I’ve recently run across an article on changing water use efficiency in bristlecones, which pretty much put the nail in the coffin on any lingering ideas that 20th century bristlecone ring widths might be a temperature proxy. Tang et al. , "The dC13 of tree rings in full-bark and strip-bark bristlecone pines in the White Mountains of California", shows a remarkable non-climatic increase of dC13 ratios of bristlecones at Sheep Mountain, the most important site in the MBH98 PC1. The dC13 values are stated to show an equally remarkable nonclimatic increase in water use efficiency at Sheep Mountain. Improved water use efficiency was the predicted mechanism for CO2 fertilization. See their Figure 3 below. Any attempt to argue that bristlecones are a temperature proxy on scientific grounds (something that has been conspicuously absent from any response by realclimate or their associates) would need to adjust for non-climatic changes in dC13 ratios and water use efficiency.
Bristlecone pines (and Gaspé cedar) have obviously featured prominently in our critique of MBH98. In our EE article, we included an extensive literature review of issues affecting the validity of these ring widths as temperature proxies, but weere then unaware of Tang et al  (which I’ve posted up here). To my knowledge, neither Mann nor any realclimate associate has attempted to defend bristlecones as temperature "proxies" other than by arguing [Wahl and Ammann] that their use improves the reconstruction RE score ( a statistical argument leading directly into questions of spurious significance which they avoid.)
Where are the defences of bristlecones as valid proxies in tree ring terms? While we’ve pointed out a number of possible mechanisms for fertilization in the 20th century, the most prominent theory is CO2 fertilization [Graybill and Idso, 1993; Lamarche at al, 1984.] The usual mechanism by which CO2 fertilization is hypothesized to lead to increased ring widths is through increased water use efficiency. Water use efficiency changes are studied in a series of remarkable articles by Xiahung Feng of Dartmouth, of which Tang, Feng and Funkhouser  is notable. "Plant water use efficiency" was defined by Farquhar et al 1989 (cited by Tang et al) as the ratio of net carbon fixed to the total water cost. The connection between CO2 fertilization and increased water use efficiency (W) is summarized by Tang et al. as follows:
Experimental work has strongly demonstrated the positive response of photosynthesis and plant water use efficiency to increasing CO2 concentration [ e.g. Strain and Cure 1985; Bazzaz 1990; Mooney et al 1991; Idso 1992; Korner and Arnone 1992; Norby et al 1992; Polley et a 1993; Wullscheger et al 1995)] and the negative response of stomatal conductance of plant leaves [Woodward 1987; Beerling and Woodward 1993; van de water et al 1994]. For example, by studying a number of C3 and C4 species, Polley et al 1993 showed that both plant water use efficiency and biomass increased with increasing ambient CO2 concentration. This led to the idea that CO2 fertilization may be evaluated by measuring plant water use efficiency.
Tang et al. discuss findings of Graybill and Idso and proceed to discuss whether water-use efficiency of bristlecones has increased at this site; and the relationship of stem growth to strip bark forms. They cored trees in June 1994, 5 of which were the SAME trees as studied by Graybill and Idso 1993 and analyzed 4 trees. Tang et al. :
To separate the atmospheric CO2 signal [direct fertilization] from the climatic noise, we assume the variation of dC13 with CO2 concentration is long-term and of low frequency, while climatic and other environmental factors contribute only to high frequency fluctuations in a dC13 tree ring series. This assumption has been shown to be essentially valid for this site [Feng and Epstein 1995; Feng 1999]. The superposition of high-frequency with low-frequency variations can be easily identified from the dC13 time series in Figure 3. The long-term trends mimic the trend of dC13(atm) and form a mirror image compared with the CO2 concentration of the atmosphere. We consider that the trends of carbon isotopes of tree rings contains the signal of atmospheric conditions [CO2 concentrations] for the past 200 years. …
Our results compare well with previously published results. Using carbon isotope data of tree rings from the White Mountains published by Epstein and Krishnamurthy 1990 and Leavitt and Long , we have calculated water use efficiently for these trees and found that the water use efficiency shows the same trends as trees we present in their study. gh frequency variations of dC13 of tree rings for trees in an arid environment often correlate with he amount of annual precipitation..
