(2012 Update…) CA was the first blog to discuss the recent papers by Wilmking and his associates on “positive and negative responders” – the opposite response of trees to recent warming in which – at the same site – some trees responded positively and some negatively. This is an important observation in trying to provide an explanation for the “divergence” problem – ring widths going down while temperatures go up. Previous discussions included:Wilmking in Alaska and Positive and Negative Responders. Other related posts include Upside Down Quadratic, Twisted Tree Heartrot Hill , bender on Gaspé , Survivorship Bias .
Today I’m going to report on some hot-off-the-press results from Pisaric et al in the March 2007 GRL. (Pisaric is the dendroclimatologist who commented on White Spruce updates.)
Wilmking et al 2004 had reported:
Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate.
Wilmking et al 2005 extended these findings to many of the classic northern sites:
Here we present evidence that this phenomenon is not a regional abnormality, but is operating in several dominant tree species in forests across the circumpolar North (Figure 1).
They went on to say ( a finding that went unreported in the 2nd Draft of the IPCC 4AR):
Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate .”¬⤠Our findings suggest that the observed divergent response to climate at circumpolar treeline, overlapping the warming of recent decades, could be important for a significant proportion of the circumpolar forests and their dominant tree species.
Pisaric et al studied sites at the latitudal limit in the Mackenzie Delta, Canada. Whereas Wilmking et al had observed 40% negative responders in the Brooks Range at the Alaska latitudinal limit, Pisaric et al observed no less than 75% negative responders in their study.
Analysis of white spruce tree ring series in the Mackenzie Delta region of northwestern Canada indicates two distinct growth patterns during the 20th century. Approximately 25% of the trees exhibit significant and positive relations with growing season temperatures in the Mackenzie Delta region and with Northern Hemisphere summer temperature anomalies (Table 1 and Figure 2b). The NEG [negative] chronology, characterizing 75% of the trees, shows significant inverse relations with growing season temperature during both the current and previous years (Table 1).
They observed that, if a chronology were made from Positive responders only, they could obtain a chronology that matched NH temperature:
The low frequency trends in the POS tree ring record match the Northern Hemisphere temperature anomalies remarkably well, especially from the early 1900s onwards.
If you need to be able to segregate “positive” and “negative” responders in order to produce a chronology, I have no idea how the authors would propose that reconstructions in the medieval period be done. How can one tell by looking at a past core whether it was a positive or negative responder? Having said that, the identification of these traits in modern populations makes it incumbent on practitioners to re-examine data sets used in MWP reconstructions to see if there is evidence of positive and negative responders in those data sets. (Although we do already know that Briffa is a negative responder to requests that he archive the key data sets from Yamal, the Tornetrask update and Taimyr.)
Driscoll, W. W., G. C. Wiles, R. D. D’Arrigo, and M. Wilmking (2005), Divergent tree growth response to recent climatic warming, Lake Clark National Park and Preserve, Alaska, Geophys. Res. Lett., 32, L20703, doi:10.1029/2005GL024258. url
Pisaric,1 Michael F. J.,Sean K. Carey,1 Steven V. Kokelj,2 and Donald Youngblut1 , Anomalous 20th century tree growth, Mackenzie Delta, Northwest Territories, Canada GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L05714, doi:10.1029/2006GL029139, 2007
Wilmking, M., R. D’Arrigo, G. C. Jacoby, and G. P. Juday (2005), Increased temperature sensitivity and divergent growth trends in circumpolar boreal forests, Geophys. Res. Lett., 32, L15715, doi:10.1029/2005GL023331
Wilmking, M., and G. P. Juday (2005), Longitudinal variation of radial growth at Alaska’ northern treeline: Recent changes and possible scenarios for the 21st century, Global Planet. Change, doi:10.1016/j.gloplacha.2004.10.017,
Wilmking, M., G. P. Juday, V. A. Barber, and H. S. Zald (2004), Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through temperature thresholds, Global Change Biol., 10, 1724– 1736.