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A Treeline Story

Filed under: — Ray Bradley @ 17 November 2009

Some of the highest growing trees in the world are also the oldest—bristlecone pines (Pinus longaeva) from the Great Basin in the western United States (eastern California, Nevada and Utah). The oldest example is more than 4800 years old. Because of their longevity and growth at high elevations (where the growth of trees is generally known to be limited by temperature) bristlecone pines have been of particular interest to dendroclimatologists (paleoclimatologists who study tree rings to reconstruct past climate). Numerous ecological studies carried out at treeline sites all over the world show that temperature imposes a critical limitation on the ability of trees to produce new tissue; mean daily temperatures of 8-9°C are required, so recent warming will have particular benefits for those trees that have managed to eke out an existence for so long, living “on the edge”.

An interesting characteristic of the western bristlecone pines is that their recent growth has markedly increased—ring widths have been higher than in previous decades. Previous studies have debated to what extent this “fact” is real, or just an artifact of the way tree-ring data are analyzed. Because the growth of trees is radial, as trees get older and the diameter of a tree increases, annual ring widths decline in thickness. This is the normal “growth function” that is commonly removed from measurements before further analysis is carried out. The trick is to do this carefully so that as much climate information is retained while the growth function is discarded, and dendroclimatologists know how to do this quite well. However, sometimes the “standardization” procedure can introduce spurious results. This led some to regard the apparent growth increase in bristlecone pines to be a meaningless result of the data processing. In a new article in the Proceedings of the U.S. National Academy of Sciences (PNAS) Matthew Salzer (Laboratory of Tree Ring Research, University of Arizona) and colleagues examine this issue head on. They studied hundreds of trees from treeline sites in the Great Basin, aligned all the samples according to date, and simply averaged the results (Figure 1). Given that these trees are all long-lived, the complicating factor of growth function (which is strongest for the early growth of a tree) was not significant for assessing the most recent growth. Their results show that mean ring width in the last 50 years has been greater than in any previous 50 year period over the last 3700 years. You have to go all the way back to ~1900-1300 B.C. to find mean ring widths approaching recent values. Furthermore, the recent increase in ring widths is seen in trees at the upper forest border at sites hundreds of km away (even when the treelines there were at lower elevations)—but not in trees below the upper forest border. Below the zone closest to treeline, wide rings are formed in cool, wet years, and narrow rings in warm, dry years, and trees from this lower zone do not show the 20th century growth surge.

It is thus clear that the bristlecone pines from the highest regions, close to their growth limit, are showing a very strong response to recent warming, and indicating just how unusual it has been in the context of the last few millennia. Previous explanations have focused on possible CO2 fertilization effects (increasing water use efficiency) but there is no obvious reason why such factors would have affected only trees within approximately 150m of local treeline in different locations. Rather, the high elevation trees, close to the limit of growth, have responded positively to the recent increase in temperature just as ecological studies would have predicted.

One final note: bristlecone pines often have an unusual growth form known as “strip bark morphology” in which annual growth layers are restricted to only parts of a tree’s circumference. Some studies have suggested that such trees be avoided for paleoclimatic purposes, a point repeated in a recent National Academy of Sciences report (Surface temperature reconstructions for the last 2,000 years. NRC, 2006). However Salzer et al’s study shows that there is no significant difference in their results when the data are divided into two classes—strip bark and non-strip-bark cases –when the raw unstandardized data are compared. So that particular issue has apparently had people barking up the wrong tree…

Figure 1: Median ring-widths (non-overlapping 50-year means) of upper forest border Pinus longaeva from 3 sites in western North America, plotted on first year of interval (from Salzer et al, PNAS, 2009)

144 Responses to “A Treeline Story”

  1. 1
    GFW says:

    That’s some very nice work by Salzer. Nothing like increasing the amount of data to get rid of troublesome issues like the growth factor.

    Two other reactions
    1. Could you give a graph that doesn’t look out-of-focus?
    2. What medieval warm period? :-)

  2. 2
    Jim Bouldin says:

    So that particular issue has apparently had people barking up the wrong tree…

    Multivariate puns, the best kind.

