild and domestic herbivores. The good news is that past densities of herbivores can be estimated from historic records, and from the fossilised remains of spores from fungi that live on dung.”" [James D. M. Speed, Gunnar Austrheim, Alison J. Hester, Atle Mysterud 2011: Functional Ecology]