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We homogenized a tree-ring width network comprising 2337 single-species records from the ITRDB and 373 records from (18). Quality criteria for the retained chronologies included a minimum sample replication of five trees, significant (P < 0.05) mean correlations between the constituting tree-level measurements, and full coverage of the 1930–1990 CE period. The network includes 213 species (78.5% gymnosperms and 21.5% angiosperms), 50% of which belong to the genera Pinus, Picea, and Quercus. The age/size-related trends in the raw data were removed from all series using cubic smoothing spline detrending with a 50% frequency cutoff at 30 years. The heteroscedastic variance structure was stabilized using adaptive power transformation prior to detrending. This procedure preserved interannual growth variability in the resulting tree-ring indices while removing long-term trends. The detrended tree-level series were averaged into site-level chronologies using a biweight robust mean. We estimated a common growing season for all sites based on the monthly course of GPP from 51 forested FLUXNET2015 sites that offer more than 5 years of data (Tier 1 data only; see data S4). These sites represent the temperature domain covered by the tree-ring network well (fig. S2A) and indicate a growing season between April and October (between October and April) for the Northern (Southern) Hemisphere when GPP is markedly positive (fig. S2B).

It has been hypothesized that the climate sensitivity of the ITRDB could be inflated because a subset of sites has been sampled for dendroclimatic purposes (35). To evaluate this possible bias with respect to our study, we obtained two independent tree-ring datasets from Canada’s National Forest Inventory (36) and from a European biomass network [(37); data S3] and detrended them in the same way as our global network. These reference datasets have not been collected for dendroclimatological purposes, and we used them to benchmark ITRDB’s climate sensitivity. We found that (i) the scatter of the three networks overlaps greatly, indicating that the ITRDB is not a marginal sample (fig. S7); (ii) ITRDB was not the most climate sensitive of the three networks; and (iii) effects of “dataset” in a series of ANCOVA analyses were mostly insignificant (table S2). Hence, we consider our tree-ring network to be suitable for the purpose of this study.

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