    I think Schulman, or maybe it was Douglas, used strip-barked bristlecones extensively in developing chronologies, for the very reason that they were less susceptible to size effects. No detrending needed.

    Thanks for the heads up Ray.

  3. 3
    Richard K says:

    I am not quite sure what this shows. If this rise took place in 1300BC then that means it has happened before. Is there any data or studies to reflect the CO2 content of the atmosphere during the last upswing?

  4. 4
    GFW says:

    To elaborate on my MWP comment, the MWP actually does show up as a warming from the couple hundred years previous, but the LIA is less apparent – it appears as a couple out outliers, although strong ones. Still, this particular reconstruction has the LIA as on average warmer than say, 850 AD. OTOH, this reconstruction is only relevant high on mountainsides, so the climatic patterns experienced by most humans in the MWP and the LIA probably better conform to other reconstructions. Can’t argue with the clear signal that what we’re doing to the planet right now is far in excess of what the MWP was like though.

  5. 5
    arch stanton says:

    Wouldn’t the difference between the current relative growth rates also indicate that if a tested tree was not in the higher part of the grove at the time of the warming, evidence of the warming might not have been recorded? Have the heights of the groves varied enough over the last couple thousand years to make a difference?


  6. 6
    Rolan O. Clark says:

    Would it be possible for the rings now ‘wide’ to constrict in width in, say, 100 years, as more growth occurs?

    It seems there may be a lot of liberty taked to assign fixed characteristics to factors that may show a lof of deviation over time.

  7. 7
    Geno Canto del Halcon says:

    The data appears to show that climate was nearly as warm in the second millenium b.c.e. as now. Who can say why, and do we have corroborating evidence of melting glaciers and sea level rise rate that was higher during that time period?

    [Response: Keep in mind that the evidence is regional, and not necessarily indicative of hemispheric or global mean temperatures (though there are a number of studies that suggest that western North America is about as good as any single region if you’re trying to estimate larger-scale temperature trends). As for the other evidence available for corroboration, we reviewed this recently. As shown in that review, (a) it is only possible to reconstruct global or hemispheric mean temperatures from glacier records for the past 400 years or so (sea level is even trickier on these timescales!), and (b) existing reconstructions of extratropical Northern Hemisphere mean temperatures using the various available proxy data, only go back about 2000 years. In the latter case, the long-term trend is indeed one of cooling, prior to the industrial era, and the authors (Kaufman et al, 2009) interpret that trend as consistent with the expected response of extratropical temperatures (during summer, but this also applies more weakly to annual mean owing to land surface feedbacks) to the long-term earth orbital-related decline in extratropical northern hemisphere summer insolation. This interpretation would also seem to apply to the observations from the present study, recognizing that (i) there is likely to be substantial century-scale variability related to other forcings (solar and volcanic) about the longer, multi-millennial orbitally induced temperature trend, and (ii) this variability (and any internal variability) is likely to be more pronounced at the regional scale of the present study. – mike]

  8. 8
    Joseph says:

    This tree ring study is a very interesting indirect way (proxy?) of proving the theory of global warming. The following could be considered off topic but I think it is in the same spirit of showing indirectly the effects of global warming. This is (perhaps) a new suggestion to accomplish the same thing.
    I live in the foothills of the rocky mountains at latitude 51 north. The layout of the land coming from the east is low prairie, elevated prairie, rolling hills and finally foothills and the mountains. In other words the terrain gets more mountainous with steeper terrain as one travels from east to west towards the rocky mountains.
    I go on daily hikes with my 2 dogs and look at various things in nature. There are 4 artesian wells which I have gone by regularly for the last few years because my dogs like to drink from them. I have never before seen these wells run totally dry. Even in midwinter with temperatures down to minus 20 or 30 these wells keep running sometimes building nice ice terraces etc. I mean they always provide a relatively copious amount of water.
    Indeed, such wells are commonly used here by people who raise cattle, horses or sheep etc. as a hobby or a sideline. People will install a drinking water trough or just create a dugout so the water can be collected for the animals.
    This is the first time since I moved here 9 years ago that these wells have gone either totally dry (2 wells) or almost stop running i.e. just a drip of water .
    The climate here is essentially dry because we are in the precipitation shadow of the Rockies. After July it rains very little if at all until late next spring. The snows usually come late in the year after the ground has been frozen (november,december)and again in the early spring before the ground thaws out.
    I therefor tend to think that these wells get their water from the mountain snow and ice melting all through the summer and that there is not enough snow and ice left in the mountains late in the season to keep these wells properly fed with water. I know that all attention is directed towards the large glaciers who are slowly or rapidly melting away. But there must be hundreds if not thousands of small valleys or depressions in the mountains filled with snow and ice which have no natural creek to discharge in and the water can only percolate through the ground and run down the mountains under the ground only to surface in the artesian wells along the foothills.
    I wonder if a survey could be done to determine if this decrease in flow of artesian wells is a common occurrence in the foothills along the length of the Rocky mountains.
    I know also that it may be better to keep quiet etc. as per Mark Twain. Based on some of the reactions to other suggestions I have read on this website I should probably look for cover.

