Mechanisms of mast seeding: resources, weather, cues, and selection

Seed size and dispersal mode select mast seeding in perennial plants

Reproduction by perennial plants varies from being relatively constant over years to the production of massive and synchronous seed crops at irregular intervals, a reproductive strategy called mast seeding. The sources of interspecific differences in the extent of interannual variation in seed production are largely unknown. We conducted a global meta-analysis of animal-dispersed species to quantify how the interannual variability in seed crops produced by plants can be explained by the seed mass, dispersal mode, phylogeny, and climate. Phylogenetic analysis indicated that the interannual variations in seed production and seed mass tended to be similar in related species due to their shared evolution. The interannual variation in seed production was 1.22 times higher in synzoochorous species dispersed by scatter-hoarders compared with endozoochorous species dispersed by frugivores. Furthermore, the production of small seeds was associated with higher interannual variation in seed production, although synzoochorous species produced larger seeds than endozoochorous species. Precipitation rather than temperature had a significant positive effect on the interannual variation in seed production. The seed mass and dispersal mode contributed more to the interannual variation in seed production than phylogeny, climate, and fruit type. Our findings support a long-standing hypothesis that interspecific variation in the masting intensity is largely shaped by interactions between plants and animals.

Mast seeding is stronger in taller plants

Introduction

Two economies of scale, predator satiation and pollination efficiency, have been proposed to explain the evolutionary mechanisms of mast seeding adopted by some long-lived plants. Plant height is strongly selected by pollination vectors and may also provide economies of scale; however, it remains unknown whether there is a close relationship between adult plant height and mast seeding intensity.

Methods

Here, we analyzed mast seeding intensity of 158 plant species to test if adult plant height can select for mast seeding.

Results

We show that mast seeding intensities are higher in taller plant species irrespective of phylogeny, life form, pollination vector, and type of Spermatophytes. We also show that anemophily rather than entomophily selects for taller plant species and higher mast seeding intensities.

Discussion

The linear correlations and evolutionary links between adult plant height and mast seeding intensity provide evidence that mast seeding could have evolved as an adaptation to taller strategy of perennial plant species.

Tail-dependence of masting synchrony results in continent-wide seed scarcity

Spatial 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.

Evolutionary ecology of masting: mechanisms, models, and climate change

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.

Mast seeding in European beech (Fagus sylvatica L.) is associated with reduced fungal sporocarp production and community diversity

Mast seeding is a well-documented phenomenon across diverse forest ecosystems. While its effect on aboveground food webs has been thoroughly studied, how it impacts the soil fungi that drive soil carbon and nutrient cycling has not yet been explored. To evaluate the relationship between mast seeding and fungal resource availability, we paired a Swiss 29-year fungal sporocarp census with contemporaneous seed production for European beech (Fagus sylvatica L.). On average, mast seeding was associated with a 55% reduction in sporocarp production and a compositional community shift towards drought-tolerant taxa across both ectomycorrhizal and saprotrophic guilds. Among ectomycorrhizal fungi, traits associated with carbon cost did not explain species' sensitivity to seed production. Together, our results support a novel hypothesis that mast seeding limits annual resource availability and reproductive investment in soil fungi, creating an ecosystem 'rhythm' to forest processes that is synchronized above- and belowground.

Widespread breakdown in masting in European beech due to rising summer temperatures

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.

Hurricanes: an unexpected mechanism linking disturbance and seed production in trees

This article is a Commentary on Cannon et al. (2024), 242: 289–301.

Tropical cyclone winds and precipitation stimulate cone production in the masting species longleaf pine (Pinus palustris)

Many trees exhibit masting - where reproduction is temporally variable and synchronous over large areas. Several dominant masting species occur in tropical cyclone (TC)-prone regions, but it is unknown whether TCs correlate with mast seeding. We analyzed long-term data (1958-2022) to test the hypothesis that TCs influence cone production in longleaf pine (Pinus palustris). We integrate field observations, weather data, satellite imagery, and hurricane models to test whether TCs influence cone production via: increased precipitation; canopy density reduction; and/or mechanical stress from wind. Cone production was 31% higher 1 yr after hurricanes and 71% higher after 2 yr, before returning to baseline levels. Cyclone-associated precipitation was correlated with increased cone production in wet years and cone production increased after low-intensity winds (≤ 25 m s-1 ) but not with high-intensity winds (> 25 m s-1 ). Tropical cyclones may stimulate cone production via precipitation addition, but high-intensity winds may offset any gains. Our study is the first to support the direct influence of TCs on reproduction, suggesting a previously unknown environmental correlate of masting, which may occur in hurricane-prone forests world-wide.

Masting, fire-stimulated flowering, and the evolutionary ecology of synchronized reproduction

Synchronized episodic reproduction among long-lived plants shapes ecological interactions, ecosystem dynamics, and evolutionary processes worldwide. Two active scientific fields investigate the causes and consequences of such synchronized reproduction: the fields of masting and fire-stimulated flowering. While parallels between masting and fire-stimulated flowering have been previously noted, there has been little dialogue between these historically independent fields. We predict that the synthesis of these fields will facilitate new insight into the causes and consequences of synchronized reproduction. Here we briefly review parallels between masting and fire-stimulated flowering, using two case studies and a database of 1870 plant species to facilitate methodological, conceptual, geographical, taxonomic, and phylogenetic comparisons. We identify avenues for future research and describe three key opportunities associated with synthesis. First, the taxonomic and geographic complementarity of empirical studies from these historically independent fields highlights the potential to derive more general inferences about global patterns and consequences of synchronized reproduction in perennial plants. Second, masting's well developed conceptual framework for evaluating adaptive hypotheses can help guide empirical studies of fire-stimulated species and enable stronger inferences about the evolutionary ecology of fire-stimulated flowering. Third, experimental manipulation of reproductive variation in fire-stimulated species presents unique opportunities to empirically investigate foundational questions about ecological and evolutionary processes underlying synchronized reproduction. Synthesis of these fields and their complementary insights offers a unique opportunity to advance our understanding of the evolutionary ecology of synchronized reproduction in perennial plants.

