Briffa’s Ph.D. student, Thomas Melvin, discusses the important impact of “modern sample bias” on RCS chronologies, discussing Tornetrask and Finnish sites in detail.
His PhD thesis, “Historical Growth Rates and Changing Climatic Sensitivity of Boreal Conifers” (May 2004) is online here and contains many interesting discussions of tree ring standardization. Here are some key comments, particularly from chapter 8:
Problems of the RCS technique are identified which are associated with tree age and diameter-related bias, arising from the use of ring-width to establish tree growth rates, regardless of tree diameter. These problems are manifest as “end effects” in chronology development and are most significant in the most recent century… Variation in growth rates between different trees growing at any specific point in time appears to be the normal situation and it is this variation which leads to modern sample bias….
The potential for modern sample bias occurs because the fastest growing trees of the earliest centuries and the young slower growing trees of the most recent centuries are generally missing from dendroclimatic samples taken from living trees at one point in time…
If samples are restricted to trees whose radius is 10cm at breast height, the samples will be from trees of varying ages (Enquist 1999)…
Samples will be from trees above some minimum radius, small trees are not generally sampled because they may suffer excessive damage from coring, and short ring sequences may be difficult to crossdate.
The impact of this on Briffa’s key sites of Polar Urals and Tornetrask is obvious as soon as the matter is stated. If, as Melvin says, there is a distribution of growth rates including both slow-growing and fast-growing trees, the minimum diameter for sampling living trees filters out slow-growing 20th century trees. The impact can be illustrated below. Figure 1 shows, for the Polar Urals site, a plot of cumulative ring widths by tree. The minimum cumulative ring width in 1990 is a little under 5 cm (scale at left is x100). The existence of a minimum sampling diameter is neatly illustrated by the lack of any samples in 1990 with cumulative ring width under the minimum. In subfossil collections, this is not the case: showing the important lack of homogeneity in sampling method. The bottom panel shows the simple mean restricted to trees with cumulative diameter above the 1990 minimum. Does 1032 look like the “coldest year of the millennium”, as claimed by Briffa et al ?
FIGURE 1. POLAR URALS. Top: Cumulative ring width by tree, with red horizontal line showing minimum in sample year. Figure 2 shows the same thing for Tornestrask, with two different minimums – the higher being the minimum in the last year of sampling (1980) and the lower one being a lower value for sensitivity purposes.
FIGURE 2. TORNETRASK RING WIDTH. Top: Cumulative ring width; middle- chronology of mean ring widths for trees with diameter exceeding 1980 minimum.
Now consider the effect on Briffa’s arbitrary adjustment of the density chronology to the ring width chronology. Both chronologies are probably biased by this effect, but intuitively it seems to me that the bias is direct on the ring width chronology and probably much greater (since the age impact on MXD is less as well.) Thus an important contributor to the divergence of 20th century Tornetrask RW and MXD chronologies would be the impact of modern sampling bias – which would, in turn, not justify any coercion of the MXD chronology towards RW values.