Abstract
High interannual variation in seed production in perennial plants can be synchronized at subcontinental scales with wide consequences for ecosystem functioning, but how such synchrony is generated is unclear1,2,3. We investigated the factors contributing to masting synchrony in European beech (Fagus sylvatica), which extends to a geographic range of 2,000 km. Maximizing masting synchrony via spatial weather coordination, known as the Moran effect, requires a simultaneous response to weather conditions across distant populations. A celestial cue that occurs simultaneously across the entire hemisphere is the longest day (the summer solstice). We show that European beech abruptly opens its temperature-sensing window on the solstice, and hence widely separated populations all start responding to weather signals in the same week. This celestial ‘starting gun’ generates ecological events with high spatial synchrony across the continent.
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Data availability
The data used in this study have been deposited in the Open Science Framework (https://doi.org/10.17605/OSF.IO/S2CD4). The full MASTREE+ dataset is available in ref. 64. The climate data were extracted from E-OBS at https://cds.climate.copernicus.eu/cdsapp#!/dataset/insitu-gridded-observations-europe?tab=form.
Code availability
R statistical software v.4.3.0 was used in this work70. All analyses used published R packages.
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Acknowledgements
This study was supported by the European Union (ERC, ForestFuture, 101039066). The views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. V.J. was supported by project no. 2021/43/P/NZ8/01209 co-funded by the Polish National Science Centre and the EU H2020 research and innovation programme under the MSCA GA No. 945339. For the purpose of open access, the authors have applied a CC-BY public copyright licence to any Author Accepted Manuscript version arising from this submission.
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M.B. conceived the study. M.B., V.J., J.S., J.F., A.H.-P. and D.K. designed the study. V.J. and J.S. performed the analysis. M.B. led the writing of the manuscript. All authors contributed critically to the interpretation of the analysis and drafts and gave final approval for publication.
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Extended data
Extended Data Fig. 1 Site-specific correlations with temperature.
Moving window Spearman correlations between temperature and masting across 61 sites distributed over the European beech range. Black dashed lines indicate the summer solstice (21 June) in each year (one and two years prior to seed fall). Each line represents one site sorted by its latitude (43.03∘ - 56.13∘). The number on the right side of the plot (grey color) indicates site-level sample size (monitoring duration in years). The observations are from the period 1952 - 2019.
Extended Data Fig. 2 Summer solstice as a celestial “starting gun".
a) Correlations between European beech seed production and temperature at 7-day windows abruptly decrease just after the solstice in year T2. The purple color shows correlations and GAM model prediction for days before the summer solstice and green after the solstice. The prediction lines are based on the GAM model with Spearman correlation coefficients as a response and interaction between relative day length (at the particular time window) and the summer solstice as predictors. Each point is the per site per window correlation coefficient. The predicted correlation at maximum day length is at -0.013 before the solstice and -0.19 after the solstice. The model is summarized in Extended Data Table 1. b) Correlations before and after the solstice shown at a) binned into 0.95-1 (maximum) day length before and after the solstice. The asterisks (***) indicate a significant difference between bins tested with GAM (p < 0.0001). The median correlation before the solstice is -0.028 and the median correlation after the solstice is -0.25. Box plots show medians (internal lines) and first and third quartiles (lower and upper hinges, respectively); whiskers show 1.5 times the interquartile range from the respective hinge and the points are outliers. c) Results of a null model testing whether the abrupt increase in the correlation coefficients (between masting and summer temperatures) is highest at the summer solstice or any other day in the year. Histogram shows the distribution of beta coefficients under the null hypothesis that is the before-after difference in correlation coefficients in any other day of the year considered instead of the solstice. The red dashed line shows the observed effect that is, how correlation coefficients “after solstice" differ as compared to “before solstice". The null model was rejected (p < 0.001; the randomization procedure is described in Methods). All three graphs show the effects for summer T2 (two years before seedfall), effects for summer T1 are presented in Fig. 2.
Extended Data Fig. 3 Masting correlations with precipitation.
Mean rolling Spearman correlations between precipitation and masting averaged across all 61 sites. The graph shows correlations in two (T2) and one (T1) years before seed production, up until September when seed fall happens. The size of the precipitation window is 7 days, with a 1-day step, and correlations are plotted according to the day of the year at the end of each 7-day window. Black dashed lines close to the sun icon indicate the summer solstice (21st June). Correlations are coded blue for positive, and red for negative. The black solid lines represent the standard error of the correlation coefficients across the sites for each window. Correlations for each site separately are reported in Extended Data Fig. 4.
Extended Data Fig. 4 Site-specific correlations with precipitation.
Moving window Spearman correlation between precipitation and masting across 61 sites distributed over the European beech range. Black dashed lines indicate the summer solstice (21 June) in each year (one and two years prior to seed fall). Each line represents one site sorted by its latitude (43.03∘ - 56.13∘). The number on the right side of the plot (grey color) indicates site-level sample size (monitoring duration in years). The observations are from the period 1952 - 2019.
Supplementary information
Supplementary Video 1
The animation showing the scale of synchrony is provided as a video file. The video shows annual seed production for each site between 1995 and 2019, scaled within each site to values between 0 and 1 (to facilitate among-site comparisons). 1995 is shown as the first year as the spatial coverage at that period is highest. Point size is scaled to the size of annual seed production, while the colour shows the within-year synchrony among sites. That synchrony is calculated as the inverse coefficient of variation.
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Journé, V., Szymkowiak, J., Foest, J. et al. Summer solstice orchestrates the subcontinental-scale synchrony of mast seeding. Nat. Plants 10, 367–373 (2024). https://doi.org/10.1038/s41477-024-01651-w
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DOI: https://doi.org/10.1038/s41477-024-01651-w