Summer solstice orchestrates the subcontinental-scale synchrony of mast seeding

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 unclear1-3. 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.

No evidence of carbon storage usage for seed production in 18 dipterocarp masting species in a tropical rain forest

Most canopy species in lowland tropical rain forests in Southeast Asia, represented by Dipterocarpaceae, undergo mast reproduction synchronously at community level during a general flowering event. Such events occur at irregular intervals of 2-10 years. Some species do not necessarily participate in every synchronous mast reproduction, however. This may be due to a lack of carbohydrate resources in the trees for masting. We tested the hypothesis that interspecific differences in the time required to store assimilates in trees for seed production are due to the frequency of masting and/or seed size in each species. We examined the relationship between reproductive frequency and the carbon accumulation period necessary for seed production, and between the seed size and the period, using radiocarbon analysis in 18 dipterocarp canopy species. The mean carbon accumulation period was 0.84 years before seed maturation in all species studied. The carbon accumulation period did not have any significant correlation with reproductive frequency or seed size, both of which varied widely across the species studied. Our results show that for seed production, dipterocarp masting species do not use carbon assimilates stored for a period between the masting years, but instead use recent photosynthates produced primarily in a masting year, regardless of the masting interval or seed size of each species. These findings suggest that storage of carbohydrate resources is not a limiting factor in the masting of dipterocarps, and that accumulation and allocation of other resources is important as a precondition for participation in general flowering.

Seed masting as a mechanism for escape from pathogens

Seed masting, a reproductive strategy characterized by variable and synchronous investment in reproduction among years, has attracted much attention. Masting trees incur a cost in delayed reproduction, and thus masting requires an ecological or evolutionary explanation. The two broad causal mechanisms to explain seed masting are resource availability and economies of scale (EOS); the former assumes reproductive investment simply covaries with environment, the latter suggests an adaptive advantage. Two of the most commonly proposed EOS for masting are predator satiation and pollination efficiency. Here we suggest an additional EOS: pathogen escape. We borrow from the disease ecology literature to describe alternative models of pathogen-mediated masting. By comparing and contrasting their ecological dynamics, we show how predator satiation and pathogen escape may favour masting through similar mechanisms of mass-action interactions and temporal delays. However, pathogen- and predator-mediated dynamics may also diverge as a result of host epidemiological structure and the spatial scale of the interaction. We propose that pathogen escape should be considered among the list of putative mechanisms to help explain the many diverse observations of masting across space and phylogeny.

Tree-ring isotopic imprints on time series of reproductive effort indicate warming-induced co-limitation by sink and source processes in stone pine

Increasing evidence indicates that tree growth processes, including reproduction, can be either sink- or source-limited, or simultaneously co-limited by sink and source, depending on the interplay between internal and environmental factors. We tested the hypothesis that the relative strengths of photosynthate supply and demand by stem growth and reproduction create variable competition for substrate that is imprinted in the tree-ring isotopes (C and O) of stone pine (Pinus pinea L.), a masting gymnosperm with large costs of reproduction, under warming-induced drought. Across five representative stands of the Spanish Northern Plateau, we also identified reproductive phases where weather drivers of cone yield (CY) have varied over a 60-year period (1960-2016). We found that these drivers gradually shifted from winter-spring conditions 3 years before seed rain (cone setting) to a combination of 3- and 1-year lagged effects (kernel filling). Additionally, we observed positive regional associations between carbon isotope discrimination (Δ13C) of the year of kernel filling and CY arising at the turn of this century, which progressively offset similarly positive relationships between Δ13C of the year of cone setting and CY found during the first half of the study period. Altogether, these results pinpoint the increasing dependence of reproduction on fresh assimilates and suggest sink and source co-limitation superseding the sink-limited functioning of reproduction dominant before 2000. Under climate warming, it could be expected that drier conditions reinforce the role of source limitation on reproduction and, hence, on regeneration, forest structure and economic profit of the nutlike seeds of the species.

Granivore abundance shapes mutualism quality in plant-scatterhoarder interactions

Conditional mutualisms involve costs and benefits that vary with environmental factors, but mechanisms driving these dynamics remain poorly understood. Scatterhoarder-plant interactions are a prime example of this phenomenon, as scatterhoarders can either increase or reduce plant recruitment depending on the balance between seed dispersal and predation. We explored factors that drive the magnitude of net benefits for plants in this interaction using a mathematical model, with parameter values based on European beech (Fagus sylvatica) and yellow-necked mice (Apodemus flavicollis). We measured benefits as the percentage of germinating seeds, and examined how varying rodent survival (reflecting, e.g. changes in predation pressure), the rate of seed loss to other granivores, the abundance of alternative food resources, and changes in masting patterns affect the quality of mutualism. We found that increasing granivore abundance can degrade the quality of plant-scatterhoarder mutualism due to increased cache pilferage. Scatterhoarders are predicted to respond by increasing immediate consumption of gathered seeds, leading to higher costs and reduced benefits for plants. Thus, biotic changes that are detrimental to rodent populations can be beneficial for tree recruitment due to adaptive behavior of rodents. When scatterhoarder populations decline too drastically (< 5 individuals ha-1 ); however, tree recruitment may also suffer.

