New poster bender has written some very thoughtful posts, including some comments on Gaspé which I I’d like to recover from deep in a political thread. The growth pulse in Gaspé cedars seemed very improbable to me as a temperature proxy; bender has some detailed knowledge on the topic.
Bender Post #1:
Re: #233 Have you yet seen the threads discussing the relationship of tree ring width to temperature ? As in, it is not a linear relationship, but an inverse quadratic; i.e., in really hot years, trees don’t grow well, so hot years do not show up in the tree ring record. Any thoughts on this?
The oft-cited Ste-Anne Gaspé “northern treeline” Thuja occidentalis tree-ring chronology used by MHB98, which is hockey-stick shaped, is only one of three for this species from eastern Canada. The other two (S. Ontario, AD 594-1990 and Duparquet, Quebec AD 1186-1987), not used by MBH98, indicate exactly what you describe: a *negative* response to warm temperatures during the 20th century (Kelley et al. CJFR 24: 1049, Archambault & Bergeron CJFR 22: 674). The reason these chronologies were ingnored by MBH98 is probably that they were not “20th century treeline”, and they exhibit significant precipitation sensitivity. (But they also are not hockey-stick shaped either.)
But because the position of the treeline varies over time, so the temperature response function coefficients of a given tree in a fixed location should vary accordingly. This point has been made by: Cook, E.R., Esper, J. and D’Arrigo, R.D. 2004. Extra-tropical Northern Hemisphere land temperature variability over the past 1000 years Quaternary Science Reviews 23:2063-2074. who describe a “loss of climate sensitivity in the northern boreal zone” in the 20th century.
And the reason this is relevant because it is exactly the point picked up on by Wegman et al. p. 28:
In MBH98/99 the authors make a simple seemingly innocuous and somewhat obscure calibration assumption. Because the instrumental temperature records are only available for a limited window, they use instrumental temperature data from 1902-1995 to calibrate the proxy data set. This would seem reasonable except for the fact that temperatures were rising during this period.
Although Wegman et al. don’t say as much, a change in a tree’s temperature sensitivity coefficient over time will bias a tree-ring based temperature reconstruction. I realize the above question was probably rhetorical. I just thought I would supply a concrete and relevant example of how such a quadratic response to temperature is not only real, but could lead to time-series nonstationarity that violates assumptions made in the inferential leap one takes in going from calibration to reconstruction.
Bender Post #2:
Re #272 Jacoby’s lost Gaspé cedars (posting from 25 April 2005 in a different thread) from Riviere-Ste-Anne, Quebec would most likely be located in the high-elevation Parc Quebecois de la Gaspésie, south of Ste-Anne-des-Monts – in which case these trees were probably never cut down. (They’re likely protected, and of no commercial value.) Granted, this is not a GPS pin-pointing, so you could not revisit the exact stands sampled in 1983. But you wouldn’t need to. All the cedars in that whole area should be responding similarly to temperature, if that is what they are responding to.
But one has to wonder about the possible release of these Gaspé cedars under heavy defoliation by spruce budworms in the 1950s and 1980s. One look at that chronology will confirm sharp spikes in growth during those two well-documented outbreaks of unprecedented severity – spikes which account for much of the hockey-stick shape of that chronology. (Maybe that’s what the sampler was doing there in 1983 – checking out the incredible damage during the peak of that outbreak?) Unfortunately the ITRDB stand metadata are insufficient to determine if the sampled trees were growing in mixed stands that could have been subject to release by insect disturbance. Possibly a moot point if it was a barren “treeline” site. Light is not usually limiting in those circumstances, but – come to think of it – these growth pulses could be a result of increased nitrogen (which is severely limiting in the boreal alpine environment) from insect frass. That might explain why the other cedar chronologies (from low-elevation cliffsides in Ontario and rocky lakeshores in northwestern Quebec; see #272) do not show the hockey-stick shape: no budworm, no frass, no nitrogen, no 20th century release.
