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Another confounding factor?

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.

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Reader Comments (6)

The negative effects were apparently due to grazing damage - something that affects young trees much more than old ones and trees with leaves much more than conifers (though there are herbivores that do eat conifers - moose for example).
Since there are essentially no trees less young than adult bristlecones and no megaherbivores that browse pine needles where they grow, I should think that positive effects of grazing (manure, decreased competition for nutrients) would probably predominate for bristlecones.

Aug 12, 2011 at 11:11 AM | Unregistered Commentertty

"in lowland regions tree rings are less likely to have been affected by herbivores..."

I thought that the "best" tree rings -- that is, the ones with the greatest temperature sensitivity and hence better signal-to-"noise" ratio in their widths -- were those from the treeline? [I put the scare quotes around noise because I object to calling all confounding factors noise. They're interfering signals.]

I wonder if the statement ["This study does not mean that using tree rings to infer past climate is flawed..."] was a concession to reviewers to obtain an imprimatur.

Aug 12, 2011 at 12:33 PM | Unregistered CommenterHaroldW

"Somebody needs to repeat this study on the bristlecones."

Oh Come on Bish! Keyboard/coffee warning before a line like that! Do you know how much coming here costs me in I.T.!

Aug 12, 2011 at 2:59 PM | Unregistered CommenterPete H

"and from the fossilised remains of spores from fungi that live on dung" - the bullshit factor?

Aug 12, 2011 at 3:27 PM | Unregistered CommenterDavid Chappell

"Somebody needs to repeat this study on the bristlecones."

The bristlecones in the Colorado Rockies are subject to herbivore browsing by mountain goats, mule deer, and Big Horn Sheep.

Aug 12, 2011 at 7:12 PM | Unregistered CommenterOrson

The introduction of the European earthworm completely changed the North American soil structure. North American earthworms were killed in the ice age and the incoming colonists reintroduced the earthworm the early 16C. Where ever the settlers moved, they bought plants, and earthworms.
The change of bioproductivity and the effect on tree rings can be observed in the few forests they have not yet invaded.
During the initial invasion, the tree rings shrink, as the trees roots are being disturbed.
After a decade, the trees put on a growth spurt.
You don't always get the same pattern, some trees do well and some worse. There is a huge literature on this, but it gets ignored.

Assessing temperature effects on tree-ring widths without knowing about changes in the eco-system, like invasive species or extinction evens (passenger pigeon) is rather silly.
Earth worms will tend to increase water and nitrogen availability, so something else becomes limiting.

Aug 12, 2011 at 10:58 PM | Unregistered CommenterDocMartyn

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