Update: The rwl data at WDCP is a mixture of measurement formats with one set of measurements in units 1/10 of the other set. This accounts for the bimodal distribution. These graphs need to be redone. End Update 2006] .
Thinking about some of these tree ring sites reminds me of some stuff I did on Cook’s Tasmania while I was learning the tree ring data, while I was first looking at Mann. Cook is on the Hockey Team, on the same line as Jacoby. Cook’s Tasmania reconstruction is a staple of nearly every multiproxy study: Mann, Jones, Crowley, etc. Here are some graphics that I did a couple of years ago, which I’ve never quite figured out what to do with. Here’s a histogram of all ring widths at Lake Johnston (there are over 170,000 measurements in this figure). A simple histogram is usually the first thing that a statistician would do, but I’ve never seen a simple histogram like this in any tree ring study. Yet it says volumes.
Figure 1. Histogram of All Ring Widths at Lake Johnston, Tasmania
Most histograms that I’ve drawn have a unimodal distribution – I think that they look like a type of gamma-distribution. But this one — a very long series back to –2136 — has a unmistakable bimodal distribution. It was a fluke that I noticed it, since it’s a very small bump in the default histogram plot, but it obviously sharpens up when by specifying the breaks as I’ve done here. It looks like a mixture of two gamma distributions, since the "main" mode is also more like a gamma distribution than a normal distribution. (I have no idea why tree rings chronologies assume normal distributions when they never are.)
The left mode is not an artifact since the dataset has about 170,000 individual measurements and the left mode has about 30,000 measurements by itself — so it’s statistically there. Another plot that I do on these sites( which again I’ve never seen elsewhere) is simply plotting the cumulative ring width "rooting" each tree in its assigned start date. Here’s what this looks like for the Tasmania site: you certainly don’t get the sense that the modern trees are exceptional. There are a couple of super-giant trees in the Greco-Roman period.
Figure 2. Lake Johnston, Tasmania – "Rooted" cumulative ring width by tree.
The scale of this is squashed by accommodating the super-giant trees, so I zoomed in a bit, with very interesting results as shown below. You can see that the slow-growing trees are concentrated in the millennium just ended.
Figure 3. Lake Johnston, Tasmania – "Rooted" cumulative ring width by tree – zoom.
I illustrated this by plotting first the distribution of "narrow-ring" trees and then the distribution of "wide-ring" trees as below. As you can see, the narrow-ring trees are "modern" phenomenon. They seem to start about half way through the first millennium. Interestingly, the earliest layers in the Quelccaya glacier (the recession of which is comlained about a lot) are dated as starting about AD450 – around the same time as the commencement of the "narrow ring" trees here. At the time I wondered if there was some connection.
Figure 4. Lake Johnston, Tasmania -Distribution of "narrow-ring" ring widths
Figure 5 . Lake Johnston, Tasmania – Distribution of "narrow-ring" ring widths
There were some observations in the article articles about this stand of huon pine being rather a surprise, since it was found quite a bit above the elevation where huon pine presently grows [I’ll add in the exact quote later].
Since this was a Hockey Team study, they concluded that the 20th century was unique in some or another [again I’ll add in the quote]. [I’ll add in Cook’s temperature reconstruction so re-vist this if you’re interested].