I’ve done an interesting exercise today in which I’ve tried to do an estimated cross-section of Tree 31 – the one with the highly discrepant core widths. Pete has some new pictures and site information that I haven’t seen, so it will be interesting to see how they reconcile.
As noted before, Almagre Tree 31 has a large difference in growth rates between two cores only 45 degrees apart -see here . Here’s the measurement plot, showing a dramatic difference between a fast-growing radius and a slow-growing radius.
I’ve done a rough cross-section of Tree 31 to translate the information from the core into a cross-section. Mining engineers and geologists always convert drill core information into cross-sections and level plans. I realize that professionals in the dendroclimatology don’t need to do this – of course, traditionally they didn’t make location maps either, waiting apparently for the invention of GPS rather thanlow-brow methods like plotting sample locations on Forest Service topo maps, as a geologist might do. So at the risk of offending a dendroclimatologist by doing something as amateur as estimating a cross-section from available core, I’ve done the exercise anyway, articulating the assumptions as I go.
Up to the first half of the 19th century, the two cores were roughly in tandem. So I’ve hypothesized that the tree was roughly circular in 1835 with a radius of about 13 cm. Between 1875 and 2007, the slow-growing radius (31B – SW) has grown about 2.9 cm (the strip bark portion obviously growing at a rate of 0), while the fast-growing radius (31A – W) has grown about 10.0 cm. We only have two cores and, for the sake of argument, I’ve interpreted the fast-growing core as being more or less in the center of the bark strip. As an experiment, here’s a contour map showing a contour map of ring widths – you can see the consistent low growth leadgin up to 1875, with the lensy growth in the strip bark in the late 19th and 20th century. (This diagram illustrates some features, but the actual tree cross section is more rectangular – Pete’s sent me a sketch. It will take a little time to re-do this diagram to incorporate a more accurate cross-section and I’ll update this when I get a chance.)
So the tree is becoming more elliptical. In old strip bark trees, there’s plenty of evidence of the trees projecting along the axis of the ellipse so that they become increasingly elliptical and ultimately almost rectangular. That part’s been noticed. But I haven’t seen any specific discussion in the literature of the relative thickening in what seem to be the “early” strip bark stages. In this case, it’s almost like the strip bark tree is “trying” to achieve a circular arc, intersecting at the strip bark boundaries. That is the principle of this diagram – I hope that we’ve got enough information to reconstruct a cross-section as it doesn’t look like another examination will be possible before spring.
In Craig Brunstein’s interesting technical report on bristlecone morphology (online – see his Figure 4 for a wide variety of growth forms), his Figure 6 shown below illustrates a somewhat similar process of ellipticization for strip-barked limbs. This isn’t a trunk, but it shows rather nicely an ellipticization process.
Again, the distinction that I’m making here is this: the strip bark growth is not radially uniform, but appears to be much thicker in the center of the strip bark – at least during the first couple of centuries. In the case of Tree 31, it appears that the strip bark formed around 1835 plus-minus 15 years. So how does one obtain a homogeneous measurement for this sort of tree? If you have massive radial asymmetry as here, a simple average may not be very accurate. In addition, in examining the information from strip bark sites, it is remarkable how few trees have more than one core. My impression is that the available cores seem to come from the center of the strips and thus there is potentially a powerful bias here, about which I’ll have more to say. If bark stripping occurred randomly over the history of the site, then the problem might not be too severe; however, as I’ll discuss in my next post on this topic, we are getting the impression that strip bark occurrence is not random and intimately related to 19th century events.