  9. 9
    tharanga says:

    These rings give annual records, don’t they? I’m curious about the sharp dip at 1450 AD, and how it might look with a 20 year mean, instead of 50.

    Are there analogous trees in the Southern Hemisphere? It seems to me that more SH proxies would be especially helpful, to establish a better global reconstruction.

    Richard K, reply 3: Not all temperature rises have to be caused by the same thing. And any feature you see on the plot could only be regional; you have to combine these results with results from other places to get a global view.

  10. 10
    Hank Roberts says:

    Joseph, you might ask around in your own area — there will be a county agricultural office, or something of the sort, there. And just a quick Google turned up quite a few recent references discussing what’s going on with water in Colorado.

    There’s almost certainly a document somewhere listing artesian wells that you can find for your area.

    I’d be very surprised if there isn’t serious attention being given to water flows — particularly because for example pumping water out of an aquifer can reduce pressure so artesian wells elsewhere quit running, and this raises legal questions.

    One quick example from one search — you can do better I’m sure:

  11. 11
    Hank Roberts says:

    > it has happened before. Is there any data or studies
    > to reflect the CO2 content …
    Yes. The “Start Here” button at the top of the page is a good place to start. Also click the first link under Science in the right sidebar.
    For pictures that are a good lead-in to reading, have a look here:
    Also have a look at this thread (Ruddiman’s work):

    > Would it be possible for the rings now ‘wide’ to
    > constrict in width in, say, 100 years

    No. The wood inside the thin outside growing layer of a tree is dead and stable; trees don’t shrink in diameter over time.

  12. 12
    David B. Benson says:

    Geno Canto del Halcon (7) — As Mike’s response indicates, that temperature high about 4000 years ago amy well have been but regional. However, something similar is found in the temperature proxies from GISP2, suggesting a somewhat similar warm period in the North Atlantic region around then. This might just be happenstance, but you could consider W.F. Ruddiman’s early anthropogenic hypothesis, as presented in his popular “Plows, Plagues and Petroleum”, in light of it. He has more in his professional papers, available from his web site; do note his hypothesis is not universally accepted.

  13. 13
    DrCarbon says:

    wrt #3: The lack of standardization means that the juvenile growth is likely in samples that go back to the center of the tree. This definitely is not at play in the 20th century. It is however possible in late second millennium BC.

    wrt #4: This paper isn’t a temperature reconstruction but a report on widths of tree rings and their possible causes. This is quite different than a formal paleoclimate reconstruction.

    wrt #9: Look at the paper and see figure 4. the paper is open access and viewed by anybody. The 1450 dip is clear at three different high elevation sites.

  14. 14
    Jim Bouldin says:

    re #7:

    In addition to what Mike has said, read the section on pg 4 titled “Ring width and tree age, with emphasis on the final sentence therein. The point is that the analysis may well be conservative wrt to temperature increase, in the sense that the pre-1900 ring widths may be somewhat inflated, relative to post-1900, by tree age (actually size) effects, given that the individual series were not detrended.

  15. 15

    Couldn’t local weather records also verify whether ring widths expand in warm years at treeline? I know the University of California has a research station in the White Mountains, so I assume they’d have pretty good data.