Resource-explicit interactions in spatial population models

Continuous-space population models can yield significantly different results from their panmictic counterparts when assessing evolutionary, ecological, or population-genetic processes. However, the computational burden of spatial models is typically much greater than that of panmictic models due to the overhead of determining which individuals interact with one another and how strongly they interact. Though these calculations are necessary to model local competition that regulates the population density, they can lead to prohibitively long runtimes. Here, we present a novel modeling method in which the resources available to a population are abstractly represented as an additional layer of the simulation. Instead of interacting directly with one another, individuals interact indirectly via this resource layer. We find that this method closely matches other spatial models, yet can dramatically increase the speed of the model, allowing the simulation of much larger populations. Additionally, models structured in this manner exhibit other desirable characteristics, including more realistic spatial dynamics near the edge of the simulated area, and an efficient route for modeling more complex heterogeneous landscapes.

Low but significant evolutionary potential for growth, phenology and reproduction traits in European beech

Local survival of forest tree populations under climate change depends on existing genetic variation and their adaptability to changing environments. Responses to selection were studied in European beech (Fagus sylvatica) under field conditions. A total of 1087 adult trees, seeds, 1-year-old seedlings and established multiyear saplings were genotyped with 16 nuSSRs. Adult trees were assessed for phenotypic traits related to growth, phenology and reproduction. Parentage and paternity analyses were used to estimate effective female and male fecundity as a proxy of fitness and showed that few parents contributed to successful regeneration. Selection gradients were estimated from the relationship between traits and fecundity, while heritability and evolvability were estimated using mixed models and the breeder's equation. Larger trees bearing more fruit and early male flowering had higher total fecundity, while trees with longer growth season had lower total fecundity (directional selection). Stabilizing selection on spring phenology was found for female fecundity, highlighting the role of late frosts as a selection driver. Selection gradients for other traits varied between measurement years and the offspring cohort used to estimate parental fecundity. Compared to other studies in natural populations, we found low to moderate heritability and evolvability for most traits. Response to selection was higher for growth than for budburst, leaf senescence or reproduction traits, reflecting more consistent selection gradients across years and sex functions, and higher phenotypic variability in the population. Our study provides empirical evidence suggesting that populations of long-lived organisms such as forest trees can adapt locally, even at short-time scales.

Seed predation-induced Allee effects, seed dispersal and masting jointly drive the diversity of seed sources during population expansion

The environmental factors affecting plant reproduction and effective dispersal, in particular biotic interactions, have a strong influence on plant expansion dynamics, but their demographic and genetic consequences remain an understudied body of theory. Here, we use a mathematical model in a one-dimensional space and on a single reproductive period to describe the joint effects of predispersal seed insect predators foraging strategy and plant reproduction strategy (masting) on the spatio-temporal dynamics of seed sources diversity in the colonisation front of expanding plant populations. We show that certain foraging strategies can result in a higher seed predation rate at the colonisation front compared to the core of the population, leading to an Allee effect. This effect promotes the contribution of seed sources from the core to the colonisation front, with long-distance dispersal further increasing this contribution. As a consequence, our study reveals a novel impact of the predispersal seed predation-induced Allee effect, which mitigates the erosion of diversity in expanding populations. We use rearrangement inequalities to show that masting has a buffering role: it mitigates this seed predation-induced Allee effect. This study shows that predispersal seed predation, plant reproductive strategies and seed dispersal patterns can be intermingled drivers of the diversity of seed sources in expanding plant populations, and opens new perspectives concerning the analysis of more complex models such as integro-difference or reaction-diffusion equations.

Increased aridity is associated with stronger tradeoffs in ponderosa pine vital functions

Trees must allocate resources to core functions like growth, defense, and reproduction. These allocation patterns have profound effects on forest health, yet little is known about how core functions trade off over time, and even less is known about how a changing climate will impact tradeoffs. We conducted a 21-year survey of growth, defense, and reproduction in 80 ponderosa pine individuals spanning eight populations across environmental gradients along the Colorado Front Range, USA. We used linear mixed models to describe tradeoffs among these functions and to characterize variability among and within individuals over time. Growth and defense were lower in years of high cone production, and local drought conditions amplified year-to-year tradeoffs between reproduction and growth, where trees located at sites with hotter and drier climates showed stronger tradeoffs between reproduction and growth. Our results support the environmental stress hypothesis of masting, which predicts that greater interannual variation in tree functions will be associated with more marginal environments, such as those that are prone to drought. With warming temperatures and increased exposure to drought stress, trees will be faced with stronger interannual tradeoffs, which could lead to further decreases in growth and defensive efforts, ultimately increasing risks of mortality.

Masting is uncommon in trees that depend on mutualist dispersers in the context of global climate and fertility gradients

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands.

