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.

Mysterious Bamboo flowering phenomenon: A literature review and new perspectives

Bamboo, a globally distributed non-timber forest resource, plays a critical role in local ecosystems and economies. Despite its significance, the understanding of bamboo's long and unpredictable flowering cycles remains limited. Our bibliometric analysis of bamboo flowering-related literature from the Web of Science database reveals an initial focus on regeneration studies, with a recent trend shifting towards microscopic and molecular perspectives. Furthermore, our narrative review emphasizes the importance of considering factors such as the proportion of flowering culms and the duration of flowering in classifying bamboo flowering phenomena. While numerous studies have endorsed the predator saturation hypothesis as a suitable explanation for the synchronicity of bamboo flowering, no existing theory explains bamboo's prolonged flowering cycles. We propose a new natural selection hypothesis as a potential explanation for these extraordinary cycles, underscoring the need for further research in this area. Despite the substantial volume of data accumulated on bamboo flowering, these resources have not been fully exploited in recent research. Future studies would benefit from more comprehensive data collection methods, encompassing field observations, satellite remote sensing data, and omics data. The convergence of traditional ecological studies with molecular techniques may pave the way for significant advancements in bamboo flowering research.

Does monocarpic Phyllostachys nigra var. henonis regenerate after flowering in Japan? Insights from 3 years of observation after flowering

Phyllostachys nigra var. henonis, a monocarpic bamboo with a 120-year flowering interval, is next predicted to flower in Japan in the 2020s. Because a huge area of the country is presently covered by stands of this species, post-flowering dieback of these stands and ensuing drastic changes in land cover may cause serious social and/or environmental problems. No study on the regeneration of this bamboo species was conducted during the last flowering event in the 1900s, and the regeneration process of this species is thus still unknown. In 2020, we encountered a localized flowering of P. nigra var. henonis in Japan and used this discovery as a rare opportunity to study the initial regeneration process of the species. Over 3 years, more than 80% of culms in the study site bloomed, but no seed was produced. In addition, no established seedlings were located. These facts strongly suggest that P. nigra var. henonis lacks the ability to produce seeds and cannot undergo sexual regeneration. Some bamboo culms were produced after flowering but died within 1 year of emergence. Small, weak culms (dwarf ramets) also appeared after flowering, but most died within 1 year as well. Three years after flowering, all culms had died, with no sign of regeneration detected. According to our 3 years of observation, this bamboo appears to be hard to regenerate-an idea completely contradicted by the fact that this species has long persisted in Japan. We thus considered other possible regeneration modes for P. nigra var. henonis.

The selective advantage of mast flowering in Veratrum album subsp. oxysepalum: Implications of the predator satiation hypothesis

PREMISE

Synchronous, highly variable flower or seed production among years within a population (i.e., masting) has been reported in numerous perennial plants. Although masting provides ecological advantages such as enhancing pollination efficiency and/or escape from predator attack, little is known about the degree of these advantages and variations in masting behavior among populations of conspecific plants.

METHODS

We determined flowering ramet density and reproductive success (fruit-set success and herbivorous damage) of a perennial herb, Veratrum album subsp. oxysepalum, across six lowland and six alpine populations in northern Japan during 2-3 years. We then analyzed the relationship between floral density and reproductive success to assess the ecological significance of mast flowering. Flowering intervals of individual plants were estimated by counting annual scars on rhizomes.

RESULTS

Most populations had mast flowering, but the intervals between flowering for individual plants were shorter in the alpine populations than in the lowland populations. Floral damage by stem borers (dipteran larvae) and seed predation by lepidopteran larvae were intense in the lowland populations. Seed production of individual ramets increased with higher floral density owing to the effective avoidance of floral-stem damage and seed predation. Although stem borers were absent in the alpine habitat, seed predation decreased with higher floral density also in the alpine populations. Pollination success was independent of floral density in both of the alpine and lowland populations.

CONCLUSIONS

These results strongly support the predator satiation hypothesis for mast flowering by this species.

