date: Thu, 16 Nov 2006 14:36:14 +0100
from: Jan Esper <esperatXYZxyz.ch>
subject: Re: Mitrie: Bristlecones
to: Keith Briffa <k.briffaatXYZxyz.ac.uk>, Martin Juckes <m.n.juckesatXYZxyzac.uk>, "Myles Allen" <allenatXYZxyz.ox.ac.uk>
...no, no, not a lot to add from my side. This is much more than I could have said. Except,
I once looked at strip bark growth trees in Central Asia, and at least there the cause for
this growth form was clear to me (Esper 2000, The Holocene):
"Strip-bark growth forms (Ferguson, 1968; Fritts, 1969; Graybill and Idso, 1993; Kelly et
al., 1992; Wright and Mooney, 1965) also appear in older Juniper trees. This condition
develops as the cambium is damaged locally and will no longer be overgrown. Mechanical
damage by rockfall seems to be the principle stimulus for cambial dieback and unilateral
growth. In extreme cases only a narrow strip on the stem is still active, creating these
eccentric growth forms."
I didn't visit the Bristlecone sites yet, but the mechanism might be the same (some
I believe that over time the crown and root system are reduced, but not at the same rate
than the reduction in circumference covered by the cambium. This would be the key for strip
bark tree rings being wider than "normal" rings.
I am not very convinced that there are long-term fertilization effects by CO2 (but have of
course no proof for this). As far as I know, (most) results from free air CO2 enrichment
experiments suggest that there is no long-term effect.
I Cc Rob Wilson to the mail, as he might have looked at Bristlecone data recently. Pehaps
he wants to add something.
At 11:57 Uhr +0000 16.11.2006, Keith Briffa wrote:
Martin and all,
I know Franco very well - but he has not worked extensively with the Bristlecones. I
still believe that it would be wise to involve Malcolm Hughes in this discussion -
though I recognise the point of view that says we might like to appear (and be)
independent of the original Mann, Bradley and Hughes team to avoid the appearance of
collusion. In my opinion (as someone how has worked with the Bristlecone data hardly at
all!) there are undoubtedly problems in their use that go beyond the strip bark problem
(that I will come back to later).
The main one is an ambiguity in the nature and consistency of their sensitivity to
temperature variations. It was widely believed some 2-3 decades ago, that high-elevation
trees were PREDOMINANTLY responding to temperature and low elevation ones to available
water supply (not always related in a simple way to measured precipitation) . However,
response functions ( ie sets of regression coefficients on monthly mean temperature and
precipitation data derived using principal components regression applied to the
tree-ring data) have always shown quite weak and temporally unstable associations
between chronology and climate variations (for the high-elevations trees at least). The
trouble is that these results are dominated by inter-annual (ie high-frequency)
variations and apparent instability in the relationships is exacerbated by the shortness
of the instrumental records that restrict analyses to short periods, and the large
separation of the climate station records from the sites of the trees. Limited
comparisons between tree-ring density data (which seem to display less ambiguos
responses) imply that there is a reasonable decadal time scale association and so
indicate a real temperature signal , on this time scale .The bottom line though is that
these trees likely represent a mixed temperature and moisture-supply response that might
vary on longer timescales.
The discussion is further complicated by the fact that the first PC of "Western US"
trees used in the Mann et al. analyses is derived from a mixture of species (not just
Bristlecones ) and they are quite varied in their characteristics , time span, and
effective variance spectra . Many show low interannual variance and a long-term
declining trend , up until about 1850 , when the Bristlecones (and others) show the
remarkable increasing trend up until the end of the record. The earlier negative trend
could be (partly or more significantly) a consequence of the LACK of detrending to allow
for age effects in the measurements (ie standardisation) - the very early sections of
relative high growth were removed in their analysis, but no explicit standardistion of
the data was made to account for remaining slow width changes resulting from tree
aging. This is also related to the "strip bark" problem , as these types of trees will
have unpredictable trends as a consequence of aging and depending on the precise nature
of each tree's structure .