  16. 16
    Jim Bouldin says:

    Brian, the White Mtn station data are almost certainly incorporated into the PRISM data, which is a complex station data interpolater, and which is much the better data to use here because of its ability to estimate temps at all 3 sites, and its ability to account for topographic and elevational effects.


  17. 17
    mike roddy says:

    Increased growth may not indicate future survivability, or we wouldn’t be seeing dead pine trees all over the West. Bristlecones are a high desert species, similar to joshua trees and, to a lesser extent, pinon pines. Joshuas are predicted to become extinct in Joshua Tree National Park, because their seeds will not germinate at the predicted temperature levels of 50 years from now.

    It’s quite likely that similar problems will beset the ancient bristlecones. They inhabit an even narrower geographic range, also at arid higher elevations. I’m from California, and rarely see them anywhere outside their small habitat east of Bishop in the Sierras, at I would guess a 5,000 foot elevation. A 2C or more increase may doom them, though it may be tough to find dendro evidence to that effect. Besides germination problems, other factors could be shorter, milder winters, leading to increased pest colonization, or changes in the hydrological cycle. I welcome knowledgeable speculation on this subject. It will be tough for them to move upslope as the climate warms, because the mountains in their range are not very high.

  18. 18
    Richard says:

    They may be showing a decent increase in temperature over the last century – but how does that correlate with the raw temperature record from nearby weather stations in rural locations? I don’t recall information on that anywhere.

  19. 19
    Rattus Norvegicus says:

    It’s pretty damn funny watching bender get all unhinged over this paper over at CA. Saying (this is a paraphrase): “strip-bark vs. whole bark was never the issue: six sigma trees!! NAS just dodged six sigma trees”. In another thread he says that the only thing that matters is sensitivity to precipitation but that Saltzer, et. al., just “dodged the issue”. Of course they didn’t dodge the issue and you can see the analysis in figure 3(c).

    It seems pretty clear that the crowd over there hasn’t come up with a good line of attack that might make one think for a second before answering their unfounded claim. CA needs a better class of denialist.

  20. 20
    John Mashey says:

    1) All this is a nice example of the general case of Liebig’s Law of the Minimum.

    2) I certainly recommend a visit to Bristlecone Pine Forest in the White Mountains and actually see these things live.

    3) And finally, Edmund Schulman and the “Living Ruins” – Bristlecone Pines, Tree Rings, and Radiocarbon Dating, by Donald J. McGraw, is a nice book on some of the history of this. McGraw was actually there signing books the day we visited. I hadn’t seen the time ‘cosmic schwung’ before, so it’s always nice to add to one’s vocabulary.

  21. 21
    Rattus Norvegicus says:

    Oops, that was figure 4(c).

  22. 22
    Bruce Tabor says:

    Re Geno Canto del Halcon and Mike’s response.

    “The data appears to show that climate was nearly as warm in the second millenium b.c.e. as now.”

    Is it possible that this is an artefact? The ring widths were merely averaged, not corrected for the “growth function”, but as the article suggests, “as trees get older and the diameter of a tree increases, annual ring widths decline in thickness.”

    The point of averaging is to average over the effects of the age of trees, but most of the trees from which rings can be measured for 3500-4500 years ago would have been relatively “young” at that time. This may be a “boundary effect”.

    To put it another way, the population being sampled for the 3500-4500 year estimate would have a much younger age distribution at that time than those sampled later.

  23. 23
    oakwood says:

    Couple of observations. You can see that each grey dot represents 50 years of time on the horizontal axis.
    The latest upward trend appears to start around 1830. So what could have caused the start of that trend that seems to continue to the end?
    Why do the data end around 1950? This is a long time ago in terms of current CO2 trends.

  24. 24
    Edward says:

    Could you relabel the date axis please. ” wide rings are formed in cool, wet years, and narrow rings in warm, dry years” So the downward trend since the year 200 means the climate is getting both warmer and drier or warmer or drier. It is easy to see this upside down if you don’t read carefully. I’m not sure where you are starting your trend line since it isn’t on the graph. The effect is subtle but a good confirmation.

    Is there a meaning to the sawtoothiness?

    Thanks for showing exactly how the climate change operates on these trees.

