• Energy Research

AbstractClimate change effects on tree reproduction are poorly understood, even though the resilience of populations relies on sufficient regeneration to balance increasing rates of mortality. Forest‐forming tree species often mast, i.e. reproduce through synchronised year‐to‐year variation in seed production, which improves pollination and reduces seed predation. Recent observations in European beech show, however, that current climate change can dampen interannual variation and synchrony of seed production and that this masting breakdown drastically reduces the viability of seed crops. Importantly, it is unclear under which conditions masting breakdown occurs and how widespread breakdown is in this pan‐European species. Here, we analysed 50 long‐term datasets of population‐level seed production, sampled across the distribution of European beech, and identified increasing summer temperatures as the general driver of masting breakdown. Specifically, increases in site‐specific mean maximum temperatures during June and July were observed across most of the species range, while the interannual variability of population‐level seed production (CVp) decreased. The declines in CVp were greatest, where temperatures increased most rapidly. Additionally, the occurrence of crop failures and low seed years has decreased during the last four decades, signalling altered starvation effects of masting on seed predators. Notably, CVp did not vary among sites according to site mean summer temperature. Instead, masting breakdown occurs in response to warming local temperatures (i.e. increasing relative temperatures), such that the risk is not restricted to populations growing in warm average conditions. As lowered CVp can reduce viable seed production despite the overall increase in seed count, our results warn that a covert mechanism is underway that may hinder the regeneration potential of European beech under climate change, with great potential to alter forest functioning and community dynamics.

Many perennial plants show mast seeding, characterized by synchronous and highly variable reproduction across years. We propose a general model of masting, integrating proximate factors (environmental variation, weather cues, and resource budgets) with ultimate drivers (predator satiation and pollination efficiency). This general model shows how the relationships between masting and weather shape the diverse responses of species to climate warming, ranging from no change to lower interannual variation or reproductive failure. The role of environmental prediction as a masting driver is being reassessed; future studies need to estimate prediction accuracy and the benefits acquired. Since reproduction is central to plant adaptation to climate change, understanding how masting adapts to shifting environmental conditions is now a central question.

AbstractClimate warming increases tree mortality which will require sufficient reproduction to ensure population viability. However, the response of tree reproduction to climate change remains poorly understood. Warming can reduce synchrony and interannual variability of seed production (“masting breakdown”) which can increase seed predation and decrease pollination efficiency in trees. Here, using 40 years of observations of individual seed production in European beech (Fagus sylvatica), we showed that masting breakdown results in declining viable seed production over time, in contrast to the positive trend apparent in raw seed count data. Furthermore, tree size modulates the consequences of masting breakdown on viable seed production. While seed predation increased over time mainly in small trees, pollination efficiency disproportionately decreased in larger individuals. Consequently, fecundity declined over time across all size classes, but the overall effect was greatest in large trees. Our study showed that a fundamental biological relationship—correlation between tree size and viable seed production—has been reversed as the climate has warmed. That reversal has diverse consequences for forest dynamics; including for stand‐ and biogeographical‐level dynamics of forest regeneration. The tree size effects suggest management options to increase forest resilience under changing climates.

AbstractSpatial synchrony may be tail‐dependent, meaning it is stronger for peaks rather than troughs, or vice versa. High interannual variation in seed production in perennial plants, called masting, can be synchronized at subcontinental scales, triggering extensive resource pulses or famines. We used data from 99 populations of European beech (Fagus sylvatica) to examine whether masting synchrony differs between mast peaks and years of seed scarcity. Our results revealed that seed scarcity occurs simultaneously across the majority of the species range, extending to populations separated by distances up to 1800 km. Mast peaks were spatially synchronized at distances up to 1000 km and synchrony was geographically concentrated in northeastern Europe. Extensive synchrony in the masting lower tail means that famines caused by beech seed scarcity are amplified by their extensive spatial synchrony, with diverse consequences for food web functioning and climate change biology.

