Jacoby: Northern Sites, Southern Exposure

I was browsing through some Jacoby articles for a different purpose and was reminded of the very interesting references to a relationship between high-latitude tree ring growth and southern exposure to the sun in two of his 1980s articles, which do not get mentioned any more once the GW campaign is on.

Jacoby et al. [1988] discusses a site chronology at Cri Lake, Quebec on the eastern shore of Hudson Bay, which has no 20th century growth trend, concluding that the difference from other northern sites with a 20th century growth trend pertained to its not having a southern exposure:

The reconstruction of summer temperatures for this site contains the expected high-frequency year-to-year changes and also two- significant low-frequency features — an abrupt cooling beginning in 1816 and an absence of trend since about 1880. Many of the high latitude tree-ring chronologies display a long-term warming trend over the past 150 years (Jacoby et al, 1985 and Jacoby and D’Arrigo, 1987.) The chronologies which show a recent long-term warming are obtained almost invariably from sites obtained on flat terrain or southerly facing slopes. We attribute the absence of this trend at Kuujjuarapik to the geomorphology of the tree sampling site. The fact that Cri Lake is a narrow, steep-sided ravine, blocked from low-angle spring and fall insolation seems to have an important impact on the way trees respond to climate. This observation is in agreement with information that we have from white spruce trees sampled at two other sites situated in the same general areas. Similar to Cri Lake, the chronology at Richmond Gulf, another somewhat sheltered site doesn’t show any long-term growth trend. In contrast, the chronology from Castle Peninsula, a south-facing slope presents the strong increased-growth (inferred warming) tend over the past 150 years characteristic of most good high-latitude North American chronologies. We apply the term good to chronologies with strong common variance between trees at both high and low frequencies. The recorded NH air temperatures (Jones and Kelly, 1983) also indicate warming in the average annual temperature for this period, but little trend is present in the summer air temperature series (Wigley et al 1985). The trees at the sampling site apparently respond primarily to summer temperatures. Thus the non-summer warming is not integrated into growth at this site. [Jacoby et al, PPP 1988]

This point was mentioned for what seems to be the only other occasion in Jacoby and D’arrigo [1989] as follows:

Also among the sites that we have collected in the boreal forests, only trees growing on sites with good exposure to the south and hence low-angle sun have strong common low-frequency variance…The eleven chronologies were selected as those with the bet common variance in the red-noise analyses. All sites had good common variance in the white noise analyses. Of the 36 northern boreal forest tree-ring sites we have sampled within the past decade, ten are thus judged to provide the best record in time and space of temperature-influenced tree growth for this region of North America. The one chronology from a lower latitude on the Gaspé peninsula had similarly good common low-frequency variance and is included due to the scarcity of other data in the eastern region.

The relationship between southern exposure for northern spruce sites and a 20th century growth trend sure seems like an interesting relationship. If the relationship was simply between growth and temperature, then you would think that the shadier sites with northern exposure would also show increased growth. In fact, arguably, the relationship would be stronger for them. If the effect is limited to sites with southern exposure, perhaps something else is involved: perhaps there is some relationship between cloudiness and enhanced 20th century growth. Intuitively, I could see how something like this could possibly explain the different responses between sites with and without southern exposure. Whatever the ultimate explanation, the question surely shouldn’t have been dropped.

One of the difficulties in pursuing this is Jacoby’s practice of selective archiving discussed previously here and here. At the NGDC archive , Jacoby archived the results for Castle Peninsula (which has increased 20th century growth), but not for Cri Lake or Richmond Gulf, which don’t. (You can easily confirm this by going to the search function, selecting tree ring, the Canada-Quebec and setting 55N as the southern limit.

Jacoby, G.C., Ileana S. Ivanciu and Linda D. Ulan, A 263-year record of summer temperature for nother n Quebec reconstructed from tree-ring data and evidence of a major climate shift in the early 1800s, PPPP 64, 69-78.
Jacoby, Gordon C. and Rosanne D’Arriog, Reconstructed northern hemisphere annual temperature since 1671 based on high-latitude tree-ring data from North America, Clim Chg 14, 39-59.


  1. Peter Hartley
    Posted Jun 18, 2005 at 11:55 AM | Permalink

    Is it possible that the connection between souhern exposure and growth might be related to the aerial fertilizer effect? I believe that the FACE experiments have shown that one effect of higher CO2 concentration is that it enables plants to take better advantage of low light conditions. Perhaps trees with southern exposure can take better advantage of spring and fall low light conditions with more CO2 in the air? For trees without southern exposure, the additional CO2 cannot work its magic because there is simply not enough light.

  2. TCO
    Posted Sep 19, 2005 at 9:57 PM | Permalink

    Very interesting and shows that you can get different effects by tree selection when sampling. Goes to the point both that (you need to know what you are doing to get decent surveys) or alternately that you can skew the results by cherrypicking. I don’t like the cherrypicking. Even if you have some argument for counfounding factors, better to do a multiple regression or a correction, than to change the sampling pattern.

    Also, I can see how trees that vary together (“good” chronologies) might reduce noise. not sure that they would be more relevant on mean in terms of a temp proxy though.

  3. Steve McIntyre
    Posted Sep 19, 2005 at 10:34 PM | Permalink

    But isn’t it worrying to you that he picked southern exposure sites? A southern site would be affected jointly by the amount of sun, cloudiness, …If you’re trying to measure temperature effect, you’d think that northern exposure sites should show the effect as well. So if you’re getting a systemic difference between northern and southern sits, maybe the effect is not due to temperature, but due to something else. Ergo, Jacoby should archivve everything. He’s archived 1 out of 96 sites that I’ve seen references to him sampling in North America in the 1990s.

  4. TCO
    Posted Sep 19, 2005 at 11:30 PM | Permalink

    Of course I think he should archive everything (have I ever said otherwise)? And I’ve already said that one should use all the data: just do a multiple regression (include exposure as a variable). I’m sure that his excuse for using southern exposure is less “tree to tree noise”. That has some rationality to it, but it still worries me as possibly producing bias.

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