Beyond annual rings: Multi-year cycles in tree-ring indices demonstrate long-term reparations of drought in European forests

It is difficult to predict tree resilience to drought because the underlying mechanisms are not fully understood. Furthermore, while tree resilience to drought can be affected by drought duration, the drought duration in most dendrochronological studies is limited to annual or intra-annual scales, with few studies exploring multi-year drought effects. Consequently, the capacity for tree resilience to multi-year droughts remains largely unknown. To further complicate our understanding, there is likely a complex interplay between multi-year droughts and legacy effects from preceding years, which can convolute the drought signals recorded in the tree ring archive. It is essential to enhance our understanding of long-term tree response to drought, to better understand the capacity for tree resilience to withstand multi-year droughts. We aimed to help fulfil this knowledge gap with the first study to explore multi-year cycling in tree ring indices, and their long-term relationships to soil drought. Our focus was on the responses of ring width index (RWI), and the isotope variability in tree-ring cellulose (δ 18 O cel and δ 13 C cel ), in forests across Europe. We hypothesised that RWI, δ 18 O cel and δ 13 C cel exhibit significant multi-year cycling (H1), which have chronic responses to soil drought over several years (H2), and we can untangle multi-year drought effects from drought legacy effects using temporal modelling (H3). We performed a data synthesis, using a compilation of multiple published datasets, targeting samples collected within 20km of forest sites within the Integrated Carbon Observation System (ICOS) station network (ICOS RI 2024). The largest contributors to RWI data were the Tree Ring Network (Babst et al. 2012), GenTree (Martínez-Sancho et al. 2020) and the International Tree Ring Data Bank, and the combined dataset had a large spatiotemporal coverage of Europe. The data for δ 18 O cel and δ 13 C cel analyses were sourced from ISONET (Members et al. 2023a, Members et al. 2023b) and individual studies. Data on drought conditions were sourced from the ICOS Carbon Portal (Integrated Carbon Observation System (ICOS) station network, 2006) and a comprehensive drought index dataset for ICOS sites (Pohl et al. 2023). Dendrochronological data was filtered to select time series >100 years. We evaluated all dendrochronological data for multi-year cycles, and then selected the dendrochronological data which had substantial drought index data available (≥ 50 years). Preliminary findings indicate significant multi-year cycles in tree-ring indices, with multiple cycles at each site, ranging from 2 years to more than 25 years. In a 659-year time series at Fontainebleau (FR), there are multiple multi-year cycling signals, the most pronounced being a 2-year cycle and a 10-year cycle. In a subset of the time series which aligned with Standardized Soil Moisture Index (SSMI) data, δ 18 O cel correlated to SSMI after time lags, which could be observed after more than 10 years. However, the correlation was inversed between lag periods, suggesting a significant role of legacy effects, further explored using temporal modelling. This is the first study to identify decade-scale cycling in tree-rings, and it is an important stepping stone towards understanding tree resilience to multi-year drought. These findings can help to evaluate tree resilience to non-lethal drought from the perspective of a tree’s lifetime, by untangling legacy effects from multi-year drought effects. Nonetheless, there remains scarce alignment between dendrochronological data and comprehensive drought index data, therefore future studies should aim to gather tree-ring index data alongside comprehensively monitored forest sites such as ICOS sites.