Why are some plant species missing from restorations? A diagnostic tool for temperate grassland ecosystems

The U.N. Decade on Ecosystem Restoration aims to accelerate actions to prevent, halt, and reverse the degradation of ecosystems, and re-establish ecosystem functioning and species diversity. The practice of ecological restoration has made great progress in recent decades, as has recognition of the importance of species diversity to maintaining the long-term stability and functioning of restored ecosystems. Restorations may also focus on specific species to fulfill needed functions, such as supporting dependent wildlife or mitigating extinction risk. Yet even in the most carefully planned and managed restoration, target species may fail to germinate, establish, or persist. To support the successful reintroduction of ecologically and culturally important plant species with an emphasis on temperate grasslands, we developed a tool to diagnose common causes of missing species, focusing on four major categories of filters, or factors: genetic, biotic, abiotic, and planning & land management. Through a review of the scientific literature, we propose a series of diagnostic tests to identify potential causes of failure to restore target species, and treatments that could improve future outcomes. This practical diagnostic tool is meant to strengthen collaboration between restoration practitioners and researchers on diagnosing and treating causes of missing species in order to effectively restore them.

Diverse ecological functions and the convergent evolution of grass awns

The awn of grasses is a long, conspicuous outgrowth of the floral bracts in a grass spikelet. It is known to impact agricultural yield, but we know little about its broader ecological function, nor the selective forces that lead to its evolution. Grass awns are phenotypically diverse across the extant ~12,000 species of Poaceae. Awns have been lost and gained repeatedly over evolutionary time, between and within lineages, suggesting that they could be under selection and might provide adaptive benefit in some environments. Despite the phylogenetic context, we know of no studies that have tested whether the origin of awns correlates with putative selective forces on their form and function. Presence or absence of awns is not plastic; rather, heritability is high. The awns of grasses often are suggested as adaptations for dispersal, and most experimental work has been aimed at testing this hypothesis. Proposed dispersal functions include soil burial, epizoochory, and aerial orientation. Awns may also protect the seed from drought, herbivores, or fire by helping it become buried in soil. We do not fully understand the fitness or nutrient costs of awn production, but in some species awns function in photosynthesis, providing carbon to the seed. Here we show that awns likely provide an adaptive advantage, but argue that studies on awn function have lacked critical phylogenetic information to demonstrate adaptive convergent evolution, are taxonomically biased, and often lack clear alternative hypotheses.

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.

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.

Molecular control of the floral transition in the mast seeding plant Celmisia lyallii (Asteraceae)

Mast flowering (or masting) is synchronous, highly variable flowering among years in populations of perennial plants. Despite having widespread consequences for seed consumers, endangered fauna and human health, masting is hard to predict. While observational studies show links to various weather patterns in different plant species, the mechanism(s) underpinning the regulation of masting is still not fully explained. We studied floral induction in Celmisia lyallii (Asteraceae), a mast flowering herbaceous alpine perennial, comparing gene expression in flowering and nonflowering plants. We performed translocation experiments to induce the floral transition in C. lyallii plants followed by both global and targeted expression analysis of flowering-pathway genes. Differential expression analysis showed elevated expression of ClSOC1 and ClmiR172 (promoters of flowering) in leaves of plants that subsequently flowered, in contrast to elevated expression of ClAFT and ClTOE1 (repressors of flowering) in leaves of plants that did not flower. The warm summer conditions that promoted flowering led to differential regulation of age and hormonal pathway genes, including ClmiR172 and ClGA20ox2, known to repress the expression of floral repressors and permit flowering. Upregulated expression of epigenetic modifiers of floral promoters also suggests that plants may maintain a novel "summer memory" across years to induce flowering. These results provide a basic mechanistic understanding of floral induction in masting plants and evidence of their ability to imprint various environmental cues to synchronize flowering, allowing us to better predict masting events under climate change.

Does masting scale with plant size? High reproductive variability and low synchrony in small and unproductive individuals

BACKGROUND AND AIMS

In a range of plant species, the distribution of individual mean fecundity is skewed and dominated by a few highly fecund individuals. Larger plants produce greater seed crops, but the exact nature of the relationship between size and reproductive patterns is poorly understood. This is especially clear in plants that reproduce by exhibiting synchronized quasi-periodic variation in fruit production, a process called masting.

METHODS

We investigated covariation of plant size and fecundity with individual-plant-level masting patterns and seed predation in 12 mast-seeding species: Pinus pinea, Astragalus scaphoides, Sorbus aucuparia, Quercus ilex, Q. humilis, Q. rubra, Q. alba, Q. montana, Chionochloa pallens, C. macra, Celmisia lyallii and Phormium tenax.

KEY RESULTS

Fecundity was non-linearly related to masting patterns. Small and unproductive plants frequently failed to produce any seeds, which elevated their annual variation and decreased synchrony. Above a low fecundity threshold, plants had similar variability and synchrony, regardless of their size and productivity.