Reproduction alternation in trees: testing the resource depletion hypothesis using experimental fruit removal in Quercus ilex

The keystones of resource budget models to explain mast seeding are that fruit production depletes tree stored resources, which become subsequently limiting to flower production the following year. These two hypotheses have, however, rarely been tested in forest trees. Using a fruit removal experiment, we tested whether preventing fruit development would increase nutrient and carbohydrates storage and modify allocation to reproduction and vegetative growth the following year. We removed all the fruits from nine adult Quercus ilex L. trees shortly after fruit set and compared, with nine control trees, the concentrations of nitrogen (N), phosphorus (P), zinc (Zn), potassium (K) and starch in leaves, twigs and trunk before, during and after the development of female flowers and fruits. The following year, we measured the production of vegetative and reproductive organs as well as their location on the new spring shoots. Fruit removal prevented the depletion of N and Zn in leaves during fruit growth. It also modified the seasonal dynamics in Zn, K and starch in twigs, but had no effect on reserves stored in the trunk. Fruit removal increased the production of female flowers and leaves the following year, and decreased the production of male flowers. Our results show that resource depletion operates differently for male and female flowering, because the timing of organ formation and the positioning of flowers in shoot architecture differ between male and female flowers. Our results suggest that N and Zn availability constrain flower production in Q. ilex, but also that other regulatory pathways might be involved. They strongly encourage further experiments manipulating fruit development over multiple years to describe the causal relationships between variations in resource storage and/or uptake, and male and female flower production in masting species.

Correlation between airborne pollen data and the risk of tick-borne encephalitis in northern Italy

Tick-borne encephalitis (TBE) is caused by a flavivirus that infects animals including humans. In Europe, the TBE virus circulates enzootically in natural foci among ticks and rodent hosts. The abundance of ticks depends on the abundance of rodent hosts, which in turn depends on the availability of food resources, such as tree seeds. Trees can exhibit large inter-annual fluctuations in seed production (masting), which influences the abundance of rodents the following year, and the abundance of nymphal ticks two years later. Thus, the biology of this system predicts a 2-year time lag between masting and the incidence of tick-borne diseases such as TBE. As airborne pollen abundance is related to masting, we investigated whether inter-annual variation in pollen load could be directly correlated with inter-annual variation in the incidence of TBE in human populations with a 2-year time lag. We focused our study on the province of Trento (northern Italy), where 206 TBE cases were notified between 1992 and 2020. We tested the relationship between TBE incidence and pollen load collected from 1989 to 2020 for 7 different tree species common in our study area. Through univariate analysis we found that the pollen quantities recorded two years prior for two tree species, hop-hornbeam (Ostrya carpinifolia) and downy oak (Quercus pubescens), were positively correlated with TBE emergence (R² = 0.2) while a multivariate model with both tree species better explained the variation in annual TBE incidence (R² = 0.34). To the best of our knowledge, this is the first attempt at quantifying the correlation between pollen quantities and the incidence of TBE in human populations. As pollen loads are collected by widespread aerobiological networks using standardized procedures, our study could be easily replicated to test their potential as early warning system for TBE and other tick-borne diseases.

Reproductive collapse in European beech results from declining pollination efficiency in large trees

Climate 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.

Pollen production of downy birch (Betula pubescens Ehrh.) along an altitudinal gradient in the European Alps

High-altitude environments are highly susceptible to the effects of climate change. Thus, it is crucial to examine and understand the behaviour of specific plant traits along altitudinal gradients, which offer a real-life laboratory for analysing future impacts of climate change. The available information on how pollen production varies at different altitudes in mountainous areas is limited. In this study, we investigated pollen production of 17 birch (Betula pubescens Ehrh.) individuals along an altitudinal gradient in the European Alps. We sampled catkins at nine locations in the years 2020-2021 and monitored air temperatures. We investigated how birch pollen, flowers and inflorescences are produced in relation to thermal factors at various elevations. We found that mean pollen production of Betula pubescens Ehrh. varied between 0.4 and 8.3 million pollen grains per catkin. We did not observe any significant relationships between the studied reproductive metrics and altitude. However, minimum temperature of the previous summer was found to be significantly correlated to pollen (r_s = 0.504, p = 0.039), flower (r_s = 0.613, p = 0.009) and catkin (r_s = 0.642, p = 0.005) production per volume unit of crown. Therefore, we suggest that temperature variability even at such small scales is very important for studying the response related to pollen production.

Mechanisms driving interspecific variation in regional synchrony of trees reproduction

Seed production in many plants is characterized by large interannual variation, which is synchronized at subcontinental scales in some species but local in others. The reproductive synchrony affects animal migrations, trophic responses to resource pulses and the planning of management and conservation. Spatial synchrony of reproduction is typically attributed to the Moran effect, but this alone is unable to explain interspecific differences in synchrony. We show that interspecific differences in the conservation of seed production-weather relationships combine with the Moran effect to explain variation in reproductive synchrony. Conservative timing of weather cues that trigger masting allows populations to be synchronized at distances >1000 km. Conversely, if populations respond to variable weather signals, synchrony cannot be achieved. Our study shows that species vary in the extent to which their weather cueing is spatiotemporally conserved, with important consequences, including an interspecific variation of masting vulnerability to climate change.

Mast seeding: Study of oak mechanisms carries wider lessons

Variable acorn crops in oaks were thought to reflect variable pollination success, but a new study shows local climates determine whether pollination or flower production drives acorn crops. This affects forest regeneration under climate change, and cautions against dichotomous summaries of biological phenomena.

