This is a week or so old now, but this Mother Jones article looks at a new paper which reports the effect of livestock on tree rings. The study is focused on birch, so it's not directly relevant to the paleo studies, but the McIntyre and McKitrick paper in E&E in 2005 discussed the possibility of the introduction of livestock having brought about the twentieth century spike in bristlecone growth that underpins the hockey-stick shape of so many of the millennial temperature reconstructions.
Interestingly, the new paper's conclusion is that livestock can reduce tree ring widths by a factor of three or so, but according to the press release, "past densities of herbivores can be estimated from historic records, and from the fossilised remains of spores from fungi that live on dung". In other words, you can control for the effect. As the paper's authors say in their press release:
This study does not mean that using tree rings to infer past climate is flawed as we can still see the effect of temperatures on the rings, and in lowland regions tree rings are less likely to have been affected by herbivores because they can grow out of reach faster.
Somebody needs to repeat this study on the bristlecones.