Tree ring widths more affected by sheep than temperature

From Wiley-Blackwell, via Eurekalert, something just plain surprising.

These are sheep in the Norwegian mountains. Credit: Atle Mysterud

Nibbling by herbivores can have a greater impact on the width of tree rings than climate, new research has found. The study, published this week in the British Ecological Society’s journal Functional Ecology, could help increase the accuracy of the tree ring record as a way of estimating past climatic conditions.

Many factors in addition to climate are known to affect the tree ring record, including attack from parasites and herbivores, but determining how important these other factors have been in the past is difficult.

Working high in the mountains of southern Norway, midway between Oslo and Bergen, a team from Norway and Scotland fenced off a large area of mountainside and divided it into different sections into each of which a set density of domestic sheep was released every summer.

After nine summers, cross sections of 206 birch trees were taken and tree ring widths were measured. Comparing these with local temperature and the numbers of sheep at the location where the tree was growing allowed the team to disentangle the relationship between temperature and browsing by sheep and the width of tree rings.

According to lead author Dr James Speed of the NTNU Museum of Natural History and Archaeology: “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.”

The impact of large herbivores on tree rings has, until now, been largely unknown, so these findings could help increase the accuracy of the tree ring record as a way of estimating past climatic conditions, says Dr Speed: “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.”

“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,” he explains.

Tree rings give us a window into the past, and have been widely used as climate recorders since the early 1900s. The growth rings are visible in tree trunk cross sections, and are formed in seasonal environments as the wood is laid down faster in summer than winter. In years with better growing conditions (in cool locations this usually means warmer) tree rings are wider, and because trees can be very long-lived and wood is easily preserved, for example in bogs and lakes, this allows very long time-series to be established, and climatic conditions to be estimated from the ring widths.

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The study was funded by the Norwegian Research Council and the Norwegian Directorate for Nature Management.

James D. M. Speed, Gunnar Austrheim, Alison J. Hester and Atle Mysterud (2011), ‘Browsing interacts with climate to determine tree ring increment’, doi: 10.1111/j.1365-2435.2011.01877.x, is published in Functional Ecology on 27 July 2011.

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When asked for comment about their effects on tree ring widths, possibly affecting paleoclimate studies based on tree rings, the sheep denied complicity and said repeatedly “Maaaa aaa  aann Maaaa aaa  aann“.

For those wanting a primer on all the things that can affect tree growth, may I suggest this primer.

71 thoughts on “Tree ring widths more affected by sheep than temperature

  1. Judging by the decrease in the number of people who believe in AGW, I would have to say that sheep density is declining.

  2. Ah, so Mann wasn’t counting tree rings in his sleep–he was counting sheep instead. I wish he’d wake up! Or should his “hockey stick” be renamed to “lamb chops”?

  3. Did he say what the effect was? I didn’t see it. Does browsing stunt the ring growth or does sheep manure increase it? Or don’t they know?

  4. “…numbers of sheep at the location where the tree was growing…”

    This number was difficult to ascertain as the research team had a tendency to fall asleep whilst collecting this data.

  5. Interested readers re-examine McIntyre and McKitrick 2005 (EE), where the potential impact of sheep on SW US trees was considered. I think that we even quoted John Muir, the founder of the Sierra Club.

  6. Now when caught, they’ll claim that pulling the wool over our eyes is normal practice.

  7. Why do I not find this [sur]prising and depressing at the same time?
    We’re doomed ya know?
    We’ll keep pointing out that the whole AGW scene is a crock of lies, untruths, misinformation and deceit, but the MSM isn’t buying in and therefore politicians will continue to delude themselves while they believe they’re on a winner.

    Meanwhile the west is spinning towards the plug hole as the bathwater empties from our economies.

  8. All along, we’ve known that Mann & Jones, poor lambs, were relying on sheep in the media and their sheer flocculent thinking.
    When it’s warming it’s climate, when it’s cooling it’s wethers.

  9. “This shows the density of herbivores affects the tree ring record.” Is Mann a vegetarian?

  10. Sheep? Sheep eat trees? I thought sheep eat grass.

    Now moose, on the other hand, reach down and eat the tops off maple saplings. Or they’ll step on one and it will never be the same. Sometimes in the spring I can use sapling height as a proxy for snow depth – when I see all the saplings trimmed down to the same height, I figure that’s where the snow line was.

    Climate-wise it’s tough to be a tree in New England. Add moose and all of a sudden the race to the sunny level becomes a lot more challenging.

    Just one thing bothers me – once a tree gets to be a couple centimeters in diameter, the moose leave it alone and so I’d expect most of the tree rings will unaffected, it’s just the first few years that are problematic.

