Not in so many words, of course. However, Briffa et al 2013 took a position on the use of radially deformed tree ring cores that would prohibit the use of strip bark bristlecones in temperature reconstructions, thereby emasculating Mann’s reconstructions. And not just the Mann reconstructions, but the majority of the IPCC reconstructions used by Briffa in AR4.

I’ll report on this issue in today’s post. I’ve been looking closely at Briffa et al 2013 over the past 10 days and unsurprisingly there is issue after issue. According to CRU, they’ve been working on this article for over seven years and, needless to say, it is impossible to fully observe the pea in only a few days, especially when the adjustments have become so baroque that the chronology style is most aptly described as East Anglia Rococo, making the weary reader long for the classic simplicity of earlier CRU illusions like the Briffa Bodge and Hide the Decline. But more on this on another occasion.



Briffa et al 2013 on Radial Deformation



Even though Briffa was the AR4 Lead Author responsible for assessing recent reconstructions, Briffa has more or less steered clear of bristlecones and radial deformation. However, one of the objectives of Briffa et al 2013 was to try to dismiss or discount the high medieval values of the Polar Urals series (see tag “urals”) when the additional data of the Polar Urals “Update” was included. Previously Briffa had just ignored the inconsistency between Polar Urals and Yamal, pretending that the issue had never occurred to them. This obtuseness has long been criticized at CA, where reconciliation of such inconsistencies has been recommended as one of the highest priorities for specialists.

B13 attributed the high medieval values (in part) to a bias arising from the inclusion of “root collar” samples:

The reanalysis of these data here show that the apparent evidence for high tree growth in medieval times in the Polar Urals region (circa 980-1040) was exaggerated due to inclusion of root-collar wood samples…We have shown here that that version of the Polar Urals TRW chronology (also combining larch and spruce data) is biased by the inclusion of multiple root-collar-derived sample data.

Briffa concluded that it was simply not “appropriate” to use radially dilated samples:

As these root collar samples appear highly variable in terms of cross-sectional dimensions, rather than being generally symmetric, it is not appropriate simply to process them with a separate (root-collar) RCS curve and it was considered necessary to remove these samples from the Polar Urals TRW RCS chronology despite the already low chronology replication (see SM4 description and Fig. PC02).

Passive voice exclusions and adjustments abound in B13 – a point that I will return to in another post. As an aside, the passive voice used to disappear the data reminds me of a famous phrase from the Vietnam War “It became necessary to destroy the town to save it” – a slogan that seems all too applicable to CRU’s approach to dendroclimatology.

Briffa’s interest in root collars arose from their examination of previously unreported metadata which showed that numerous medieval samples in the Polar Urals Update were taken from “root collars” rather than the more usual stems (because of more advanced rotting in the stems as opposed to the root collars). See in particular PU06 in SM4, which reproduces a letter to Schweingruber (presumably from Shiyatov). B13 summarized this as follows:

Closer examination of the details of the sample material reveals that the Polurula samples comprise a relatively large proportion of root-collar wood. The root-collar (or root crown) refers to the lower section of the tree bole (stem), generally near the soil surface, where the bole meets the upper parts of individual roots ..

Briffa observed that there were two related problems with samples taken from root collars: (1) increased width relative to samples taken higher up on the stem; (2) greater variability around the circumference, depending on the location of the major roots:

[root collars are] frequently associated with an expansion in the stem diameter at this point. It would be expected that ring width dimensions in such root-collar samples would be systematically larger than equivalent rings measured higher in the boles of the same trees. It is also the case that average ring dimensions in the root-collar vary greatly when measured at different positions around the circumference, according to the positions of the major roots. (91/9)

In section 6.1, Briffa re-iterated the issue in similar terms:

The root-collar samples have more variable (and generally larger) ring dimensions than regular stem samples, here sampled at varying levels above ground depending on the height of remnant tree boles.

