Identifying drivers of non-stationary climate-growth relationships of European beech

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species' ecological amplitude. Our results show that drought sensitivity is highly variable and non-stationary, though consistently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees' rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech's drought sensitivity, which needs to be taken into consideration when assessing this species' response to climate change.

This project is funded by the Bavarian Ministry of Science and the Arts in the context of the Bavarian Climate Research Network (BayKliF). The following authors acknowledge funding support. AMP: Project PN23090301, within FORCLIMSOC program (Ministry of Research, Innovation and Digitization of Romania). ASJ: Supported by the UK Natural Environment Research Council under grants NE/V00929X/1 and NE/S010041/1. BS: Supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia within the framework of the program technological development [451-03-65/2024-03/200169]. CCR: Grant of the Ministry of Research, Innovation and Digitization, under the “Romania's National Recovery and Resilience Plan - Founded by EU –NextGenerationEU” program, project “Compound extreme events from a long-term perspective and their impact on forest growth dynamics (CExForD)” number 760074/23.05.2023, code 287/30.11.2022, within Pillar III, Component C9, Investment 8. FB: Funded, in part, by the Experiment Station of the College of Agriculture, Biotechnology, and Natural Resources at the University of Nevada, Reno (USA). IP: CNCS-UEFISCDI, project number PN-III-P4-PCE-2021–1002. IDL: Proyectos de Generación de Conocimiento, MICIN (#PID2021-128759OA-I00) and Proyectos de Consolidación Investigadora 2022 (#CNS2022-135228). JJC: Projects PID2021-123675OB-C43 and TED2021-129770B-C21 (Spanish Ministry of Science and Innovation). JK: DFG GRK 2010 Response. KC: ARIS -Slovenian Research and Innovation Agency, program P4-0015. LMW: DFG GRK 2010 Response. MB: MCID nucleu program, project PN23090303. MK: Supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia within the framework of the program technological development [451-03-65/2024-03/200169] and Science Fond of the Republic of Serbia [6686]. MP: Supported by University of Forestry, Sofia, Project No. B1219. MS: Czech Science and the project TRANSFER. TZ - D01-320/30.11.2023 LTER-BG. PJ: Czech Science and the project TRANSFER. TZ - D01-320/30.11.2023 LTER-BG. TZ: “D01-320/30.11.2023 LTER-BG”. WH: National Natural Science Foundation of China (No. 32271973).

Peer reviewed