All this to say: MBH98 is based on at least one tree-ring chronology whose local ecology and dynamics were never formally studied. Archiving data for future use is one thing. Understanding it is quite another. Does it make sense to publish based on archived data which have never actually been analyzed?
Short answer: Weather influences insect population dynamics significantly, but the degree and form of the effect varies from system to system. Statistical evidence is hard to come by, but that’s not surprising given the way these systems work. There is no “one size fits all” answer.
Longer: People have tried to make the link between all kinds of weather indices and insect outbreaks and they usually don’t pan out. It is thought that this is because of the action of other variables in the system (the forest, insect predators, etc.) – that weather is far from the only driver, such that the correlations are typically weak. That being said, there are many systems, and situations, where drought can act as a trigger to insect outbreak. The idea is that (1) insects like the warm weather, and (2) they do well on the weakened, drought-stressed trees. The spruce bark beetles currently in Alaska are a great example. All insects are cold-blooded and in temperate areas are limited by temperature, so as we move out of the Holocene most forest-dwelling species are expanding their range with the retreat of the glaciers.
Dendrochronologists like Tom Swetnam know all about this, and so people trained in dendrochronology at LTRR and elsewhere generally have a good appreciation for the potential importance of insects in affecting ring width, density, and character. (In fact his 1993 published work on western spruce budworms tried to make the link between budworm outbreaks and wet summers.) But I would not say that the appreciation for millenial-scale variability in outbreak dynamics is very good. Insects are viewed as a random noisy nuisance which they are happy to be rid of (usually by cherry-picking hypothetically insect-free sites). One problem is that the signal associated with some insect species, often periodic, has a way of fading in and out in time and space.
The argument against insects as a key driver of ring characteristics goes like this: we’ve never seen any on this tree species in this particular site type in the 20th century, so therefore they were never here, and therefore weather is the primary driver. The problem is that we now know that climates vary naturally. We know that insects move in when a region warms up (spruce beetles in Alaska, mountain pine beetle in British Columbia, hemlock wooly adelgid in the NE US). So in theory it is possible that these insects, though absent now from the system, were at one time prevalent.
A dendroclimatologist will argue that the proof is in the pudding: weather variable correlates with ring width/density, therefore it causes it. The problem is, as you know, that these correlations are never all that strong, and they even go through phases of strengthening and weakening. Maybe the weakening is the result of some other limiting factor kicking into play? Who knows what. But where there’s uncertainty, there’s doubt.
I can’t comment on the specific case of BP as it is a system I don’t know anything about. But I was interested to learn about 10 year cyclic outbreaks on cedar leaf miners on northern white cedar in eastern Canada (Jacoby’s lost Gaspé cedars). The outbreaks can be quite intense and are triggered by drought – such as the most recent outbreak from 2000-02 in Ontario. Dead cedars in Guelph not 10km from where Kelly et al did their cliffside cedar studies. This insect has never been seen at high elevations in Gaspé in the 20th century. But that doesn’t mean it wasn’t there during the MWP.
I’m not suggesting insects everywhere are the major driver and that dendroclimatologists are in gross error. I’m suggesting that they shouldn’t be so quick to dismiss insects, as there is a chance they could be compromising reconstruction quality during some parts of the tree ring record.
Given the importance of BP to MBH98 it might be interesting to know something about the insects of BP. As I say, I don’t know thing myself. High elevation sites are often too cold for defoliating insects. On the other hand, climates do vary
(On the uptick of BP growth in the 20th century: has a nitrogen pulse been excluded yet? That it is only in the NE US BP is interesting. Looks like there are actually two upticks – 1950, 1980 – coinciding with eastern spruce budworm outbreaks.)
This is one of my areas of research (but NOT BP), and so I can say firsthand that the state of knowledge evolves continually.
Long answer to a simple question. Wish that nature were simpler.