And again in Supplementary Material 3, Briffa emphasized the variability around the circumference at the root collar:

The rings in the trunk of Siberian larch are generally concentric whereas the rings in the root-collar samples can vary considerably, being larger in the direction of a major root and smaller in directions between roots (Figure PU07). The allocation of material to individual roots will partly depend on mechanical strength requirements within the tree and growth can favour one root (direction) over several decades. Hence the dimensions of TRW measurements for one year taken in different directions from root-collar samples can be very variable…

These concerns seem entirely reasonable to me. Inhomogeneity between sample populations are a very serious problem in Briffa’s RCS methodology. B13 asserts that need to test subpopulations for homogeneity, but are completely and irredeemably at sea in actually carrying out statistical tests for homogeneity – procedures on which many statistical specialists could have assisted. Nonetheless, their concern over inhomogeneity between root collar and stem samples seems valid to me based on my own first examination of the data. However, they are obtusely blind to other inhomogeneities of equal or larger magnitude. One wonders whether inhomogeneities between sites are in fact so severe as to swamp whatever Briffa is trying to do. An issue for another day.



Figure PU07

B13 illustrated the supposed severity of radial asymmetry at Polar Urals in their Figure PU07 (Supp Mat 4) commenting as follows in its caption: “As sampling approaches the root the variation of ring-width measured along radii in different directions increases considerably.”



Original Caption: Figure PU07 Wood sample from the base of the stem of larch (Larix sibirica Ledeb.) from the Polar Urals. 22-43C 66°49.07’N 65°33.94′ E, 269 m.a.s.l., 1170-1412 CE. As sampling approaches the root the variation of ring-width measured along radii in different directions increases considerably. {Note the sample ID (22-43C) does not match any archived ID either in number or format.)

Strip Bark and Radial Deformation

By bristlecone standards, the radial deformation evidenced in figure PU07 is very slight indeed. Indeed, it is doubtful whether a single bristlecone chronology can survive the implicit standards of Briffa et al 2013.

The diagrams below show two examples (pages 32, 33) from Brunston of the extraordinary deformation in bristlecones – not even remotely “concentric”.





The effect of scarring as a proximate cause of radial deformation was neatly illustrated in a graphic below (ironically from the thread on which the first notice of Climategate occurred). The cross section is from an Engelmann spruce which was scarred (see bottom of graphic) by a glacier. In areas adjacent to the scar (dated to 1846), there was a huge growth pulse (see bottom right part of tree). Variability around the circumference of this tree is far more extreme than that shown in Briffa’s PU07. In addition, the growth enhancement is much larger than in PU07.



Figure 2. Deformation in engelmann spruce scarred by glacier in 1846. From presentation by Brian Luckman. See CA post ^.

At Climate Audit, strip bark has been a longstanding issue, but, since our work at Almagre, on the basis of mechanical deformation rather than the carbon dioxide fertilization hypothesized by Graybill, Lamarche and Idso (which may exist but as a much lesser effect.)

At Almagre, Pete Holzmann observed six-sigma differences in ring widths in cores drilled only a few inches apart on the same tree. See, for example, here and various presentations. The difference between adjacent cores was illustrated through the following diagram of Pete’s cores from Graybill Almagre tree 56:



Figure 1. Ring widths in two 2007 cores in Almagre tree 31 (Graybill 56). The difference in widths is due to radial deformation.

Pete’s theory – one that I endorse – was that the strip bark event caused or corresponded to mechanical deformation in the surviving part of the tree that was evident in the subsequent growth pulse. Curiously, in the Climategate emails, MBH coauthor Hughes privately expressed a similar thought to Briffa (though not in any public statements):

A further problem arises from the observation that radial increment may increase rather dramatically in the period after most of the bark dies back, but of course we don’t know when that was.

At Almagre, Pete observed that strip bark formation could be connected to bark being torn off by falling limbs due to snow weight, a hypothesis that might connect the notoriously severe winters in the US west in the 1840s to the apparent bristlecone growth pulse commencing in the 1850s.

As long ago as 2006, the NAS panel hadrecommended that strip bark data be “avoided” in temperature reconstructions, but this recommendation was flouted by Briffa in IPCC AR4 and subsequently by Mann (Mann et al 2008) and other specialists.

Commenter Salamano at realclimate, in one of the earliest comments, asked whether the strip bark problem was comparable to the root collar problem, but unfortunately did not receive a reply that was responsive to the important issue.

Nonetheless, it is an important issue.

I very much welcome the strong position taken by Briffa and coauthors against the use of radially deformed tree ring data. I look forward to the prompt application of these standards by Mann and others to strip bark chronologies. Given realclimate’s endorsement of B13, I presume that realclimate will urge that all reconstructions relying on bristlecones be recalled pending assessment of radial deformation and inhomogeneity according to B13 standards.



