Exorcizing Divergence in Tree‐Ring Density Along the Rocky Mountains

Abstract

Abstract Briffa et al. (1998), https://doi.org/10.1038/35596 published a seminal paper on the reduced sensitivity of annual tree growth to temperature across Northern Hemisphere treeline stands. By averaging tree‐ring chronologies to sub‐continental means, they found decade‐long trends in maximum latewood density (MXD) progressively diverging from observed warming temperatures since the 1960s. This divergence challenges the reliability of the proxy, but the lack of an intercontinental network of up‐to‐date MXD chronologies extending into the 21st century hindered large‐scale evaluations of the phenomenon, leaving it unresolved. Here, we introduce nine new MXD chronologies along the North American Rocky Mountains between 38° and 69°N and analyze their trends after applying novel approaches to preserve low‐frequency variability. When following the original Briffa et al. (1998), https://doi.org/10.1038/35596 methodology, the divergence between increasing temperatures and MXD chronologies reaches offset values greater than 1°C by 2020 CE. However, divergence markedly decreases and even disappears entirely when MXD chronologies are (a) based on high‐replication data sets including differently old trees, (b) detrended using signal‐free age‐dependent splines instead of Hugershoff curves, and (c) calibrated against optimum season instead of April–September temperatures. MXD chronologies north of 60°N exhibit stable relationships with regional summer temperatures on interannual to multi‐decadal timescales, but at the southern sites, a lack of high‐frequency proxy‐target coherency is evident starting in the second half of the 20th century. This study emphasizes the importance of careful site and target selection, sampling design, and chronology development for overcoming the divergence problem and reconstructing summer temperatures from MXD data in North America. Plain Language Summary Trees growing in cold regions of the Northern Hemisphere often produce denser wood during warm summers. Because of this, wood density from such locations can be used to estimate past climate conditions. However, previous research has found that many of these tree‐ring records do not reflect the strong warming trend observed in recent decades. This problem, known as “divergence,” questions how reliable tree rings are for reconstructing past climate. Here, we present new wood density data from nine locations along the Rocky Mountains in western North America. We demonstrate that divergence in these sites can be minimized by using a large number of trees of different ages, applying suitable techniques for analysis, and selecting the part of the year that most strongly affects tree growth at each site. Our results show that the link between instrumental temperatures and tree‐ring data has remained stable in the northern half of our study area across both short and long periods. In contrast, in the southern Rocky Mountains, changes in tree growth from year to year are increasingly influenced by dry conditions rather than temperature. Key Points Tree‐ring density continues to track recent warming trends in Western North America Divergence between proxy and instrumental data is minimized by thorough site and target selection and suitable methodology Interannual variations in tree growth in the southern Rocky Mountains are increasingly determined by water availability