Just when you think that you’ve heard of all the possible problems with tree rings, the newest issue comes from “positive” and “negative” responders to temperature within the same site. These issues are discussed in a number of articles by various post-docs associated with Jacoby and D’Arrigo with the latter as co-authors. So in fairness to them, they are not just ignoring the problem of the failure of tree rings to respond to post-1980 temperature – although the issue is dealt with more candidly in specialist articles than in the general literature. Also in fairness to Jacoby who I’ve criticized for failing to archive information (and will continue to criticize), unlike Mann, he’s collected a lot of information. When I twitted him for supplying data to Mann for use in Mann and Jones  and then refusing to supply to me, he said that he didn’t supply it to Mann and that he had no idea how Mann got the data and seemed to have antipathy towards all data re-processors. However as I’ve said before, exploration geologists also go into remote spots of the world and it would never occur to them that this labor made them owners of the data. But it’s the compoanies that manage the geologists. I’ve got a bigger beef with NSF being co-opted by dilatory archivers than even the dilatory archivers. But on to positive and negative responders.
Driscoll et al  report of Alaskan sites:
During standardization, it became clear that the chronologies from Fish Trap Lake (FT) and Portage Lake (PO) contained two subpopulations within each site and hence they were separated into four chronologies, two of which responded positively to increased temperatures in the latter half of the 20th century and two of which appeared to experience an overall decline after the 1950s (Figure 2)….
Growth of the divergent subpopulations correlate well prior to 1950 and diverge thereafter, supporting the idea that contemporary warming has introduced unprecedented stresses on some trees and thus impeded growth. Furthermore, strong positive correlations with August precipitation among negative responders suggest that temperature induced drought stress may be relieved in these populations by higher levels of precipitation late in the growing season.  The interplay between climate variables (e.g. temperature, precipitation), highly localized non-climate variables (e.g. competition, depth to permafrost) and individual tree growth rates and allometry is not well understood in the context of recent warming at high latitude tree-line sites. It appears that unprecedented climatic stresses are triggering diverse growth responses between and within study sites that may greatly complicate dendroclimatic reconstructions of past climate conditions. Such intervening variables, if undetected, may seriously threaten the accuracy and validity of dendroclimatic reconstructions of past climate conditions.
Wilmking et al  summarize two earlier studies as follows:
In a recent study of treeline sites in northern Alaska, [Wilmking et al., 2004], where pronounced warming has taken place in recent decades [Hansen et al., 1999], we systematically sampled 1558 white spruce (Picea glauca) at thirteen sites in the Brooks Range and the Alaska Range. Opposing types of tree growth response (positive or negative) to temperature were demonstrated, with both types of trees occurring within a given sample site. In such cases, some individual tree-ring series contributed to a statistically significant relationship of the chronology with a particular predictive function of climate, while others degraded it. Even though these opposing growth responses were present in all sampled sites, their relative proportion varied between sites following patterns of regional and local moisture availability [Wilmking and Juday, 2005].
Then they proceed to generalize the phenomenon to many sites, including Tornetrask (TK) and Polar Urals (PU):
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).All trees with a significant positive correlation to site-specific mean monthly temperatures were grouped into “Å”Åresponder chronology A”. All tree ring series with a statistically significant negative correlation to a site-specific mean monthly temperature (mostly July; an indication of drought stress) were grouped into what we call “Å”Åresponder chronology B”. Trees could only be members of one responder chronology, and possible overlaps occurred only very infrequently. Trees with no significant correlation with the main climate indices were excluded from further analysis (for number of trees in each group; see Table 1).
As you see from Table 1, there are a lot of opposite responses in all the sites considered except Labrador (where there was a cooling trend):
They go on to conclude:
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.
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
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’s 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.