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 influence of summer temperature dominates growth. A mean April–September response is optimum for describing the major forcing signal over the whole densitometric 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 influences, though a negative response to winter precipitation in northern Siberia is consistent with the suggestion of an influence 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 temperature 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.