Tree-ring width and density data around the Northern Hemisphere: Part 1, local and regional climate signals

Abstract

A detailed description is presented of the statistical patterns of climate forcing of tree growth (annual maximum latewood density and ring-width time series), across a network of 387 specially selected conifer sites that circle the extra-tropical Northern Hemisphere. The in‘ uence of summer temperature dominates growth. A mean April–September response is optimum for describing the major forcing signal over the whole densito-metric network, though a shorter June–July season is more relevant in central and eastern Siberia. The ring- width chronologies also have a shorter optimum (June–August) seasonal signal, but this is much weaker than the density signal. The association between tree-ring density and precipitation variability (as measured by partial correlations to account for the correlation between temperature and precipitation) is considerably weaker than with temperature. The ring-width response to precipitation is dominated by ‘noise’ and local site in‘ uences, though a negative response to winter precipitation in northern Siberia is consistent with the suggestion of an in‘ uence of delayed snowmelt. Average correlations with winter temperatures are small for all regions and correlations with annual temperatures are positive only because of the strong link with summer temperatures. Reconstructions of summer temperature based on composite regional density chronologies for nine areas are presented. Five regions (northwestern North America, NWNA; eastern and central Canada, ECCA; northern Europe, NEUR; northern Siberia, NSIB; and eastern Siberia, ESIB) constitute an arbitrary ‘northern’ division of the network, while the four other regions (western North America, WNA; southern Europe, SEUR; central Asia, CAS; and the Tibetan Plateau, TIBP) make up the ‘southern’ part. We also present two larger composite regional reconstructions comprising the data from the five higher-latitude (HILAT) and four lower-latitude (LOLAT) areas respectively; and a single series made up of data from all regions (ALL), which is highly correlated with Northern Hemisphere mean summer temperature. We calculate time-dependent uncertainty ranges for each of these reconstructions, though they are not intended to represent long timescales of tempera ture variability (>100 years) because the technique used to assemble the site chronologies precludes this. Finally, we examine in more detail the reduced sensitivity in the tree-growth data to decadal-timescale summer-temperature trends during the last 50 years, identified in earlier published work.

Extending North Atlantic oscillation reconstructions back to 1500

“…Prediction for each time step is obtained by ®tting a regression model with the predictor data available for that particular month (season). This allows time-dependent estimations of uncertainty that illustrate how addition or removal of speci®c predictors affects the strength of the calibration (Briffa et al, 2001;Jones et al, 2001a). The residuals of the regression do not show any signi®cant autocorrelation such that the uncertainty ranges were not further adjusted.…”

“…Uncertainty ranges for the predicted winter NAO index values were computed in terms of + 2 standard error (SE; for calculation see von Storch and Zwiers, 1999;Briffa et al, 2001) using statistics from the ®nal calibration period (1901±1995). SE quanti®es the uncertainty in the regression coef®cients and the residual variance that is not captured by the reconstructions.…”

Temperature reconstructions and comparisons with instrumental data from a tree-ring network for the European Alps

“…The exceptionally cold temperatures reconstructed in the early nineteenth century, are found in many regional to global scale temperature reconstructions (e.g. Briffa et al, 2002a). Cold temperatures, but of a lesser severity, are also recorded in the early instrumental data and reconstructed in the A-S density based reconstruction.…”

“…Prior tree-ring characterizations of temperature variability have relied upon a selection of maximum latewood density data from this region, which have been analyzed as part of larger European-wide or global networks (e.g. Briffa et al, 1988Briffa et al, , 1998Briffa et al, , 2002aBriffa et al, , 2002bSchweingruber et al, 1987;Schweingruber and Briffa, 1996), or more local to subregional reconstructions using ring-width data (SerreBachet et al, 1991;Nicolussi and Schiessling, 2001;Wilson and Topham, 2004;Büntgen et al, 2005). Herein, we consider large networks of both ring-width and density data from high-elevation sites across the central and western Alps to develop June-August (JJA) and April-September (A-S) mean temperature reconstructions.…”

A palaeotemperature record for the Finnish Lakeland based on microdensitometric variations in tree rings

“…In consequence of their cold habitat, the tree-ring width chronologies provide excellent proxy records of past temperature variability (Grudd et al, 2002;Helama et al, 2002Helama et al, , 2009aHelama et al, , 2009bHelama et al, , 2010aHelama et al, , 2010bHelama et al, , 2012bLinderholm and Gunnarson, 2005). In many regions, however, an improved dendroclimatic signal of summer temperature can be attained using X-ray based microdensitometric series of maximum latewood densities (MXD), rather than using annual widths of the rings (Briffa et al, , 2002a(Briffa et al, , 2002b. This dendroclimatic connection has been exploited also in the mountainous and timberline areas of Fennoscandia where the MXD chronologies have been exploited for summer temperature reconstructions to detail the past climate variability through the Common Era (Schweingruber et al, 1988;Briffa et al, 1988Briffa et al, , 1992Grudd 2008;Büntgen et al, 2011;Gunnarson et al, 2011;Esper et al, 2012).…”