  11. Isnt it about time that we just go with tree ring growth is only a good proxy for the thickness of the tree ring?

  12. So tree rings are a proxy for the density of herbivores. The density of herbivores is a proxy for the density of foliage. The density of foliage is a proxy of CO2. CO2 is a proxy for temperatures. See? No problem. Just another subroutine in the model.

  13. Many factors in addition to climate are known to affect the tree ring record, including attack from parasites and herbivores, but determining how important these other factors have been in the past is difficult.

    You know, I have NEVER read ANY tree ring temperature proxy EVER corrected for the rise in CO2 that the Idso family have so cleverly identified as causing a 12 – 27% INCREASE in the growth rates of every living green thing on earth.

    Could his hated CO2 be the cause of Mann’s “decline” in the supposed temperature proxy in the past 50 years? If he is expecting an increase in tree rings only from temperature effects, what happens when his trees begin growing faster because they have more plant food? If/when the opposite happens, and temperatures (start to rise from the LIA in 1650) rise AND plant food starts to rise but from a different date (the mid-1950’s), what happens to his conversions?

    When they have competing effects of less water and more CO2? Has the CO2 levels really been smooth and even the past 2000 years? Can tree rings establish a different record than what Mann hopes/dreams/lives to find?

  14. Kinda feel sorry for Mann et. al. Devote your career to this business and then get thrown for a loop like this. Another variable for the climate modelers. If sheep can have this effect, what about all the other critters?

  15. “This study does not mean that using tree rings to infer past climate is flawed…”

    Yes it does, and it’s worse than we thought.

  16. “The impact of large herbivores on tree rings has, until now, been largely unknown, so these findings could help increase the accuracy of the tree ring record as a way of estimating past climatic conditions, says Dr Speed:”

    Thats utter crap. New Zealand scientists proved this relationship back in the 70s and introduced agroforestry.. the integration of sheep and trees to take advantage of the extra nitrogen made available by the animals. The increase in diameter (and thus ring width) and wood volumes in such circumstances are of the order of 40%. That scientific information is well known throughout the farming and forestry communities as well as published in international scientific journals.

    JC

  17. And I’ll say this again, this also calls into question the whole idea of dendrochronology.

    If trees are more influenced by local conditions, such as canopy cover and herbivores, then they are not so influenced by global or regional climatic issues. If that is the case, you cannot use tree-ring widths to measure climate – and you cannot match them all up, one tree against another, into a great long series that measures chronology.

    Even if you could find some pristine forest where the tree rings measured climate and you could form a historical series – the wood sample you are measuring and ‘dating’ is not from a pristine forest. In which case, its rings may have been influenced more by its overbearing neighbour and dolly the sheep, than the climate that created your wonderful tree-ring series. In which case, how on earth can you match the rings in the sample, with the pristine tree ring record, and thus derive a date?

    .

  18. Bob L says:
    July 26, 2011 at 8:38 pm

    Judging by the decrease in the number of people who believe in AGW, I would have to say that sheep density is declining.

    That’s certainly true in two areas familiar to me: Perthshire, Scotland and Haute Vienne France. In Haute Vienne the last couple of years has seen a huge change from sheep and cows to Maize and Sunflowers.

  19. “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.”

    Oh for F@#$ sake.

    Of course you can, dear.

  20. Hopefully and eventually some people will feel sheepish for thinking that temperature trends can be demonstrated with changes in tree ring growth. As shown here and in other posts and comments, there are just too many other and important influences on tree ring growth. But surely the three most important are water, (sun)light and suitable environment (some trees like alkaline soils, acid soils, nutrient rich, nutrient poor, lots of water, little water) which can change.
    It looks like a tall order to identify temperature changes of such small magnitude over a 100 years using trees. Added to that, we do seem to be in a period of science where there is a kind of machismo in showing how incredibly fine one can measure: Global sea level rises of a few millimeters; Global temperature rises of 10ths of a degree over 10s of years,
    Meanwhile global fish stocks are declining in numbers that are firmly to the left of the decimal point and there are over 23,500 nuclear warheads which exceeds the number of cities with a million people by over 23,000.

  21. Ric Werme and JC make very interesting points. What is the real mechanism behind this effect on tree rings? If it is only the first few years then it is irrelevant, if it is over the liftime of the tree then what has it got to do with nibbling?

  22. I hope you all realise that this is progress.
    Climate scientists performed an actual experiment. It was designed, set up, maintained, measured and interpreted.

    Well done.

  23. Build up a temperature record from tree ring data. Use the temperature record to verify the tree ring data.

    ugh?