Oak masting drivers vary between populations depending on their climatic environments

Large interannual variation in seed production, called masting, is very common in wind-pollinated tree populations and has profound implications for the dynamics of forest ecosystems and the epidemiology of certain human diseases.^1,2,3,4,5 Comparing the reproductive characteristics of populations established in climatically contrasting environments would provide powerful insight into masting mechanisms, but the required data are extremely scarce. We built a database from an unprecedented fine-scale 8-year survey of 150 sessile oak trees (Quercus petraea) from 15 populations distributed over a broad climatic gradient, including individual recordings of annual flowering effort, fruiting rate, and fruit production. Although oak masting was previously considered to depend mainly on fruiting rate variations,^6,7 we show that the female flowering effort is highly variable from year to year and explains most of the fruiting dynamics in two-thirds of the populations. What drives masting was found to differ among populations living under various climates. In soft-climate populations, the fruiting rate increases initially strongly with the flowering effort, and the intensity of masting results mainly from the flowering synchrony level between individuals. By contrast, the fruiting rate of harsh-climate populations depends mainly on spring weather, which ensures intense masting regardless of the flowering synchronization level. Our work highlights the need for jointly measuring flowering effort and fruit production to decipher the diversity of masting mechanisms among populations. Accounting for such diversity will be decisive in proposing accurate, and possibly contrasted, scenarios about future reproductive patterns of perennial plants with ongoing climate change and their numerous cascading effects.

Large investment of stored nitrogen and phosphorus in female cones is consistent with infrequent reproduction events of Pinus koraiensis, a high value woody oil crop in Northeast Asia

Pinus koraiensis is famous for its high-quality timber production all the way and is much more famous for its high value health-care nut oil production potential since 1990's, but the less understanding of its reproduction biology seriously hindered its nut productivity increase. Exploring the effects of reproduction on nutrient uptake, allocation and storage help to understand and modify reproduction patterns in masting species and high nut yield cultivar selection and breeding. Here, we compared seasonality in growth and in nitrogen ([N]) and phosphorus ([P]) concentrations in needles, branches and cones of reproductive (cone-bearing) and vegetative branches (having no cones) of P. koraiensis during a masting year. The growth of one- and two-year-old reproductive branches was significantly higher than that of vegetative branches. Needle, phloem and xylem [N] and [P] were lower in reproductive branches than in vegetative branches, although the extent and significance of the differences between branch types varied across dates. [N] and [P] in most tissues were high in spring, decreased during summer, and then recovered by the end of the growing season. Overall, [N] and [P] were highest in needles, lowest in the xylem and intermediate in the phloem. More than half of the N (73.5%) and P (51.6%) content in reproductive branches were allocated to cones. There was a positive correlation between cone number and N and P content in needles (R² = 0.64, R² = 0.73) and twigs (R² = 0.65, R² = 0.62) of two-year-old reproductive branches. High nutrient sink strength of cones and vegetative tissues of reproductive branches suggested that customized fertilization practices can help improve crop yield in Pinus koraiensis.

El Niño-Southern Oscillation affects the species-level temporal variation in seed and leaf fall in a mixed temperate forest

El Niño-Southern Oscillation (ENSO), the variation between anomalously cold (La Niña) and warm conditions (El Niño), is one of the most prominent large-scale climate patterns with worldwide effects. Elevated seed and leaf fall has been found at the positive phase of ENSO (El Niño) in tropical forests. However, how seed and leaf fall respond to ENSO at species level is understudied, especially in temperate forests. In this study, we monitored seed and leaf fall at the species-level at 150 points across a 25-ha temperate forest in northeastern China over a span of 12 years. Using time series and wavelet analyses, we assessed three hypotheses: 1) temperate tree species' seed and leaf fall are strongly, but differently, correlated with ENSO and, 2) community synchrony in seed and leaf occurred both at seasonal and ENSO scales; finally, 3) local climatic modulated the effects of ENSO on seed and leaf fall. We found that ENSO was significantly correlated with seed and leaf fall of all species, although correlation strength varied across species (r = 0.206-0.658). Specifically, ENSO indices (ENSO12 or ENSO34) accounted for the most variation in seed and leaf fall of Acer pseudo-sieboldianum (40 % and 34 %, respectively) and ranged 4 %-31 % in all other species. Leaf fall was synchronous with ENSO cycles with a period of 2-7 years, but community synchrony of seed fall was only detected at seasonal scales. ENSO influenced seed fall of Fraxinus mandshurica and Tilla amurensis by mediating rainfall and relative humidity, respectively, highlighting the interactive effects of local climate and ENSO. Our findings highlight the potential effects of ENSO on ecosystems outside of tropical regions and improve our ability to predict regeneration dynamics and nutrient cycling of temperate forests under the context of global change.

AN, Johnson AJ, Govindan B, Baby S. Flower-fruit dynamics, visitor-predator patterns and chemical preferences in the tropical bamboo, Melocanna baccifera

