A standard technique of dendroclimatologists is to calculate coefficients between ring width chronologies and monthly temperature and precipitation for 12-18 months relevant to the annual growth. twq has reiterated to us that Gou et al 2007, which is a few minutes off the press, has claimed high correlations to temperature of a site in the upper reaches of the Yellow River that is within several hundred miles of Dulan. Today I report on response functions reported by other authors in the area, which consistently report positive correlations with spring precipitation and negative correlations with summer temperature – what you’d expect from precipitation limited growth in arid and semi-arid regions like Dulan. I reiterate that I’m not trying to adjudicate between specialists here – I’m only observing that the results of Gou et al 2007 are inconsistent with authors reporting on Dulan junipers.
Zhang et al 2003
Here are the response coefficients for Zhang et al 2003. This is a very important study, since 55 of the cores were used in Kang et al 1997 (which is the source of the Dulan chronology used in Yang et al 2002 and the multiproxy studies.) Black- temperature; white – precipitation. The authors interpret these results as justifying a reconstruction of spring precipitation. Recently a study involving Cook of chronologies in the Tienshan use similar correlations to reconstruct the Palmer Drought Index, which is a positive function of precipitation and negative function of summer temperature – an approach applied by Cook in the United States. (Cook’s precipitation work appears to me to be much the best work of anything done by the Team.) There is a weak positive correlation to temperature in the preceding fall, so he ring width is obviously a complicated integral.
Original Caption: Figure 2. Response function coefficients showing the relationships between radial growth of Sabina przewalskii Kom. and monthly mean air temperature and total monthly precipitation for the regional climatic data set for the period 1953—2000. Lag 1 represents the tree-ring growth in the previous year. Black bars stand for temperature, white bars for precipitation, and x for significance at the 0.05 level as tested by bootstrap method.
Shao et al 2004
Here is a similar plot from Shao et al, also showing positive correlations.
Sheppard et al 2004
Here is the response coefficients from Sheppard et al 2004, again showing a similar pattern to Zhang et al 2003 and Shao et al. – positive correlations to spring precipitation, negative correlations to spring temperature.
Original Caption: Fig. 4 Monthly correlations using ring-width indices from the combined juniper chronology and meteorological data from Dulan (1955—1988) for precipitation (black bars) and temperature (gray bars). The critical correlation value for n=30 and alpha = 0.05 is ⯰.30 (Rohlf and Sokal 1981)
Gou et al 2005
Gou et al 2005 is an interesting article about 4 sites on the north slope of the Qilianshan, which contains many insightful comments about high-elevation trees and which I’ll try to come back to on another occasion. Their table of response coefficients shows a positive correlation to spring precipitation and negative correlation to summer temperatuyre.
Gou et al 2006
Here is a table showing response coefficients from Gou et al 2006, from 3 sites in the upper headwaters of the Yellow River, near the site of Gou et al 2007. Again we see the characteristic pattern of positive correlations to spring precipitation, negative correlations to spring temperature. Gou et al used the negative correlations to temperature to reconstruct temperature inverting the chronology.
Gou et al 2007
Now we get to twq’s favorite: Gou et al 2007. This is a study of a single site in the upper headwaters of the Yellow River. The site chronology is taken from the lower border as this is said to have a higher correlation to temperature (they say that these relationships are “complicated”) and report strong positive correlations of this site to local temperatures. Unfortunately they did not archive their measurement and the meteorological station data is either not archived or stale. For an article that is essentially a data paper, it’s too bad that they don’t actually archive any data.
Original Caption: Figure 3. Correlation coefficients between the Standard (STD), Residual (RES) and Arstan (ARS) chronologies index and the observed minimum temperatures recorded at Xinhai meteorological station (1960—2001). Ave. in the Figure 3 means average minimum temperature of previous October to current April. The dashed lines indicate the 95% and the 99% significance levels.
The consistent positive relationships between temperature and ring width chronologies are a very different pattern than we observed with the Dulan junipers. There are a couple of possibilities: this site is a magic thermometer; the correlation, while seemingly significant, is nonetheless “spurious” – a topic discussed on many occasions on this blog, the type example being the “99% significant” correlation between alcoholism and Church of England marriages reported by Yule in the 1920s. Or maybe there’s something about the location of the site that makes it a better but non-magic thermometer. Regardless, the positive correlations at site HBL don’t alter the results of Zhang et al 2003, which are the ones that apply to the south-facing Dulan junipers used in Yang et al 2002.
While the correlations at site HBL are intriguing, at cliamteaudit, we don’t accept the dendroclimatological procedure of simply cherry-picking one study. Any use or reliance on Gou et al 2007 needs to include a proper accounting and reconciliation of all the other studies in the area, something that Gou et al 2007 made no attempt to do.
Let me conclude with a quote from Gou et al 2005 – an interesting article as I noted above and one which, unlike Gou et al 2007, did not have any members of the Team (Jacoby, Cook). Gou et al 2005 concluded with the following observation:
trees growing at high elevation show a lower sensitivity to climate. This conclusion is of fundamental importance for tree-ring research in arid and semi-arid regions. It is important to understand the relationship between climate change and the growth of the trees in order to develop an appropriate ecological model of plant environmental reactions and to establish a valid basis on which to reconstruct long-term climate change over wide geographical areas…. Understanding the ecology model between trees and climate will allow us to examine differences in the long-term climatic response that may be related to changes in climate.
Everyone at climateaudit endorses the view that understanding the ecological relationship between trees and climate is a prerequisite for using ring width chronologies in temperature reconstructions. Maybe Gou could send a memo to the Team.