  24. Well in archaeological dating processes ie. carbon dating. Check out the Bristle cone pines saga.
    I can’t remember exactly but tree ring growth is measured by the space between rings and how the trees growth can vary from year to year, especially in old trees and there’s been a drought. Carbon dating organic matter has a plus and minus factor of course, and is not as accurate for stuff that is tested that is younger than 2,000 years. I don’t think many trees survive
    this long maybe their trunks do of course. But something happened years ago with the Bristle
    cone pines, I can’t remember but I believe for some reason the atmosphere got bombarded with more carbon. Oh G don’t tell the alarmists?

  25. According to some research precipitation is far more important than temperature for tree ring separation. With so many probable errors in this system I still do not think tree rings are a good proxy for temperature.

  26. You’ve “virtually” said it all, you naughty, naughty people! They still have that fundametal problem, how do they “know” their assumptions are correct just becaue the model produces what they expected it to produce. A circuitous argument if ever there was one! You still have to tell a computer programme to show what you want it to show all the way down the line, it doesn’t think for itself you know!

    RockyRoad says:
    July 26, 2011 at 8:44 pm
    Ah, so Mann wasn’t counting tree rings in his sleep–he was counting sheep instead. I wish he’d wake up! Or should his “hockey stick” be renamed to “lamb chops”?

    Curiously, or even disturbingly, a chop bone bears an uncanny resemblence to chunky Hockey Stick! However I am worried, when I got some graph paper out of the draw (some of us like to draw lines & curves still in this electronic age), I plotted 7/10°Celcius rise in global temperature over 150 years to 2000 using red ink against thin black-inked base lines, thing is I could hardly see it but that was because I changed the aspect ratio & scales accordingly. Suddenly it didn’t look all that dramatic after all! Playing games with “chartmanship” is such fun!

  27. Quite a while ago, I lived in town (Chelmsford, Essex) I owned a ground-floor flat, with a small garden and was on a busy road, downhill of but slap next door to a petrol filling station.
    One day, as the local market was closing, I bought the last 8 Leylandii plants that a stall holder had and I planted them, in a line about 4ft apart, at the foot of my lawn to make a bit a hedge between me and the traffic.
    The didn’t take off spectacularly as Leylandii are want to do and 3 years later I moved jobs and rented the property out.
    6 years later I sold the property and had to clear the furnishings- it was only then I noticed how these trees had grown. The one closest to the petrol station was maybe 7ft tall and each tree was about 6″ less than the previous one, decreasing in size in a perfectly linear fashion as they got further from the filling station. Why?
    I assumed it was water/rain run-off from the forecourt of the filling station (Essex is very dry county) so that the first tree got loads of water and each one got successively less. as the water drained away downhill. But and if anything, forecourt run-off would be polluted and would stunt them if not kill them outright
    But was it extra water?
    Do trees thrive on petrol fumes, was it extra CO2, water vapour or heat from the cars as they queued to get in and out of the filling station. Was it something to do with people walking their dogs or drunks from the pub 100yards away. So, what was going on there?

  28. DCC says:
    July 26, 2011 at 8:51 pm
    Did he say what the effect was? I didn’t see it. Does browsing stunt the ring growth or does sheep manure increase it? Or don’t they know?
    ////////////////////////////////////////////////////////////////////////////////////////
    My thoughts exactly.
    Another possibility that came to mind was that perhaps the sheep weeded out very small adjacent saplings, which if left undisturbed would eventually grow and compete for light and nutrients available for nearby more substantial trees.

  29. I would not get too carried away with this paper which I see more as amusement than anything else. At most, it is a reminder that many factors which may not at first have been readily apparent can influence the accuracy of conclusions drawn from proxy evidence.

    I question the validity of the implications of this research on tree ring data used in proxy records Since this data comes more from forest than scrub land). I hate proxies generally. At best they are a rough and ready indicator of trends, but can never be reliably tuned and since so many other ‘outside’ factors are at play on the development of the proxy, they cannot be used to quantify a specific sub set of one of the ‘outside’ factors at play. In my opinion, people try to read far too much into proxies, and all proxy drawn data should always carry a substantial error bar and note the high uncertainties.

  30. Well it is time please that WUWT added
    Viv Forbes
    and
    The Galileo Movement
    to the link section.

    http://www.galileomovement.com.au/

    I know we are DOWN UNDER, and we did have the greatest industry in wool for there for a while, but now we Aussies need to show our tremendous work in the mining and agriculture sector + associated industries/workers for the many Australians, thank you. And IT.

