result of thinning stratospheric ozone (Briffa et al. 2004), or the possibility that surface instrumental temperatures are affected by an upward bias (Hoyt 2006). Elevational treeline sites in Mongolia (D’Arrigo et al. 2001) and the European Alps (Büntgen et al. 2005) are not affected by “divergence.” This geographic separation was confirmed by Cook et al. (2004), who subdivided long tree ring records for the Northern Hemisphere into latitudinal bands and found not only that “divergence” is unique to areas north of 55°N but also that the difference between northern and southern sites found after about 1950 is unprecedented since at least A.D. 900.
An especially suitable strategy to minimize confounding effects is to sample sites along ecological gradients, such as elevation or latitude (Fritts and Swetnam 1989, Bugmann 1996). For example, Naurzbaev et al. (2004) selected sites along latitudinal (from 55 to 72°N) and elevational (from 1120 to 2350 meters above sea level) transects and used the parameters of the Regional Curve Standardization to infer climatic influences and past temperature variability. Other strategies are available to improve tree ring reconstructions of surface temperature. Some of these strategies involve using maximum temperature instead of mean temperature (Luckman and Wilson 2005), combining multiple tree ring parameters related to temperature (Helle and Schleser 2004), sampling species with opposing responses to temperature (Biondi et al. 1999), and applying mechanistic models to tree ring records (Anchukaitis et al. 2006).
The possibility that increasing tree ring widths in modern times might be driven by increasing atmospheric carbon dioxide (CO2) concentrations, rather than increasing temperatures, was first proposed by LaMarche et al. (1984) for bristlecone pines (Pinus longaeva) in the White Mountains of California. In old age these trees can assume a “strip-bark” form, characterized by a band of trunk that remains alive and continues to grow after the rest of the stem has died. Such trees are sensitive to higher atmospheric CO2 concentrations (Graybill and Idso 1993), possibly because of greater water-use efficiency (Knapp et al. 2001, Bunn et al. 2003) or different carbon partitioning among tree parts (Tang et al. 1999). Support for a direct CO2 influence on tree ring records extracted from “full-bark” trees is less conclusive. Increasing mean ring width was reported for Pinus cembra from the central Alps growing well below treeline (Nicolussi et al. 1995). Free-Air CO2 Enrichment (FACE) data for conifer plantations in the Duke Forest (Hamilton et al. 2002) and at the alpine treeline (Hättenschwiler et al. 2002) also showed increased tree growth after exposure to atmospheric CO2 concentrations about 50 percent greater than present. On the other hand, no convincing evidence for such effect was found in conifer tree ring records from the Sierra Nevada in California (Graumlich 1991) or the Rocky Mountains in Colorado (Kienast and