Another serious issue to be considered relates to the fact that the PC1 time series in
the Mann et al. analysis was adjusted to reduce the positive slope in the last 150
years (on the assumption - following an earlier paper by Lamarche et al. - that this
incressing growth was evidence of carbon dioxide fertilization) , by differencing the
data from another record produced by other workers in northern Alaska and Canada
(which incidentally was standardised in a totally different way). This last adjustment
obviously will have a large influence on the quantification of the link between these
Western US trees and N.Hemisphere temperatures. At this point , it is fair to say that
this adjustment was arbitrary and the link between Bristlecone pine growth and CO2 is ,
at the very least, arguable. Note that at least one author (Lisa Gaumlich) has stated
that the recent growth of these trees could be temperature driven and not evidence of
The point of this message is to show that that this issue is complex , and I still
believe the "Western US" series and its interpretation in terms of Hemispheric mean
temperature is perhaps a "Pandora's box" that we might open at our peril!
What does Jan say about this - he is very acquainted with these issues?
At 15:01 15/11/2006, Martin Juckes wrote:
Concerning Bristlecones, I had a sympathetic reply from Prof. North, but he
deferred to the person who wrote the relevant paragraph in the NAS report
(Franco Biondi) who is firmly of the view that strip-bark bristlecones should
not be used. I've read a few of the articles cited to back up this statement
and I am surprised by the extreme weakness of the evidence. There is one
study of 27 strip-bark pines which shows that they clearly developed
anomalous growth around 1850. Attributing this to CO2 is odd, to say the
least. I'm writing a brief review of the literature which I'll send round in
a few days time.
On Sunday 12 November 2006 22:21, Myles Allen wrote:
> Although it probably doesn't feel like it, it seems to me you're doing
> rather well...
> -----Original Message-----
> From: Martin Juckes [mailto:m.n.juckesatXYZxyzac.uk]
> Sent: 10 November 2006 15:24
> To: andersatXYZxyzu.su.se; Eduardo.ZoritaatXYZxyzs.de; hegerlatXYZxyze.edu;
> esperatXYZxyz.ch; k.briffaatXYZxyz.ac.uk; Myles Allen; weberatXYZxyzi.nl;
> Subject: Mitrie
> well, I've had a few exchanges on climateaudit, and decided to leave
> them to
> it for a few days.
> I'm going to send an email to Prof. North of the NAS panel to ask if he
> meant "don't use bristlecones", as he is quoted by McIntyre. I believe
> would be incorrect to select sites on the basis of what the data from
> sites looks like, and this makes up a substantial part of the argument
> Graybill and Idso (1993).
> Does anyone know where I can get hold of the categorisation of the Sheep
> Mountain trees used by Graybill and Idso (ca534.rwl from the WDC for
> paleoclimatology I think) into "strip-bark" and "full-bark"? I've sent
> email to the WDC query address.
> I've also sent of for a publication which is cited by co2science as
> Sargasso Sea data with the dating shifted by 50 years (Loehle, 2004,
> Ecological Modelling). This appears to be a source of considerable
> among the climate sceptics. The shifted series fits nicely with the idea
> the Medieval Warm Period was warmer than the 20th century, so there is a
> widespread perception that it is being ignored to fudge the results.
> Apart from a couple of oversights in the documentation of the data files
> McIntyre hasn't come up with much yet. I need to read up a bit more on
> different Tornetraesk/Fennoscandia series. There was an interesting
> discussion on "cherrypicking", with contributors suggesting that testing
> effect of removing each proxy series in turn was "cherrypicking" and
> selecting series based on subjective analysis of what the series look
> would be much better!
> I've had a comment from the editor saying that responses to non-refereee
> comments are optional, especially if the comments are not relevant to
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.
PD Dr. Jan Esper
Swiss Federal Research Institute WSL
Zuercherstrasse 111, 8903 Birmensdorf, Switzerland
Voice: +41-44-739 2510
Fax: +41-44-739 2515