CONCLUSIONS

Our study shows that within-species variation in masting patterns is correlated with variation in fecundity, which in turn is related to plant size. Low synchrony of low-fertility plants shows that the failure years were idiosyncratic to each small plant, which in turn implies that the small plants fail to reproduce because of plant-specific factors (e.g. internal resource limits). Thus, the behaviour of these sub-producers is apparently the result of trade-offs in resource allocation and environmental limits with which the small plants cannot cope. Plant size and especially fecundity and propensity for mast failure years play a major role in determining the variability and synchrony of reproduction in plants.

Climate Change Strengthens Selection for Mast Seeding in European Beech

Climate change is altering patterns of seed production worldwide [1-4], but the potential for evolutionary responses to these changes is poorly understood. Masting (synchronous, annually variable seed production by plant populations) is selectively beneficial through economies of scale that decrease the cost of reproduction per surviving offspring [5-7]. Masting is particularly widespread in temperate trees [8, 9] impacting food webs, macronutrient cycling, carbon storage, and human disease risk [10-12], so understanding its response to climate change is important. Here, we analyze inter-individual variability in plant reproductive patterns and two economies of scale-predator satiation and pollination efficiency-and document how natural selection acting upon them favors masting. Four decades of observations for European beech (Fagus sylvatica) show that predator satiation and pollination efficiency select for individuals with higher inter-annual variability of reproduction and higher reproductive synchrony between individuals. This result confirms the long-standing theory that masting, a population-level phenomenon, is generated by selection on individuals. Furthermore, recent climate-driven increases in mean seed production have increased selection pressure from seed predators but not from pollination efficiency. Natural selection is thus acting to restore the fitness benefits of masting, which have previously decreased under a warming climate [13]. However, selection will likely take far longer (centuries) than climate warming (decades), so in the short-term, tree reproduction will be reduced because masting has become less effective at satiating seed predators. Over the long-term, evolutionary responses to climate change could potentially increase inter-annual variability of seed production of masting species.

Rapid aggregative and reproductive responses of weevils to masting of North American oaks counteract predator satiation

The predator satiation hypothesis posits that masting helps plants escape seed predation through starvation of predators in lean years, followed by satiation of predators in mast years. Importantly, successful satiation requires sufficiently delayed bottom-up effects of seed availability on seed consumers. However, some seed consumers may be capable of quick aggregative and reproductive responses to masting, which may jeopardize positive density dependence of seed survival. We used a 17-yr data set on seed production and insect (Curculio weevils) infestation of three North American oaks species (northern red Quercus rubra, white Q. alba, and chestnut oak Q. montana) to test predictions of the predation satiation hypothesis. Furthermore, we tested for the unlagged numerical response of Curculio to acorn production. We found that masting results in a bottom-up effect on the insect population; both through increased reproductive output and aggregation at seed-rich trees. Consequently, mast seeding in two out of three studied oaks (white and chestnut oak) did not help to escape insect seed predation, whereas, in the red oak, the escape depended on the synchronization of mast crops within the population. Bottom-up effects of masting on seed consumer populations are assumed to be delayed, and therefore to have negligible effects on seed survival in mast years. Our research suggests that insect populations may be able to mount rapid reproductive and aggregative responses when seed availability increases, possibly hindering satiation effects of masting. Many insect species are able to quickly benefit from pulsed resources, making mechanisms described here potentially relevant in many other systems.

Effectiveness of predator satiation in masting oaks is negatively affected by conspecific density

Variation in seed availability shapes plant communities, and is strongly affected by seed predation. In some plant species, temporal variation in seed production is especially high and synchronized over large areas, which is called ‘mast seeding’. One selective advantage of this phenomenon is predator satiation which posits that masting helps plants escape seed predation through starvation of predators in lean years, and satiation in mast years. However, even though seed predation can be predicted to have a strong spatial component and depend on plant densities, whether the effectiveness of predator satiation in masting plants changes according to the Janzen-Connell effect has been barely investigated. We studied, over an 8-year period, the seed production, the spatiotemporal patters of weevil seed predation, and the abundance of adult weevils in a holm oak (Quercus ilex) population that consists of trees interspersed at patches covering a continuum of conspecific density. Isolated oaks effectively satiate predators, but this is trumped by increasing conspecific plant density. Lack of predator satiation in trees growing in dense patches was caused by re-distribution of insects among plants that likely attenuated them against food shortage in lean years, and changed the type of weevil functional response from type II in isolated trees to type III in trees growing in dense patches. This study provides the first empirical evaluation of the notion that masting and predator satiation should be more important in populations that start to dominate their communities, and is consistent with the observation that masting is less frequent and less intense in diverse forests.