# Reproductive data Fagus sylvatica: Widespread masting breakdown in beech [https://doi.org/10.5061/dryad.qz612jmps](https://doi.org/10.5061/dryad.qz612jmps) This dataset, used in the Global Change Biology article "Widespread breakdown in masting in European beech due to rising summer temperatures", contains 50 time series of population-level annual reproductive data by European beech (*Fagus sylvatica*, L) across Europe. The dataset builds on the open-access dataset [MASTREE+](https://doi.org/10.1111/gcb.16130), and expands it for European beech. ## Description of the data The dataset column names follow that of MASTREE+. A description of MASTREE+ column names (Modified from Table 1 in the [MASTREE+ article)](https://doi.org/10.1111/gcb.16130): | *Columns* | *Description* | *Contains NA?* | | :-------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------- | | Alpha\_Number | Unique code associated with each original source of data, that is, the publication, report or thesis containing extracted data, or the previously unpublished data set included in MASTREE+. | No | | Segment | Temporal segment of a time-series containing gaps (note that years with no observations are not recorded). Individual timeseries can consist of multiple segments. | No | | Site\_number | Code to differentiate multiple sites from the same original source (Alpha\_Number/Study\_ID). | No | | Variable\_number | Code to differentiate multiple measures of reproductive output from the same species-site combination (e.g. where seeds and cones were recorded separately). | No | | Year | Year of observation. | No | | Species | Species identifier, standardised to The Plant List nomenclature. ‘spp.’ is used to indicate a record identified to the genus level only. ‘MIXED’ indicates a non-species-specific community-level estimate of annual reproductive effort. | No | | Species\_code | Six-character species identifier. | No | | Mono\_Poly | Monocarpic (semelparous) or Polycarpic (iteroparous) species. | No | | Value | The measured value of annual reproductive output. | No | | VarType | Continuous or ordinal data. Continuous time-series are recorded on a continuous scale. Ordinal series are recorded on an ordered categorical scale. All ordinal series are rescaled to start at 1 (lowest reproductive effort) and to contain only integer values. | No | | Max\_value | The unit of measurement, where VarType is continuous (otherwise: NA). | No | | Unit | The maximum value in a time-series. | No | | Variable | Categorical classification of the measured variable. Options limited to: cone, flower, fruit, seed, pollen, total reproduction organs. | No | | Collection\_method | Classification of the method used to measure reproductive effort. Options are limited to: cone count, cone scar count, flower count, fruit count, fruit scar sound, seed count, seed trap, pollen count, lake sediment pollen count, harvest record, visual crop assessment, other quantification, dendrochronological reconstruction. | No | | Latitude | Latitude of the record, in decimal degrees. | No | | Longitude | Longitude of the record, in decimal degrees. | No | | Coordinate\_flag | A flag to indicate the precision of the latitude and longitude. A = coordinates provided in the original source B = coordinates estimated by the compiler based on a map or other location information provided in the original source C = coordinates estimated by the compiler as the approximate centre point of the smallest clearly defined geographical unit provided in the original source (e.g. county, state, island), and potentially of low precision. | No | | Site | A site name or description, based on information in the original source. | No | | Country | The country where the observation was recorded. | No | | Elevation | The elevation of the sample site in metres above sea level, where provided in the original source (otherwise: NA). | Yes | | Spatial\_unit | Categorical classification of spatial scale represented by the record, estimated by the compiler based on information provided in the original source. stand = <100 ha, patch = 100–10,000 ha, region = 10,000–1,000,000 ha, super-region = >1,000,000 ha. | No | | No\_indivs | Either the number of monitored individual plants, or the number of litter traps. NA indicates no information in the original source, and 9999 indicates that while the number of monitored individuals was not specified, the source indicated to the compiler that the sample size was likely ≥10 individuals or litter traps. | No | | Start | The first year of observations for the complete time-series, including all segments. | No | | End | The final year of observations for the complete time-series, including all segments. | No | | Length | The number of years of observations. Note that may not be equal to the number of years between the Start and End of the time-series, due to gaps in the time-series. | No | | Reference | Identification for the original source of the data. | No | | Record\_type | Categorisation of the original source. Peer-reviewed = extracted from peer reviewed literature Grey = extracted from grey literature Unpublished = unpublished data. | No | | ID\_enterer | Identification of the original compiler of the data. AHP, Andrew Hacket-Pain; ES, Eliane Schermer; JVM, Jose Moris; XTT, Tingting Xue; TC, Thomas Caignard; DV, Davide Vecchio; DA, Davide Ascoli; IP, Ian Pearse; JL, Jalene LaMontagne; JVD, Joep van Dormolen. | No | | Date\_entry | Date of data entry into MASTREE+ in the format yyyy-mm-dd. | No | | Note on data location | Notes on the location of the data within the original source, such as page or figure number. If not provided, NA. | Yes | | Comments | Additional comments. If not provided, NA. | Yes | | Study\_ID | Unique code associated with each source of data. M\_ = series extracted from published literature; A\_ = series incorporated from Ascoli et al. (2020), Ascoli, Maringer, et al. (2017) and Ascoli, Vacchiano, et al. (2017); PLK\_ = series incorporated from Pearse et al. (2017); D\_ = unpublished data sets. NA is attributed if no study ID has been previously associated with this time-series in MASTREE+ v.1. | Yes | Note that the new beech reproductive data has been assigned an arbitrary Alpha_Number for the purpose of this study. Future MASTREE+ updates which incorporate this new data may alter the time series ID columns (e.g. Alpha_Number, Site_number, Variable_number). MASTREE+ updates can be found on [GITHUB](https://github.com/JJFoest/MASTREEplus). Climate change effects on tree reproduction are poorly understood even though the resilience of populations relies on sufficient regeneration to balance increasing rates of mortality. Forest-forming tree species often mast, i.e. reproduce through synchronised year-to-year variation in seed production, which improves pollination and reduces seed predation. Recent observations in European beech show, however, that current climate change can dampen interannual variation and synchrony of seed production, and that this masting breakdown drastically reduces the viability of seed crops. Importantly, it is unclear under which conditions masting breakdown occurs, and how widespread breakdown is in this pan-European species. Here, we analysed 50 long-term datasets of population-level seed production, sampled across the distribution of European beech, and identified increasing summer temperatures as the general driver of masting breakdown. Specifically, increases in site-specific mean maximum temperatures during June and July were observed across most of the species range, while the interannual variability of population-level seed production (CVp) decreased. The declines in CVp were greatest where temperatures increased most rapidly. Additionally, the occurrence of crop failures and low-seed years has decreased during the last four decades, signalling altered starvation effects of masting on seed predators. Notably, CVp did not vary among sites according to site mean summer temperature. Instead, masting breakdown occurs in response to warming local temperatures (i.e. increasing relative temperatures), such that the risk is not restricted to populations growing in warm average conditions. As lowered CVp can reduce viable seed production despite the overall increase in seed count, our results warn that a covert mechanism is underway that may hinder the regeneration potential of European beech under climate change, with great potential to alter forest functioning and community dynamics.