  25. 25
    Anne van der Bom says:

    I sort of expected error bars on the graph. Is that an omission or isn’t it supposed to have those?

  26. 26

    It sounds as if Salzer is trying to take the simplest possible reconstruction method to get an idea of the climate. I think that is a good approach to the whole RCS discussion. I have done something somewhat similar to the Yamal dataset.

  27. 27
    Jon says:

    I was watching a doco about the battle of Hastings 1066.
    The deployment of troops made no sense if one took the geography to be the same as today. When sea levels were increased the reasons for the battle formations became clear. South England is supposed to have been sinking since the last ice age so if the sea level was much higher in 1066 then the climate would likely have been warmer than today, and/or for some time, around that date.
    The programme was, I would say, completely untainted by the current controversy.
    There may be more direct evidence available than these seemingly problematic tree rings.

  28. 28
    Geoff Sherrington says:

    Somebody uner post 11 Hank Roberts says

    “No. The wood inside the thin outside growing layer of a tree is dead and stable; trees don’t shrink in diameter over time.”

    There is exrtensive literature on the curing of trees for lumber and violin making (to give just 2 leads) that indicate that water submersion can densify tree wood. Since many dendro studies use old dead trees preserved underwater, I guess one should express some doubt about that comment.

  29. 29
    Mike Smith says:

    #7: “The data appears to show that climate was nearly as warm in the second millenium b.c.e. as /now/.”

    I’ve not read the paper, but the graph’s last point is 1950. I’m not sure we can take the last data point to mean ~2000. The global temperature has increased by ~0.5degC since the middle of the last century: So most of the human-caused global warming wouldn’t really show up on the graph…

  30. 30
    shannon says:

    thanks for the great info. I just found this site. To me looking at the figure 1. we are about the peak as were years ago. To me it see cyclic. keep up the good work.

  31. 31
    Chris S says:

    Geoff #29:

    During what period are you hypothesising the trees in this study were submerged?

  32. 32
    Pat says:

    I just found this website. I’m in the midst of looking at both sides of the global warming yay/nay debate. Recent talks in my area of carbon taxes and other financial incentives to curb greenhouse gases has peaked my curiosity. Since me and the other citizens of my country will soon have another hand in our pockets, I want to see if I’m getting what I’m about to be paying for. I like this site so far, but I’d like to know, is this issue cut and dry for the most of you, the science is settled and it’s CO2 that is solely driving our climate? Thanks for your time.

  33. 33
    bigcitylib says:

    I notice the CA post on this topic has way more graphs. You will never “win” these encounters if you just have one graph.

  34. 34
    Bill J says:

    Where is the nearest surface station temperature data,for the last century ?

  35. 35
    Hank Roberts says:

    > way more graphs

    Yep, this is something I keep finding — I’ll search Google, then Scholar, then Google Images on anything about climate. The septic sites are _very_ far ahead in posting pictures that show up in the Image search results.

    It doesn’t mean they’re using them right — it means they’re using them far more.

    Advertising, basically. They’re way, way ahead in attention-grabbing.

  36. 36
    Hank Roberts says:

    Pat, you’ve gotten the basics wrong in the question you posted above.

    There’s no hand in your pocket except your own — assuming you support things like schools, public health, highways, air traffic control, public utilities.

    It takes work to understand all of the things you as a citizen do by taxation.

    School isn’t just arithmetic; public health isn’t just whooping cough prevention; highways aren’t just asphalt. Warming isn’t just CO2. All are areas where people found problems worth solving by cooperating in paying for things business can’t do.

    If you want to argue about that, this isn’t the place; try this one:

    The “Start Here” link at the top of the page is a good place to begin for the very basics. Give it at least a few minutes of your time.

  37. 37
    Jim Bouldin says:

    He who hath the most graphs wins!!!

    So McIntyre’s jumped into this one now has he? My surprise is exceeded only by my urgent desire to follow his labyrinthine path of amalgamated “insight” and innuendo. By the way, is he up to triple digits yet on the number of Yamal articles?

  38. 38
    Rattus Norvegicus says:

    The graph is for binned and averaged ring widths based on 50 year intervals. 1950 would be the last starting date of a 50 year bin, so 1950 is the last data point. Date is for the first year of the bin.