Masting in wind-pollinated trees: system-specific roles of weather and pollination dynamics in driving seed production

Masting, the highly variable production of synchronized large seed crops, is a common reproductive strategy in plant populations. In wind-pollinated trees, flowering and pollination dynamics are hypothesized to provide the mechanistic link for the well-known relationship between weather and population-level seed production. Several hypotheses make predictions about the effect of weather on annual pollination success. The pollen coupling hypothesis predicts that weather and plant resources drive the flowering effort of trees, which directly translates into the size of seed crops through efficient pollination. In contrast, the pollination Moran effect hypothesis predicts that weather affects pollination efficiency, leading to occasional bumper crops. Furthermore, the recently formulated phenology synchrony hypothesis predicts that Moran effects can arise because of weather effects on flowering synchrony, which, in turn, drives pollination efficiency. We investigated the relationship between weather, airborne pollen, and seed production in common European trees, two oak species (Quercus petraea and Q. robur) and beech (Fagus sylvatica) with a 19-yr data set from three sites in Poland. Our results show that warm summers preceding flowering correlated with high pollen abundance and warm springs resulted in short pollen seasons (i.e., high flowering synchrony) for all three species. Pollen abundance was the best predictor for seed crops in beech, as predicted under pollen coupling. In oaks, short pollen seasons, rather than pollen abundance, correlated with large seed crops, providing support for the pollination Moran effect and phenology synchrony hypotheses. Fundamentally different mechanisms may therefore drive masting in species of the family Fagacae.

Resource limitation underlying multiple masting models makes mast seeding sensitive to future climate change

Mechanistic models can help resolve controversy over the responses of mast seeding plants to future environmental change. We evaluate drivers of mast seeding by: developing and validating a new mechanistic resource-based model of mast seeding using four 40-yr Chionochloa (snow tussock) datasets; and comparing the performance of competing empirically-based statistical models, that aim to approximate the mechanisms underlying mast seeding, in explaining simulated and observed data. Our mechanistic model explained 90-99% of the variation in Chionochloa flowering, with higher rates of stored resource mobilisation and lower probability of climatic induction of flowering occurring at lower fertility sites. Inter-annual variation in floral induction and the degree to which seeding is resource-limited explained shifts in the relative performance of different empirical models fitted to data simulated from the mechanistic model. Empirical models explicitly capturing the interaction between the floral induction cue and internal resource state underlying the resource-limited induction mechanism had > 8.7× the statistical support of alternatives when fitted to Chionochloa datasets. We find support for resource-limited floral induction with multiple empirical models consistent with this same mechanism. As both resource acquisition and flowering cues are climate sensitive, we expect climate change to impact upon patterns of mast seeding.

Evolutionary drivers of mast-seeding in a long-lived desert shrub

PREMISE OF THE STUDY

The evolutionary drivers and proximal regulators of mast-seeding are well understood for species of mesic environments, but how these regulators interact with high spatial and interannual variability in growing-season precipitation for a masting species in a desert environment has never been examined.

METHOD

We followed flowering and seed production in 16 populations of the North American desert shrub blackbrush (Coleogyne ramosissima) from contrasting environments across its range over an 11-year period to determine patterns of interannual reproductive output variation.

KEY RESULT

Patterns of reproductive output in blackbrush did not track current growing season precipitation, but instead were regulated by prior-year weather cues. The strength of the response to the masting cue depended on habitat quality, with higher mean reproductive output, shorter intervals between years of high seed production, and lower CVp at more favorable sites. Wind pollination efficiency was demonstrated to be an important evolutionary driver of masting in blackbrush, and satiation of heteromyid seed predator-dispersers was supported as an evolutionary driver based on earlier studies.

CONCLUSIONS

Both the evolutionary drivers and proximal regulators of masting in blackbrush are similar to those demonstrated for masting species of mesic environments. Relatively low synchrony across populations in response to regional masting cues occurs at least partly because prior-year environmental cues can trigger masting efforts in years with resource limitation due to suboptimal precipitation, especially in more xeric low-elevation habitats.

