Yesterday, I published a press release titled Tree ring widths more affected by sheep than temperature in which researchers in Norway published a peer reviewed study stating:
“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.”
They went on to say that “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…”, but according to MM05 below, that doesn’t seem to be fully accurate.
John Muir (founder of the Sierra Club) described them as “hooved locusts”.
There is one other issue that needs to be canvassed and eliminated prior to reliance on bristlecone pines. The pulse in bristlecone pine growth is contemporaneous with a pulse in woody plant growth throughout the American Southwest, attributed to overgrazing by sheep in the late 19th Century (see Figure 8),
…which in turn followed the extension of the railroads [Allen, 1998; Allen et al., 1998]. Sheep differ from otherspecies in that they will completely destroy grasslands by eating down to the roots, leaving barrens [Allen, 1998]. Although Allen  only documented the expansion of pinyon pines and junipers into terrain formerly occupied by 19th century grasslands, Allen (2004, pers. comm.) did not exclude the possibility of a similar effect involved in anomalous 20th century growth for bristlecone pines, but was unaware of any studies on the topic.
There is a published reference to the introduction of large commercial sheep flocks in the late 19th century in the White Mountains CA [St. Andre et al. 1967], where the key sites of Sheep Mountain and Campito Mountain are located. The founder of the Sierra Club, John Muir, complained of the depredations of sheep in the Sierra Nevadas (adjacent to the White Mountains) as “hoofed locusts” [Muir, 1911]. Carl Purpus, a late 19th century botanical collector in the Sierra Nevadas, stated in 1896 that commercial flocks had cleaned out all grass to the top of Old Mt Whitney [present-day Mount Langley, which reaches 4,280 m] [Ertter, 1988]. Allen (pers. comm., 2004) said that there was a large commercial sheep trail at Jicarita Peak NM, another bristlecone pine site studied by LaMarche and Stockton . In severe high-altitude terrain, even after the departure of commercial flocks, a small population of bighorn sheep could prevent the re-establishment of grass (Leslie Thomas, Colorado Springs, landscape architect, pers. comm.)
Since grass (and other herbs) compete with pines for scarce moisture, one can hardly exclude, on a priori basis, thepossibility of a connection between anomalous 20th century growth rates of bristlecone pines and a growth release following 19th century overgrazing, as experienced elsewhere in the American Southwest.
Like the many other factors in Leibigs law affecting plant growth, sheep are just one more variable not considered by Mann et al. The evidence keeps mounting that that hockey stick is made of woody assumptions that don’t hold up under scrutiny.