Mast seeding and associated events in Melocanna baccifera, the largest fruit producing bamboo, is an enigma. So far there are no comprehensive accounts on its flowering phenology, fruiting dynamics and animal interactions. In this study, spanning over 13 years (2009 to 2022), we observed eight M. baccifera clumps in JNTBGRI Bambusetum from flowering initiation, fruiting to eventual death. Flowering phenology, floral characteristics, floret types, breeding system, bee visitation, pollination, fruit production and predators were recorded; predation patterns were correlated with fruit chemistry. Flowering duration of clumps ranged from 20 (March 2009-October 2010) to 120 (September 2012-August 2022) months. Bisexual florets are dichogamous and protogynous; and female duration (22-72 h) is many times higher than male duration (2-6 h). The highest ever fruit production for an individual bamboo clump (456.67 Kg) was recorded. Of the total fallen fruits (38371), 38.11% were predated, 43.80% good fruits (no predator hits) and 18.09% immature fruits. A positive correlation between reward (fruits) versus predation was observed, especially in short intervals of high fruit production. Pollen predators (Apis cerana indica, Halictus taprabonae, Braunsapis cupulifera, Trigona iridipennis), fruit predators, ranging from arthropods to mammals, viz., millipede (Spinotarsus colosseus), slug (Mariaella dussumieri), snails (Cryptozona bistrialis, Macrochlamys sp.), borers (Achroia grisella, Blattella germanica), mammals (monkeys Macaca radiata, rats Rattus rattus, porcupine Hystrix indica, wild boar Sus scrofa, palm civet Paradoxurus hermaphroditus), seedling predators (rabbit Lepus nigricollis, deer Axis axis), and insect/pest predators (ants Crematogaster biroi, Oecophylla smaragdina, mantis Euchomenella indica) were identified. Fruit predation is linked to its age and chemistry. Apart from new insights on flowering phenology, breeding system, pollination and fruiting dynamics, this study demonstrates the vibrant interaction between M. baccifera flowers/fruits and visitors/predators, and provides significant leads towards elucidating the cause of rat multiplication and other events associated with its gregarious flowering.

How will global change affect plant reproduction? A framework for mast seeding trends

Forest ecology traditionally focuses on plant growth and survival, leaving seed production as a major demographic process lacking a framework for how it will be affected by global change. Understanding plant reproductive responses to changing climate is complicated by masting, the annually variable seed production synchronized within populations. Predicting trends in masting is crucial, because masting impacts seed predation and pollination enough to override simple trends in mean seed production. Proximate mechanisms of seed production patterns in perennial plants are gathered to identify processes through which masting may be affected by a changing environment. Predicting trends in masting will require understanding the mechanisms that cause predictable seed failure after high-seed years, and the stochastic mechanisms that synchronize individuals in high-seed years.

Global patterns in the predator satiation effect of masting: A meta-analysis

Masting, or synchronous production of large seed crops, is widespread among plants. The predator satiation hypothesis states that masting evolved to overwhelm seed predators with an excess of food. Yet, this popular explanation faced few rigorous tests. We conducted a meta-analysis of studies that related the magnitude of seed production to the intensity of seed predation. Our results validate certain theoretical notions (e.g., that predator satiation is more effective at higher latitudes) but challenge others (e.g., that specialist and generalist consumers differ in the type of functional response to masting). We also found that masting is losing its ability to satiate consumers, probably because global warming affected masting patterns. This shift might considerably impair the reproduction of masting plants.

Predator satiation is the most commonly tested hypothesis that explains the evolutionary advantages of masting. It proposes that masting benefits plant reproduction by reducing the proportion of seed crop that is consumed by predators. This hypothesis is widely accepted, but many theoretical notions about predator satiation have not been subjected to a robust evaluation. To address this issue, we conducted a meta-analysis of studies that quantified seed predation in relation to mast seeding. We found evidence of both numerical (starvation between mast years) and functional (satiation during mast years) response of consumers to masting. These two effects reinforced each other. Masting satiated invertebrate but not vertebrate seed predators. Satiation was more pronounced at higher, temperate, and boreal latitudes, perhaps because masting is more effective in reducing seed losses when plant communities are less diverse. The effectiveness of masting in satiating invertebrate consumers declined over time (1972 to 2018), probably reflecting the impact of climate change on the frequency and intensity of masting. If masting ceases to reduce seed losses, a crucial advantage of this reproductive strategy will be lost, and sustainability of many tree populations will decline.

Does Age, Residency, or Feeding Guild Coupled with a Drought Index Predict Avian Health during Fall Migration?

Birds are good indicators of environmental change and are often studied for responses to climate. Many studies focus on breeding birds, while fewer look at the migration period, which is a critical time for many birds. Birds are more susceptible to unusual climatic events during their migration due to the metabolic stress of long-distance movements. In the fall of 2020, an unusual cold weather event coupled with drought and wildfire smoke led to a large avian mortality event in New Mexico. Later analysis pointed to the mortality being largely due to starvation. This was the impetus for our research. We used 11 years of fall bird banding data from two locations, along with local drought indices, to determine what predicts avian health during the migration period. We used fat score data from over 15,000 individual birds to assess whether drought indices, age, diet, or residency influenced avian health using multiple logistic regression. We found that the probability of positive fat scores decreased as drought severity increased for younger, insectivorous, migratory birds. Insectivores had a higher probability of receiving a fat score greater than zero relative to local drought conditions, which is important, since many North American insectivores are in steep decline. Migratory birds showed a greater response than year-round residents, and older birds showed a lower but significant response compared to hatch-year birds. Our results suggest that migratory insectivores in the southwestern United States may be less resilient to drought-related climate change.