Potential role of masting by introduced bamboos in deer mice (Peromyscus maniculatus) population irruptions holds public health consequences

We hypothesized that the ongoing naturalization of frost/shade tolerant Asian bamboos in North America could cause environmental consequences involving introduced bamboos, native rodents and ultimately humans. More specifically, we asked whether the eventual masting by an abundant leptomorphic (“running”) bamboo within Pacific Northwest coniferous forests could produce a temporary surfeit of food capable of driving a population irruption of a common native seed predator, the deer mouse (Peromyscus maniculatus), a hantavirus carrier. Single-choice and cafeteria-style feeding trials were conducted for deer mice with seeds of two bamboo species (Bambusa distegia and Yushania brevipaniculata), wheat, Pinus ponderosa, and native mixed diets compared to rodent laboratory feed. Adult deer mice consumed bamboo seeds as readily as they consumed native seeds. In the cafeteria-style feeding trials, Y. brevipaniculata seeds were consumed at the same rate as native seeds but more frequently than wheat seeds or rodent laboratory feed. Females produced a median litter of 4 pups on a bamboo diet. Given the ability of deer mice to reproduce frequently whenever food is abundant, we employed our feeding trial results in a modified Rosenzweig-MacArthur consumer-resource model to project the population-level response of deer mice to a suddenly available/rapidly depleted supply of bamboo seeds. The simulations predict rodent population irruptions and declines similar to reported cycles involving Asian and South American rodents but unprecedented in deer mice. Following depletion of a mast seed supply, the incidence of Sin Nombre Virus (SNV) transmission to humans could subsequently rise with dispersal of the peridomestic deer mice into nearby human settlements seeking food.

Large-scale spatial synchrony and cross-synchrony in acorn production by two California oaks

Seed production that varies greatly from year to year, known as "masting" or "mast-fruiting" behavior, is a population-level phenomenon known to exhibit geographic synchrony extending, at least in some cases, hundreds of kilometers. The two main nonexclusive hypotheses for the driver of such geographically extensive synchrony are (1) environmental factors (the Moran effect), and (2) the mutual dependence of trees on outcrossed pollen (pollen coupling). We tested 10 predictions relevant to these two hypotheses using 18 years of acorn production data on two species of California oaks. Data were obtained across the entire ranges of the two species at 12 sites (10 for each species) separated by up to 745 km. In general, our results provided strong support for the importance of the Moran effect as a driver of spatial synchrony in and between these two species. Particularly compelling was evidence of close concordance between spatial synchrony in acorn production and key environmental factors extending over the range of both species and significant spatial cross-synchrony between the two species, despite considerable differences in their geographical ecology. Because oaks are monoecious, female flowers are not necessarily related to pollen production, and thus, our tests do not address the role of pollen coupling in bisexual species where pollen and flower production are necessarily correlated. For the oak species considered here, however, the Moran effect is a key driver of large-scale spatial synchrony in acorn production.

Empty seeds are not always bad: simultaneous effect of seed emptiness and masting on animal seed predation

Seed masting and production of empty seeds have often been considered independently as different strategies to reduce seed predation by animals. Here, we integrate both phenomena within the whole assemblage of seed predators (both pre and post-dispersal) and in two contrasting microsites (open vs. sheltered) to improve our understanding of the factors controlling seed predation in a wind-dispersed tree (Ulmus laevis). In years with larger crop sizes more avian seed predators were attracted with an increase in the proportion of full seeds predated on the ground. However, for abundant crops, the presence of empty seeds decreased the proportion of full seeds predated. Empty seeds remained for a very long period in the tree, making location of full seeds more difficult for pre-dispersal predators and expanding the overall seed drop period at a very low cost (in dry biomass and allocation of C, N and P). Parthenocarpy (non-fertilized seeds) was the main cause of seed emptiness whereas seed abortion was produced in low quantity. These aborted seeds fell prematurely and, thus, could not work as deceptive seeds. A proportion of 50% empty seeds significantly reduced ground seed predation by 26%. However, a high rate of parthenocarpy (beyond 50% empty seeds) did not significantly reduce seed predation in comparison to 50% empty seeds. We also found a high variability and unpredictability in the production of empty seeds, both at tree and population level, making predator deception more effective. Open areas were especially important to facilitate seed survival since rodents (the main post-dispersal predators) consumed seeds mostly under shrub cover. In elm trees parthenocarpy is a common event that might work as an adaptive strategy to reduce seed predation. Masting per se did not apparently reduce the overall proportion of seeds predated in this wind-dispersed tree, but kept great numbers of seeds unconsumed.

