A CA reader has provided a link to an extremely interesting presentation by dendro Brian Luckman of U of Western Ontario (Rob Wilson’s thesis supervisor) at the 2008 Canadian Society of Petroleum Geologists. Reader Erasmus de Frigid draws attention to the inhomogeneity in the tree ring record created when the tree was scarred by a glacier, evidenced by a terrifically interesting cross-section picture of the results of glacier scarring on ring widths. It sure looks to me like the net result of glacier scarring resulted in strip bark – something that’s obviously an important issue.
The graphic below (slide 3) is remarkable in several aspects. While Luckman doesn’t use the term “strip bark”, “strip bark” trees are scarred trees. “Classic” strip bark bristlecones in the dry White Mountains end up with only a “strip” of surviving bark and are predominantly “scar”, but some of the Almagre strip bark trees have what must be a similar appearance to this Engelman spruce – half or more of the trunk covered with bark and half scar.
Note, as reader Erasmus de F observed, the tremendous growth pulse in the surviving part of the trunk immediately following the glacier scar.
If you drilled a core in the center of the surviving “strip” bark in the scarred spruce, you would get a huge growth pulse in the late 19th century; if you drilled a core at the edge of the surviving strip bark, you would get correlated but narrow widths. This is exactly the situation that we hypothesized at Almagre strip bark (our Tree 31 discussed here.) Here’s a ring width plot from the prior post. The glacier-scarred tree would yield a graphic like this:
Black – drilled from S in center of strip; red – from SE from edge of strip
At the time, I did the following pseudo-contour to illustrate the effect – surely this is almost uncannily comparable to the Luckman cross-section.
There is a final item that links the Almagre strip bark to the Luckman graphic really nicely. Luckman dated the scar precisely dated to an advance of the Dome Glacier in 1846. Now look at the date where the growth pulse at Almagre started – the late 1840s! (A photo gallery of the Almagre trees are online at http://picasaweb.google.com/Almagre.Bristlecones.2007).
Later in his presentation (slide 13), Luckman provides a couple of equally interesting graphics showing the sorts of mechanical inhomogeneities that can occur in tree ring cores and how these extremely difficult statistical problems are “handled” by dendros. Luckman provides the following equation listing factors affecting growth. A_t denotes an age effect; C_t a climate effect.
The first delta-D effect is defined as “the occurrence of disturbance factors within the forest stand (for example, a blow down or tilting of trees)”. This sort of “disturbance factor” is illustrated in the following graphic. In his oral commentary, Luckman said that the dendrochronologist needs to use trees that are “normally” grown, that the tree illustrated below had been tilted several times and that you would “exclude that sort of record because there’s been some disturbance”/
The second delta-D effect is defined as a site-wide disturbance i.e. “the occurrence of disturbance factors from outside the forest stand (for example, fire or an insect outbreak that defoliates the trees, causing growth reduction)”. Luckman also mentioned logging in his oral commentary. An example is shown below. Luckman said orally that this sort of disturbance is “much more difficult to remove”.
Luckman defined the term E_t as “random (error) processes not accounted for by these other processes.” He then stated that, if you “take care” of these four factors (age, within-site disturbance, sitewide disturbance and random error), you then get a disturbance-free chronology that is related to common climate, using the diagram:
At slide 11, Luckman described standardization as a “black art”. Another description of the statistical procedures involved in cancelling out strip bark and similar effects is provided in the diagram below.