Comment on cp-2021-2

Abstract

The unstable sensitivity of growth-climate relationships greatly restricts tree-ring-based paleoclimate reconstructions, especially in areas with frequent "divergence" problems, such as the temperate zone in northeast China. Here, we propose an original tree-species mixing method to overcome this obstacle and improve the stability and reliability of reconstruction models. We take the tree-ring based growing-season minimum temperature reconstruction for the northern Changbai Mountains in northeast China as an example to illustrate the method. Compared with previous temperature reconstruction models, our reconstruction model is more stable and reliable and explains up to 68% of the variance. It is also highly consistent with historical records and tree-ring-based temperature reconstructions from the nearby Xiaoxing'an Mountains and from across the Northern Hemisphere. Our reconstruction uses two different tree species and is more accurate than temperature reconstructions developed from a single species. Over the past 259 years (AD 1757(AD -2015, five significant cold periods and five warm periods were identified. The reconstruction indicates rapid warming since the 1980s, which is consistent with other instrumental and reconstructed records. We also found the Atlantic Multidecadal Oscillation plays a crucial role in driving the growing-season minimum temperature in the northern Changbai Mountains.climates (Fritts, 1976;Zhu et al., 2020a). However, the lack of long-term instrumental climate data severely limits our ability to understand past climates; for this reason, long climate proxy records are urgently needed.Tree rings, especially ring widths, are a critically important paleoclimate proxy (Fritts, 1976) and are widely used for reconstructing climate at a high resolution over hundreds to thousands of years (Anchukaitis et al., 2017;Wilson et al., 2016). Such records are invaluable for placing recent climatic changes in a long-term context, which can help considerably with planning appropriate responses to future climate changes and extreme events. However, the statistical calibration and verification of tree-ring based reconstructions is a rigorous process (Fritts, 1976) that requires a relatively stable growth-climate relationship over time.Over the last several decades, many studies have addressed the "divergence problem", an anomalous reduction in tree-ring indices and temperature sensitivity after rapid warming (D'Arrigo et al., 2008). Other studies have found an increased correlation between tree-ring indices and temperature after rapid warming, such as in the case of Fraxinus mandshurica (FM) (Cao et al., 2018) and Larix gmelinii (Zhang et al., 2016) in northeastern China. These two types of "unstable sensitivity" in growth-climate relationships challenge the assumption of "uniformity principle" in dendroclimatology (Fritts, 1976), and thus brings more uncertainties in treering-based inferences on past climate.The Changbai Mountains are the highest in eastern Eurasia, covering nearly 2,000 square kilomet...