Original Caption: Figure 3. Carbon isotopic compositions of tree rings for bristlecone pines from Sheep Mountain. The dC13 values were obtained from each tree for at most every 5th tree ring from 1796 to 1995. The smooth solid lines are modeled curves assuming exponential trends.
They removed the high-frequency variations and compared the low-frequency results in their Figure 5. Tang et al:
Figure 5 indicates that for all trees analyzed, water use efficiency increased with concentration of atmospheric CO2 increased. This may be related to the increase in the growth rate observed from tree ring widths by Graybill and Idso 1993 for bristlecone pine on Sheep Mountain and earlier by Lamarche et al 1983 in the White Mountains….
It is possible that the partitioning of biomass between roots and shoots was systematically different for the full-bark than for the strip-bark trees. For young full-bark trees to maintain foliage and reproductive growth, much assimilated carbon is allocated to the root system, Older strip bark trees may also be doing this to some degree but they use a substantial fraction of fixed carbon for cambial growth Graybill and Idso 1993. ..Using carbon isotope data of tree rings from the White Mountains area published by Epstein and Krishnamurthy 1990 and Leavitt and Long 1992, we have calculated water use efficiency for these trees and found the water use efficiency shows the same tends as trees we present in this study.
Original Caption: Figure 5. The relative rate of change in water-use efficiency for full-bark (solid) and strip-bark (dashed) bristlecone pine trees on Sheep Mountain. There is no significant difference in the relative rate of change in W between full-bark and strip-bark forms.
They also showed the climatic high-frequency variations after taking out the non-climatic increase in water use efficiency:
Original Caption: Figure 4. Correlation of high frequency variations among four bristlecone pines. The detrended value of dC13 represented the difference between the measured dC13 values of a given ring of a given tree and the value of the exponential trend at that year for that tree.
Funkhouser, a coauthor of Tang et al. , is an associate and sometime coauthor of Hughes, the H of MBH. The potential for CO2 fertilization to distort tree ring proxies was specifically noted as a caveat in the 1995 IPCC Second Assessment Report concerning tree ring proxies. MBH99 touched on CO2 fertilization, referring to CO2 fertilization as a potential problem, but did not discuss the dC13 evidence. Astonishingly (and only for pre-1400 results), they adjusted 19th century values of the NOAMER PC1 and bizarrely claimed that the CO2 fertilization effect had somehow reached a "saturation" point in the 20th century – a claim not supported by any evidence and obviously at odds with the explicit statements of Lamarche et al , Graybill and Idson  and the dC13 evidence of Tang et al  and predecessor studies.
Tang et al.  confirms that there has been an important non-climatic effect on bristlecone pines, creating a trend. Because of non-existent statistical control in MBH98-99 for spurious relationships, this non-climatic trend had a spurious relationship with the temperature PC1, which imprinted the NH temperature reconstruction as we’ve pointed out in our articles. Bristlecones do not just affect MBH; they also are integral to Crowley and Lowery  and Mann and Jones .
Epstein S. and Krishnamurthy R. V. (1990) Environmental information in the isotopic record in trees. Philos. Trans. R. Soc. London Ser. A 330, 427–439.
Xiahong Feng and Samuel Epstein, 1995, Carbon isotopes of trees from arid environments and implications for reconstructing atmospheric CO, concentration, Geochimica et Cosmochimica Acta, 59( 12),. 2599-2608.
Xiahong Feng, 1998. Long-term ci/ca response of trees in western North America to atmospheric CO2 concentration derived from carbon isotope chronologies, Oecologia (1998) 117:19±25 8b.
L Tang K., Feng X., and Funkhouser G. S. (1999) The d13C of tree rings in full-bark and strip-bark bristlecone pine trees in the White Mountains of California. Global Change Biology 5(1), 33–40.