  39. 39
    Hank Roberts says:

    Sorry, Geoff, I tried but I can’t support your claim by trying to look it up for you. Can you? Best summary I can find is:

    Even if submerged whole trees did somehow shrink, such wouldn’t change the relative thickness of the annual rings–it’s those patterns that are used to correlate one tree with another to establish a timeline.

  40. 40
    Pat says:

    I wasn’t arguing, I was asking a question, there is no need for insult. I did read quite a bit so far on this site and I’m still continuing to do so. Thanks Hank.

  41. 41
    Steve says:

    The last point on the graph is indeed around 1950, but the caption explicitly states that these are 50 year means (averages) plotted on the first year of the interval. In other words, the last point at 1950 is the 1950-2000 average.

    If you’re going to plot an average you have to pick a point in your range. In this case they picked the beginning of the range instead of, say, the middle or the end.

  42. 42
    Mathis says:

    off topic:
    can anyone tell me where the world’s meteorological data are stored and processed to climate change information? is it in the climate centre (such as hadley) itself or how does it work. many thanks, any answer to this is much appreciated.

  43. 43
    Alexandre says:

    One thing was not clear to me on the original post:

    On that graph, is the growth function removed? In other words: did that region experience a similar temperature today and some 4,000 years ago? Or are the rings growing so much now that they are matching the early tree´s intense ring growth?

  44. 44
    dhogaza says:

    I wasn’t arguing, I was asking a question, there is no need for insult.

    Well, when you post an obviously denialism-strawman statement like:

    I want to see if I’m getting what I’m about to be paying for. I like this site so far, but I’d like to know, is this issue cut and dry for the most of you, the science is settled and it’s CO2 that is solely driving our climate?

    Science has never, does not, and will never say that CO2 is solely driving our climate. Despite what you may’ve been told by others, climate scientists do occasionally raise their head from their computer workstations and look outside on sunny days, for instance. Scientists are aware that we’ve had ice ages in the past, warm periods in the past, etc.

    You’ll get a more understanding response here if you take the time to learn what science says *first* – and …

    Big hint coming up:

    You will not find what science actually says about climate if you read denialist sites which lie to you.

  45. 45
    Ray Ladbury says:

    Pat @32,

    It is deceptive to ask whether “the science” is settled, because science is a process that continually advances our understanding of its subject matter. The issue of whether increased CO2 will lead to warming, however, is very well settled. It is an inevitable consequence of our current theory of Earth’s climate–a theory with mountains of evidence in support of it. The naysayers (I prefer “denialists”) have neither science nor evidence on their side–only misinformation and the unfortunate human tendency to believe what we want to believe.

  46. 46
    Rattus Norvegicus says:


    The graph is of raw ring widths, no detrending was done. The paper states that the earliest portions of their data come from remnant wood and that it is impossible to get an estimate of pith offset values, so they just don’t know.

  47. 47
    Jim Bouldin says:

    re 22 and 43:
    The age-related trend is not removed in that graph, nor in any graph from the study. The implications of this for the wide ring widths over 3000 years ago are discussed in the first subsection of the Discussion section.

  48. 48
    Jim Bouldin says:

    Mathis, it’s not all in any one place. See

  49. 49
    Hank Roberts says:

    Pat, no insult intended, we do try to be helpful (and most of us here are ordinary readers, not climate scientists; the Contributors (sponsors of the site) are listed in the sidebar. You’ll get to know the other climate scientists.

    Don’t blame them for our mistakes — or occasional impatience (grin).

    We’re trying. (“Sometimes _very_ trying” as someone quite close to me is wont to say.)

    Many of us tend to reply too fast when a new userid posts FAQ about “settled” and “only CO2” — patience furthers.

  50. 50
    Jaydee says:

    32 Pat

    The way I read it is this:

    the data says that the climate is warming. The data also say that by far the most likely cause is CO2 produced by humans burning things (Coal, oil, gas).

    So to that extent the science is settled. I suppose it is possible that tommorow somebody might come up with evidence that something other theory explains things better. However, people have been trying for many years and have produced nothing that contradicts the CO2 theory.