Just spotted this at ERW:

http://environmentalresearchweb.org/cws/article/news/47108

Looks plausible and sensible to me. Makes things more complicated for paleoclimatology.
Looks like the locations of the trees would need to be assessed, although the impact is probably limited.

However I predict this will be used as an excuse in many comments and discussions.

eg. 'Tree rings can't be depended on because animals affect growth'

It might be worth doing an objective post about it?

BTW I don't see it being reported anywhere else yet.
But I suspect some mainstream media outlets might write something.

And who better to write about it than a tree warden? 

heh, heh, heh.

Good point.

I just wish the name 'tree warden' meant you had to have a lot of knowledge about trees.
In the UK a tree warden is a volunteer, where as in other places they are professionals and qualified.

If I have a go, it probably won't be until next week.

 I need to think about this a bit more.

Could this be one of the reasons for divergence in more recent times? 

Yeah blame New Zealand/Australian sheep farmers ;-)

New York Times:

http://www.nytimes.com/2011/08/02/science/02obtree.html

"Although sheep prefer to eat grass and herbs, in the mountains this is sometimes not enough, and they turn to shrubs and small trees for nourishment."

Which probably highlights how limited the research is.
I don't think they have many Giraffes in Norway and I haven't seen many tree climbing sheep. I guess a bigger issue would be large herbivores such as Giraffes and Elephants, or tree climbing herbivores such as Lemurs and Koalas.

Probably needs more work with different species in order to get estimates to see if there is any impact. Have many tree rings been analysed in Africa, Madagascar, Asia and Australia?

BTW I think it is a matter of time before this appears in forums and discussions. Skeptics are going to add it to their string of points that they churn out.

Well, to be honest, it does open up a legitimate realm of uncertainty. Until one goes back and does correlations against the amount and kind of animal droppings in the soil, it means that tree rings have yet another factor of uncertainty around them.

Unfortunately, this brings back up the issue of the "hiding the decline": The real question being, Why do we trust tree-ring proxies at all, when we know that, at some periods, that they don't match the instrument record? I think this is a reasonable question that I have not seen discussed by proxy experts.

It's a Wiley publication, behind a pay wall, so I can't access the full article:

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2435.2011.01877.x/abstract

But the ERW report would suggest that browsing would mask the increase in temperature in cooler places (assuming they reported it correctly).

ERW:

"The growth rings are visible in tree-trunk cross sections, and are formed in seasonal environments as wood is laid down faster in summer than winter. In years with better growing conditions – in cool locations this usually means warmer temperatures – tree rings are wider."

So if you have a bunch of browsing sheep chomping on short trees in the summer/spring. They will be nibbling at the leaves, preventing photosynthesis and reducing the amount of wood generated. Creating thinner gaps between rings.

http://en.wikipedia.org/wiki/Dendrochronology#Growth_rings

It's only a matter of time before we see tree climbing sheep looking for a little tipple ;-)

http://www.bbc.co.uk/news/world-europe-14842999

Just noticed this Watts post on the subject dated 26 July???

http://wattsupwiththat.com/2011/07/26/tree-ring-widths-more-affected-by-sheep-than-temperature/

So why was ERW so slow off the mark??
Appologies if members had seen this already, why didn't anyone say anything??