This monograph provides a comprehensive synthesis of dendroclimatic research based on northern old growth sites in the forests of North America and Eurasia, as conducted by the scientists at the Lamont-Doherty Earth Observatory's Tree-Ring Laboratory (TRL-LDEO) of Columbia University over the past four decades ( Figure 1.1 ).
Figure 1.1 Locations of tree-ring chronologies sampled from northern forest sites over the past few decades by TRL-LDEO scientists and colleagues. Many more have been sampled by other scientists for these areas. For color detail, please see color plate section .
We focus herein mainly on the latitudinal northern treeline (see Glossary), as well as other locations further south (such as the elevational treeline in Mongolia, and the Russian Far East) where trees are often sensitive indicators of past temperature variability. In keeping with the requirements of the "Opportunities for Promoting Understanding through Synthesis" (OPUS) program of the National Science Foundation that funded the writing of this monograph, we emphasize the research conducted by scientists at the TRL-LDEO. However, we place these studies in the context of the broader field of Northern Hemispheric dendroclimatology (Glossary), conducted by numerous other researchers and colleagues who have provided valuable and important insights into this topic over the years.
1.2. Basic Tree-Ring Principles
The research described herein adheres to the basic principles of dendrochronology, as outlined in introductory and general texts by Stokes and Smiley (1968), Fritts (1976), Schweingruber (1988), Cook and Kairiukstis (1990), Speer (2012), and others. The main premise of dendrochronology is the establishment of precise, high-resolution (annually-resolved) tree-ring chronologies, derived using the method known as cross-dating (references above; Glossary). The cross-dating technique is based upon the observation that there is a common climatic and environmental signal in the ring-width variations of samples of wood compiled from trees (of the same species) from the same site and region. Relatively narrow (wide) rings are used to infer more adverse (favorable) environmental conditions for growth. When performed correctly, this method ensures that there are no dating errors resulting from anomalous growth patterns. These include, for example, false rings (Glossary); in which growth is slowed, resulting in thicker-walled cells, for a period during the growing season due to a particular adverse event, such as drought, or missing rings (in which radial growth is not laid down in a particular wood sample or tree due to an adverse event; Glossary). Although it has been suggested by Mann and colleagues (2012), based on tree-growth model simulations, that such missing rings can occur amongst all trees at a given site following major volcanic events (e.g., in 1258, 1815), there is no actual tree-ring evidence that this is in fact the case, as indicated in several subsequent presentations and publications (Anchukaitis et al., 2012a; Esper et al., 2013, in press; D'Arrigo et al., 2012a; D'Arrigo et al., 2001b; Journal of Geophysical Research, in press).
The science of dendroclimatology evolved from the need to understand past and present climate variability as well as the factors impacting tree growth and climate response on a range of spatial and temporal scales. Determination of how climate has varied in the past is also critically important for evaluating the sensitivity of the Earth's climate system to both natural and anthropogenic forcing. Yet, instrumental observations are limited in length and spatial coverage, particularly in many remote far northern regions, where station records may only span a few decades. Overcoming