Long-term dynamics of the network structures in seed dispersal associated with fluctuations in bird migration and fruit abundance patterns

In temperate zones, seed-dispersal networks by migratory birds are formed on long time scale. In mid-October from 2005 to 2016, to explore the dynamics of the network structures, we examined interannual variability of fruit abundance, bird migration, and seed-dispersal networks in central Japan. For 12 years, the fruit abundance exhibited a remarkable fluctuation across years, with the number of fruiting plants and matured fruits fluctuating repeatedly every other year, leading to the periodic fluctuations. The abundance of migratory birds was also fluctuated. According to the abundance of fruits and migratory birds, the 12 years was classified into three types: frugivores and fruits were abundant, frugivores were abundant but fruits were scarce, and frugivores were scarce. The seed-dispersal networks were investigated by collecting faeces and vomits of migrants. Of the 6652 samples collected from 15 bird species, 1671 (25.1%) included seeds from 60 plant species. Main dispersers were composed of Turdus pallidus, T. obscurus, and Zosterops japonicus. The network structures were almost nested for 12 years. Specifically, the nested structure was developed in years when fruit abundance was low. GLM analyses showed the abundance of migrants, particularly T. pallidus and T. obscurus, had strong positive effects on nested structure. It may be caused by the fact the two Turdus species were more frequently functioning as generalist dispersers when fruit abundance was lower. Our study suggested fruit abundance and foraging behaviour of frugivores determine the network structures of seed dispersal on long time scale.

Costs and benefits of masting: economies of scale are not reduced by negative density-dependence in seedling survival in Sorbus aucuparia

Masting is a widespread reproductive strategy in plants that helps to reduce seed predation and increase pollination. However, masting can involve costs, notably negative density-dependent (NDD) seedling survival caused by concentrating reproduction in intermittent events. Masting benefits have received widespread attention, but the costs are understudied, which precludes understanding why some plant species have evolved intense masting, while others reproduce regularly. We followed seed production, seed predation (both 13 yr), and seedling recruitment and survival (11 yr) in Sorbus aucuparia. We tested whether NDD in seedling survival after mast years can reduce the benefits of pulsed reproduction that come through predator satiation. Seed predation rates were extreme in our population (mean = 75%), but were reduced by masting. The commonly accepted, but untested, assertion that pulsed recruitment is associated with strong NDD was unsupported. Consequently, the proportion of seedlings that survived their first year increased with fruit production. This provides a rare test of economies of scale beyond the seed stage. Our results provide estimation of the costs of mast seeding, and indicate that these may be lower than expected. Low masting costs, if common, may help explain why masting is such a widespread reproductive strategy throughout the plant kingdom.

Limitations to Propagule Dispersal Will Constrain Postfire Recovery of Plants and Fungi in Western Coniferous Forests

Many forest species are adapted to long-interval, high-severity fires, but the intervals between severe fires are decreasing with changes in climate, land use, and biological invasions. Although the effects of changing fire regimes on some important recovery processes have previously been considered, the consequences for the dispersal of propagules (plant seeds and fungal spores) in forest communities have not. We characterize three mechanisms by which changing fire regimes disrupt propagule dispersal in mesic temperate, boreal, and high-elevation forests: reduced abundance and altered spatial distributions of propagule source populations, less effective dispersal of propagules by wind, and altered behavior of animal dispersers and propagule predators. We consider how disruptions to propagule dispersal may interact with other factors that are also influenced by fire regime change, potentially increasing risk of forest conversion. Finally, we highlight urgent research topics regarding how dispersal limitation may shape twenty-first century forest recovery after stand-replacing fire.

Natural disturbances and masting: from mechanisms to fitness consequences

The timing of seed production and release is highly relevant for successful plant reproduction. Ecological disturbances, if synchronized with reproductive effort, can increase the chances of seeds and seedlings to germinate and establish. This can be especially true under variable and synchronous seed production (masting). Several observational studies have reported worldwide evidence for co-occurrence of disturbances and seed bumper crops in forests. Here, we review the evidence for interaction between disturbances and masting in global plant communities; we highlight feedbacks between these two ecological processes and posit an evolutionary pathway leading to the selection of traits that allow trees to synchronize seed crops with disturbances. Finally, we highlight relevant questions to be tested on the functional and evolutionary relationship between disturbances and masting. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Understanding mast seeding for conservation and land management

Masting, the intermittent and synchronous production of large seed crops, can have profound consequences for plant populations and the food webs that are built on their seeds. For centuries, people have recorded mast crops because of their importance in managing wildlife populations. In the past 30 years, we have begun to recognize the importance of masting in conserving and managing many other aspects of the environment: promoting the regeneration of forests following fire or other disturbance, conserving rare plants, conscientiously developing the use of edible seeds as non-timber forest products, coping with the consequences of extinctions on seed dispersal, reducing the impacts of plant invasions with biological control, suppressing zoonotic diseases and preventing depredation of endemic fauna. We summarize current instances and future possibilities of a broad set of applications of masting. By exploring in detail several case studies, we develop new perspectives on how solutions to pressing conservation and land management problems may benefit by better understanding the dynamics of seed production. A lesson common to these examples is that masting can be used to time management, and often, to do this effectively, we need models that explicitly forecast masting and the dynamics of seed-eating animals into the near-term future. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

The effects of ENSO and the North American monsoon on mast seeding in two Rocky Mountain conifer species

We aimed to disentangle the patterns of synchronous and variable cone production (i.e. masting) and its relationship to climate in two conifer species native to dry forests of western North America. We used cone abscission scars to reconstruct ca 15 years of recent cone production in Pinus edulis and Pinus ponderosa, and used redundancy analysis to relate time series of annual cone production to climate indices describing the North American monsoon and the El Niño Southern Oscillation (ENSO). We show that the sensitivity to climate and resulting synchrony in cone production varies substantially between species. Cone production among populations of P. edulis was much more spatially synchronous and more closely related to large-scale modes of climate variability than among populations of P. ponderosa. Large-scale synchrony in P. edulis cone production was associated with the North American monsoon and we identified a dipole pattern of regional cone production associated with ENSO phase. In P. ponderosa, these climate indices were not strongly associated with cone production, resulting in asynchronous masting patterns among populations. This study helps frame our understanding of mast seeding as a life-history strategy and has implications for our ability to forecast mast years in these species. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Delayed fertilization facilitates flowering time diversity in Fagaceae

