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35 Pags.- 7 Figs.- 4 Tabls. The definitive version is available at: https://rmets.onlinelibrary.wiley.com/journal/10970088 ; The long‐term hydroclimatic variability in Santiago (Chile) was analysed by means of a new 481‐year (1536–2016 CE) tree‐ring reconstruction of the Standardized Precipitation Evapotranspiration Index (SPEI) of August, integrating the hydroclimatic conditions during the preceding 14 months. Results show a high frequency of extreme drought events in the late 20th and early 21st centuries, while the frequency of extreme wet events was higher in the 17th–18th centuries. The mid‐20th century represents a breaking point for the hydroclimatic history in the region, including some significant changes: (a) the interannual variability increased; (b) the wet events became less intense; (c) the extreme dry events became more frequent; and (d) the most intense dry event of the entire period was identified, coinciding with the so‐called Megadrought (2006–2016). A correlation analysis between the reconstructed SPEI and three climate indices (PDO, SOI and Niño3.4) was performed at monthly scale, considering different multi‐annual aggregations. The analysis shows diverse impacts on the hydroclimatic variability, with positive correlations between SPEI and PDO as well as Niño3.4, and negative correlations between SPEI and SOI. The most significant correlations were, overall, found at multi‐annual time scales (>7 years). Results help to better understand the current hydroclimatic changes (Megadrought) in a long‐term context. ; R.S.N. is funded by a "Juan de la Cierva" postdoctoral grant FJCI-2017-31595. This work was developed under the project CAS/1900020 funded by the Spanish Ministry of Science and the Fulbright Foundation. O.M.R., P.S. and R.S.N. thank the Climatology Group (2017SGR1362, Catalan Government) and the CLICES Project (CGL2017-83866-C3-2-R). R.S.N. and M.D.L. are supported by the Government of Aragón through the "Programme of research groups" (group H38, "Clima, Agua, Cambio Global y Sistemas Naturales") and thank the project CGL2015-69985-R. E.T. and M.V. were partially supported by NSF-PIRE (OISE-1743738) and NSF-P2C2 (AGS-1702439). M.F. is funded by a postdoctoral grant CONICYT PIA AFB170008 of the Institute of Ecology and Biodiversity (IEB). ; Peer reviewed

The long-term hydroclimatic variability in Santiago (Chile) was analysed by means of a new 481-year (1536-2016 CE) tree-ring reconstruction of the Standardized Precipitation Evapotranspiration Index (SPEI) of August, integrating the hydroclimatic conditions during the preceding 14 months. Results show a high frequency of extreme drought events in the late 20th and early 21st centuries, while the frequency of extreme wet events was higher in the 17th-18th centuries. The mid-20th century represents a breaking point for the hydroclimatic history in the region, including some significant changes: (a) the interannual variability increased; (b) the wet events became less intense; (c) the extreme dry events became more frequent; and (d) the most intense dry event of the entire period was identified, coinciding with the so-called Megadrought (2006-2016). A correlation analysis between the reconstructed SPEI and three climate indices (PDO, SOI and Nino3.4) was performed at monthly scale, considering different multi-annual aggregations. The analysis shows diverse impacts on the hydroclimatic variability, with positive correlations between SPEI and PDO as well as Nino3.4, and negative correlations between SPEI and SOI. The most significant correlations were, overall, found at multi-annual time scales (>7 years). Results help to better understand the current hydroclimatic changes (Megadrought) in a long-term context. ; "Juan de la Cierva" postdoctoral grant FJCI-2017-31595 Spanish Government CAS/1900020 Fulbright Foundation CAS/1900020 Climatology Group (Catalan Government) 2017SGR1362 CLICES Project CGL2017-83866-C3-2-R Government of Aragon through the "Programme of research groups" H38 National Science Foundation (NSF) NSF - Office of the Director (OD) OISE-1743738 National Science Foundation (NSF) NSF - Directorate for Geosciences (GEO) AGS-1702439 CONICYT PIA of the Institute of Ecology and Biodiversity (IEB) AFB170008 CGL2015-69985-R

2 .pdf files: File 1. Author's article final version, Post-Print (19 Pags.- 4 Figs.- 2 Tabls.). File 2. Supplementary Materials (3 Figs.- 2 Tabls.- 1 Model Equation). ; Aim Previous work demonstrated the global variability of synchrony in tree growth within populations, that is, the covariance of the year‐to‐year variability in growth of individual neighbouring trees. However, there is a lack of knowledge about the causes of this variability and its trajectories through time. Here, we examine whether climate can explain variation in within‐population synchrony (WPS) across space but also through time and we develop models capable of explaining this variation. These models can be applied to the global tree cover under current and future climate change scenarios. Location Global. Time period 1901–2012. Major taxa studied Trees. Methods We estimated WPS values from a global tree‐ring width database consisting of annual growth increment measurements from multiple trees at 3,579 sites. We used generalized linear mixed effects models to infer the drivers of WPS variability and temporal trends of global WPS. We then predicted WPS values across the global extent of tree cover. Finally, we applied our model to predict future WPS based on the RCP 8.5 (2045–2065 period) emission scenario. Results Areas with the highest WPS are characterized by a combination of environments with both high mean annual temperature (>10°C) and low precipitation (<300 mm). Average WPS across all temperate forests has decreased historically and will continue to decrease. Potential implications of these patterns include changes in forest dynamics, such as higher tree growth and productivity and an increase in carbon sequestration. In contrast, the WPS of tropical forests of Central and South America will increase in the near future owing to reduced annual precipitation. Main conclusions Climate explains WPS variability in space and time. We suggest that WPS might have value as an integrative ecological measure of the level of environmental stress to which forests are subjected and therefore holds potential for diagnosing effects of global climate change on tree growth. ; E.T. and M.V. were partially supported by National Science Foundation ‐ Partnership in International Research and Education (OISE‐1743738) and National Science Foundation ‐ Paleo Perspectives on Climate Change (AGS‐1702439). R.S.‐N. is funded by a "Juan de la Cierva" postdoctoral grant FJCI‐2017‐31595. E.T., M.S., R.S.‐N. and M.D.L. are supported by the Government of Aragón through the "Program of research groups" (group H38, "Clima, Agua, Cambio Global y Sistemas Naturales"). A.L. was supported by grant EVA4.0, No. CZ.02.1.01/0.0/0.0/16_019/0000803, financed by OP RDE and by the U.S. Department of Agriculture Forest Service. ; Peer reviewed