Luxmoore 1988). Further evidence comes from a recent review of data for mature trees in four climatic zones, which concluded that pine growth at the treeline is limited by factors other than carbon (Körner 2003). While “strip-bark” samples should be avoided for temperature reconstructions, attention should also be paid to the confounding effects of anthropogenic nitrogen deposition (Vitousek et al. 1997), since the nutrient conditions of the soil determine wood growth response to increased atmospheric CO2 (Kostiainen et al. 2004). However, in forest areas below the treeline where modern nitrogen input could be expected to influence dendroclimatic records, such as Scotland (Hughes et al. 1984) and Maine (Conkey 1986), the relationship between temperature and tree ring parameters was stable over time.

In conclusion, tree ring science provides useful insights into past temperature variability. Promising areas of current and future research can be summarized as:

  • updating site chronologies that were collected 20–30 years ago,

  • increasing the number and geographic coverage of temperature-sensitive tree ring chronologies longer than 1,000 years,

  • quantifying the precision and accuracy of low-frequency temperature signals,

  • performing experimental studies on biophysical relationships between temperature and tree ring parameters, and

  • refining mechanistic models of temperature effects on tree ring parameters at multiple spatial and temporal scales.

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