  31. one of the postulated mechanisms is that the sheep wipe out the herbs that compete with trees, thus stimulating tree growth. Craig Allan has written on this in connection with ponderosa pine. There were huge populations of sheep in some US areas in the late 19th century.

  32. Well in Australia sheep don’t eat bark, goats do so do deer, and cattle will. I remember planting
    100 pines and couldn’t understand where they all went. Dear little cattle. Possibly even rabbits
    I don’t know but I was told to one should protect saplings from cattle? Who nose? LOL

    Anyway ‘Hide the Decline’ is still on U Tube that ridiculed M Mann it is hilarious. Even though he
    said he would sue them?

  33. Once again we have what appears to be an outstanding feat of genetic hybridization, One that has seemed to become more popular and prevalent with each passing day. I speak of course of that malignant creature which results from the combination of DNA of the crocodile with the DNA of the abalone i.e. the infamous crocabalone.

  34. While it may be possible to estimate the total number of herbivores in a given time period in a given area, it will never be possible to accurately state the number of herbivores in a particular area in a particular year. Beyond that, it will never be possible to determine whether ANY herbivores nibbled on the tree that you are sampling in any particular year.

    While it may be possible for such a study to help more precisely define the error bars regarding a particular trees ring measurements, to state that this study helps them to define the data itself better is not supported.

  35. Can’t provide another sheep pun – they’ve all been used up :-)
    Not sure if this has any relevance, but I have recently finished cutting up 18 – 22 year old rainforest acacia, cyclone damaged 3rd February 2011, at 19°11’38″S 146°40’31″E. Stand-alone trees, no herbivores, no additional nutrients, not much variation in solar here over their lifetime. Been monitoring precipitation since Aug 2004, average has been slightly above long-term average. No artificial irrigation anywhere near the youngest tree. The older tree was on imported salt-pan clay over dune sand, got to suck water from profligate irrigation next door, and has a trunk diameter at 1m of about 75cm. Younger tree is on old dune sand only. Trunk diameter at 1m is about 36cm. Last 7 years of tree rings show increasing width. If “as above so below” applies, the tree got to the ‘dry season’ minimum aquifer (6m down) by year 10 at the latest. So it looks like the latter 7 years growth could be enhanced CO2 ?
    On the other hand, although we never irrigated near the tree or applied nutrients in the area, nutrient deposition would have been occurring throughout the period.

  36. Dave Wendt says: July 27, 2011 at 4:27 am
    …….. i.e. the infamous crocabalone.
    Excellent. Do they design necklaces? And where can one purchase such?

  37. There’s another really important finding in the abstract of this paper:

    “Radial growth was negatively related to altitude and related to summer temperature in a nonlinear fashion, increasing from low temperatures and saturating and decreasing at high temperatures.”

    Nonlinear response to temperature is a major problem for tree ring-based temperature reconstructions.

  38. Jessie wool still is one of our biggest exports particularly fine wool. But remember our ‘woolly jumpers’ also provide meat. And one of the Greens objectives and Gaunaut’s suggestions was to cut methane emissions, beef and sheep should be taxed $11 per head and $7 per year respectively. NZ is considering introducing agriculture into carbon taxing that Garnaut suggested also. Gaunaut also suggested that farmers ‘farm’ kangaroos also. Ever tried to milk or shear a kangaroo, not withstanding they have defied domestication as they are marsupials. Just Google
    Ross Garnaut on methane emissions. The man is a nut! Suggested that farmers through carbon credits from soil sequestration could make more money than farming and wool production.
    Well wind farms are giving farmers $10,000 dollars a year per wind turbine as rent on their land.
    One farmer has fifteen on his land, probably more than he would recover from farming without any expenditure at all. That’s what is happening in Oz. Four corners had a good program
    on Monday 25th July about health and farmers near wind turbines. (Available from ABC TV Four Corners). Interesting stuff.

  39. richard verney says:

    July 27, 2011 at 3:32 am

    Yes, it’s amusing, stupid and all that but someone paid these idiots to do this rubbish and with whose money ?

  40. Wife’s friend says their section of Oregon has not gotten above 84F while we in Kansas have not gotten below 84F for more than a few minutes each night. What sort of proxy for world wide temperatures would come from trees in these locations?

  41. “sheep pun – they’ve all been used up”

    No, it’s airy stuff, perhaps, butt perpetrators of the rampant AGW scam wool need to be thoroughly lambasted, and there a får more plays on words and puns to horn in on yet, shor nuff (especially if we schapen our multilingual skills).