Fagaceae includes typical masting species that exhibit highly synchronized and fluctuating acorn production. Fagaceae shows an interesting feature in that fertilization is delayed by several weeks to more than 1 year after pollination. Although delayed fertilization was recorded over a century ago, the evolutionary advantage of delayed fertilization is still poorly understood. Here, we present a new hypothesis that delayed fertilization facilitates temporal niche differentiation via non-overlapping flowering times among species. Comparing flowering and fruiting times in 228 species from five genera in Fagaceae, we first show that there is a close association between a wider spread of flowering times and the likelihood of a 2-year fruiting habit in which there is a long delay from pollination to fertilization. To study the coevolution of flowering time and delayed fertilization, we developed a mathematical model that incorporates the effects of competition for pollinators, seed predator satiation and unfavourable season for reproduction on fitness. The model shows that delayed fertilization facilitates the diversification of flowering time in a population, which is advantageous for animal-pollinated trees that compete over pollinators. Our new hypothesis about the coevolution of delayed fertilization and flowering time will provide new insight into the evolution of masting. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

The ecology and evolution of synchronized reproduction in long-lived plants

Populations of many long-lived plants exhibit spatially synchronized seed production that varies extensively over time, so that seed production in some years is much higher than on average, while in others, it is much lower or absent. This phenomenon termed masting or mast seeding has important consequences for plant reproductive success, ecosystem dynamics and plant-human interactions. Inspired by recent advances in the field, this special issue presents a series of articles that advance the current understanding of the ecology and evolution of masting. To provide a broad overview, we reflect on the state-of-the-art of masting research in terms of underlying proximate mechanisms, ontogeny, adaptations, phylogeny and applications to conservation. While the mechanistic drivers and fitness consequences of masting have received most attention, the evolutionary history, ontogenetic trajectory and applications to plant-human interactions are poorly understood. With increased availability of long-term datasets across broader geographical and taxonomic scales, as well as advances in molecular approaches, we expect that many mysteries of masting will be solved soon. The increased understanding of this global phenomenon will provide the foundation for predictive modelling of seed crops, which will improve our ability to manage forests and agricultural fruit and nut crops in the Anthropocene. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

A brief history of masting research

Although it has long been recognized that seed production by many forest trees varies greatly from year to year, masting (along with 'mast fruiting', 'mast seeding' and 'masting behaviour') as a concept referring to such variability is a relatively recent development. Here, I provide a brief history of masting research, highlighting some of the early contributions by foresters, zoologists and others that paved the way for the burgeoning number of studies currently being conducted by researchers around the world. Of particular current interest is work attempting to understand the proximate mechanisms, evolutionary drivers and community effects of this important ecological phenomenon as well as the ways that climate change may influence masting behaviour in the future. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Fire history and weather interact to determine extent and synchrony of mast-seeding in rhizomatous scrub oaks of Florida

In disturbance-prone ecosystems, fitness consequences of plant reproductive strategies are often determined by the relative timing of seed production and disturbance events, but the role of disturbances as proximate drivers of seed production has been overlooked. We use long-term data on seed production in Quercus chapmanii, Q. geminata and Q. inopina, rhizomatous oaks found in south central Florida's oak scrub, to investigate the role of fire history and its interaction with weather in shaping acorn production and its synchrony. Acorn production increased with the time since last fire, combined with additive or interactive effects of spring precipitation (+) or drought (-). Furthermore, multiple matrix regression models revealed that ramet pairs with shared fire history were more synchronous in seed production than ones that burned in different years. Long-term trends suggest that increasingly drier spring weather, in interaction with fire frequency, may drive a decline of seed production. Such declines could affect the community of acorn-reliant vertebrates in the Florida scrub, including endangered Florida scrub-jays (Aphelocoma coerulescens). These results illustrate that fire can function as a proximate driver of seed production in mast-seeding species, highlighting the increasingly recognized importance of interactions among reproductive strategies and disturbance regimes in structuring plant populations and communities. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

By wind or wing: pollination syndromes and alternate bearing in horticultural systems

Cyclical fluctuations in reproductive output are widespread among perennial plants, from multi-year masting cycles in forest trees to alternate bearing in horticultural crops. In natural systems, ecological drivers such as climate and pollen limitation can result in synchrony among plants. Agricultural practices are generally assumed to outweigh ecological drivers that might synchronize alternate-bearing individuals, but this assumption has not been rigorously assessed and little is known about the role of pollen limitation as a driver of synchrony in alternate-bearing crops. We tested whether alternate-bearing perennial crops show signs of alternate bearing at a national scale and whether the magnitude of national-scale alternate bearing differs across pollination syndromes. We analysed the Food and Agriculture Organization of the United Nations time series (1961-2018) of national crop yields across the top-producing countries of 27 alternate-bearing taxa, 6 wind-pollinated and 21 insect-pollinated. Alternate bearing was common in these national data and more pronounced in wind-pollinated taxa, which exhibited a more negative lag-1 autocorrelation and a higher coefficient of variation (CV). We highlight the mutual benefits of integrating ecological theory and agricultural data for (i) advancing our understanding of perennial plant reproduction across time, space and taxa, and (ii) promoting stable farmer livelihoods and global food supply. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.