Temporal changes in seasonal climate sensitivity in a tree-ring MXD record from Northern Sweden

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Maximum latewood density (MXD) chronologies from northern Scandinavia are widely used for temperature reconstructions because of their strong relationship with growing-season temperatures. However, climate–MXD relationships may vary through time, potentially affecting reconstruction skill and interpretation. Here, we present an MXD-based temperature reconstruction from Ammarnäs, northern Sweden, spanning 1260–2021 CE, and investigate the temporal stability of the climate signal recorded in Scots pine (Pinus sylvestris L.).The chronology is based on 130 MXD series from living and drywood samples and was standardized using multiple Regional Curve Standardisation (mRCS). Calibration against April–August temperatures explains 53% of the variance in the instrumental record (1861–2021). Split-period calibration and verification analyses reveal stronger climate sensitivity during the early instrumental period (1861–1940; R² = 0.69) than during the later period (1941–2021; R² = 0.36). Progressive shortening of the calibration period indicates that the decline in model performance is primarily associated with recent decades.Monthly climate analyses show that the seasonal distribution of the temperature signal has changed through time. July became relatively more important while the August signal weakened. This redistribution coincides with declining reconstruction skill and resembles patterns reported from other high-latitude tree-ring records affected by the divergence problem.The reconstruction captures interannual to centennial variability and the overall warming trend of the twentieth century. The results demonstrate that both signal strength and seasonality can vary through time and should be explicitly considered in dendroclimatological calibration and interpretation.