  42. The biggest unknown about tree rings is the effect of precipitation and evaporation, especially where water is in short supply. Take a look at any mountainous desert area and you will see trees above a certain elevation but none below where it is hotter and drier. If you look at slopes in the Rocky Mts., you will see trees on north-facing slopes but none on south-facing slopes. The reason is that south-facing slopes in the Northern Hemisphere get more direct sunshine, hence are drier and difficult for trees to grow. For a given area, imagine comparing the relative widths of rings from trees on slopes intermediate between north and south facing. A small increase in precipitation will have a much greater effect than a change in temperature. A more telling experiment than the sheep study would be to divide an area into plots (all of which will have the same temp) and vary the amount of water each plot receives rather than the number of sheep each plot receives).

  43. Pete in Cumbria UK says:
    July 27, 2011 at 2:54 am

    The one closest to the petrol station was maybe 7ft tall and each tree was about 6″ less than the previous one, decreasing in size in a perfectly linear fashion as they got further from the filling station.

    Could be sunshine and shade. The taller one was in the sun longer.

    As co2 is always increasing I would expect bigger rings now, not because it’s warmer but because the trees have more co2 to extract.

  44. Lightning creates NOX which would create nitrogen-based acids in the humid atmosphere and with the rain which would, either or both, attack and damage leaves or soak into the ground with the rain and fertilize the trees. A gentle rain produces no lightning, so maybe trees are also a proxy for lightning activity?

  45. This is such a great site. Got my daily dose of face palm again. Where else could you find all of this stuff. Baaaa Humbug.

  46. i don’t know much about elks, moose and reindeer. But Deer do eat bristlecone pines when there is nothing else to eat. It is possible the big guys ( elk, moose, reindeer ) do to. May be someone in this site enlighten us.
    Because generally there are not a whole lot of leaves on these plants, and they don’t really grow straight up all the time, and the leaves do not regrow very fast, it is possible Deer could do a lot of damage to any one particular bristlecone tree, even if it is a couple of hundred years old tree. Eat some 1000 needles, not a whole lot left. next year’s tree ring is probably not too good.

    Don’t know if they eat the bark as well, and what the effect of de-barking them will be on the tree rings. I know nothing about the tree selection process used for the Yamal or any other dendro studies

    SO, SO many unknowns!!!

  47. Hey, what’s this business about separating the trees into areas and putting different sets of animals into the areas and controlling for other variables?

    Don’t they know the CORRECT way to do science is to make up some numbers, write some random equations, run the fake numbers through the random equations, then declare your hypothesis is proved?

    How’d they get this hopelessly obsolete “controlled experiment” stuff through peer review?

  48. Clay Marley says:
    July 26, 2011 at 10:09 pm

    This study suggests tree rings can also be a proxy for the density of mann.

  49. Jessie says:
    July 27, 2011 at 5:08 am
    Dave Wendt says: July 27, 2011 at 4:27 am
    …….. i.e. the infamous crocabalone.
    Excellent. Do they design necklaces? And where can one purchase such?

    Unfortunately, no. The creatures shell, when given a thorough professional polishing, will briefly display the iridescent luster of the abalone, but very quickly reverts to a more croc like presentation. Also, as an “unexpected” and unintended consequence it gives off a powerful smell that makes a durian seem like a bouquet of roses and lavender by comparison.

  50. As a young pup in in Cyprus I do remember seeing sheep in the lower parts of whatever olive trees they could climb. They were going after the new growth.

  51. Mutton heads! Mr. Mann can not bring himself to sheepishly admit that using tree growth rings for temperature proxies is just baaaaaaaaaaa-d science…

  52. The problems with tree ring proxies are well known which is why they are matched against other proxies.

    The link itself notes:
    Many factors in addition to climate are known to affect the tree ring record, including attack from parasites and herbivores, but determining how important these other factors have been in the past is difficult.

    The lead author states:

    The good news is that past densities of herbivores can be estimated from historic records, and from the fossilised remains of spores from fungi ..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”

  53. I am amazed, but today I noticed sheep nibbling at the base of some trees. This was more or less at head height – no joke. I couldn’t photograph it becasue I was driving at the time. So they do it for sure, but whether this is the significant nibbling or not I don’t know.

  54. Its not so much the nibbling of the trees themselves but the eating of the grasses and clover etc.

    the sheep rip out the plants at the roots, leaving barren ground behind. one result of course is that with this vegetation gone less water is absorbed down into the soil for use by the trees. it simply runs off too quickly. second is the decimation of nitrogen fixer species. less nitrogen in the soil also prohibits the tree growth.

  55. So the reason they use tree rings is because its much easier to find trees that are conform and “support” the political established UNFCCC?

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