Are global forests performing in sync? The need to account for spatiotemporal biases in tree-ring records
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Populations whose dynamics are driven by correlated environmental stochasticity face increased risk of extinc-tion. Forests in particular are being pushed to their physi-ological limits under global change; hence, the analysis of common patterns of tree performance across scales be-comes crucial to discern early warning signals and tipping points. Here, we critically evaluate customary and recent approaches for the analysis of time series of radial growth based on simple correlations as measure of direction and signal strength shared by spatially segregated forests (synchrony). By accounting for changes in spatial distri-bution of populations and temporal coverage of tree- ring records, we show that growth synchrony has not substan-tially augmented for the past 150 years across Europe, a continent with high availability of tree- ring series. We guard against the use of absolute correlations as a metric of synchrony and stress that robust analytical methods should be applied to evaluate synchrony trends when using biased spatiotemporal databases.