Reproductive success of individuals with different fruit production patterns. What does it mean for the predator satiation hypothesis?

The predator satiation hypothesis states that synchronous periodic production of seeds is an adaptive strategy evolved to reduce the pressure of seed predators. The seed production pattern is crucial to the predator satiation hypothesis, but there are few studies documenting the success of individuals that are in synchrony and out of synchrony with the whole population. This study is based on long-term data on seed production of Sorbus aucuparia and specialised pre-dispersal seed predation by Argyresthia conjugella, in a subalpine spruce forest in the Western Carpathians (Poland). At the population level, we tested whether functional and numerical responses of predators to the variation of fruit production operate. At the individual level, we tested whether individuals with higher interannual variability in their own seed crops and higher synchrony with the population have higher percentages of uninfested fruits. The intensity of pre-dispersal seed predation was high (average 70 %; range 19–100 %). There were both functional and numerical responses of predators to the variation of fruit production at the population level. We found that individuals that were expected to be preferred under seed predator pressure had higher reproductive success. With increasing synchrony of fruit production between individual trees and the population, the percentage of infested fruits decreased. There was also a negative relationship between the interannual variation in individual fruit production and the percentage of infested fruits. These results confirm selection for individuals with a masting strategy. However, the population does not seem well adapted to strong seed predation pressure and we suggest that this may be a result of prolonged diapause of A. conjugella.

Mast seeding under increasing drought: results from a long-term data set and from a rainfall exclusion experiment

Mast seeding, the synchronous, highly variable seed production among years, is very common in tree species, but there is no consensus about its main causes and the main environmental factors affecting it. In this study, we first analyze a long-term data set on reproductive and vegetative growth of Quercus ilex in a mediterranean woodland in order to identify the main environmental drivers of interannual variation in flower and seed production and contrast the impact of climate vs. adaptive factors as main causes of masting. Second, we conducted an experiment of rainfall exclusion to evaluate the effects of an increasing drought (simulating predictions of global change models) on both reproductive processes. The annual seed crop was always affected by environmental factors related to the precipitation pattern, these abiotic factors disrupting the fruiting process at different periods of time. Seed production was strongly dependent upon water availability for the plant at initial (spring) and advanced (summer) stages of the acorn maturation cycle, whereas the final step of seed development was negatively affected by the frequency of torrential-rain events. We also found clear evidence that seed masting in the study species is not only regulated by selective endogenous rhythms, but is mainly a physiological response to the variable environment. Our results from the rainfall exclusion experiment corroborated the conclusions obtained from the 26-year fruiting record and demonstrated that the high interannual variation in seed crop was mainly determined by the success in seed development rather than by the flowering effort. Under a global change scenario, it could be expected that the drier conditions predicted by climate models reinforce the negative effects of summer drought on seed production, leading to negative consequences for tree recruitment and forest dynamics.

Why do sex ratio dimorphisms exist in Quercus masting? Evolution of imperfect synchronous reproduction in Monoecious trees

Masting is synchronous intermittent production of seeds in perennial plant populations. Some self-compatible monoecious Quercus species, such as oaks, exhibit sex ratio dimorphism and produce a certain proportion of male flowers, even in a year when no seed set occurs. To investigate sex ratio dimorphism in masting trees, we introduced sexual allocation as an evolutionary trait into the Resource Budget Model and examined the evolution of the sex ratio. Analytical and numerical findings show that (1) perfectly synchronous intermittent reproduction does not evolve; (2) if the fruiting cost of female flowers R(c) is sufficiently large and the pollen limitation beta is intermediate, annual reproduction does not evolve; (3) under conditions (2), sex ratio dimorphism can evolve across a wide region of parameter space; (4) after dimorphism is established, individuals with a female-biased sex ratio receive much more pollen supply from male-biased individuals and tend to show intermittent reproduction with or without synchrony; and (5) dimorphism is maintained with irregular and nearly discontinuous changes of sex ratio. These results suggest that sex ratio dimorphism contributes to improving pollen availability and causes resource depletion and the occurrence of intermittent reproduction in female-biased individuals.

Primary and secondary resource pulses in an alpine ecosystem: snow tussock grass (Chionochloa spp.) flowering and house mouse (Mus musculus) populations in New Zealand

Context. Rodent populations in many parts of the world fluctuate in response to resource pulses generated by periodic high seed production (masting) by forest trees, with cascading effects on predation risk to other forest species. In New Zealand forests, populations of exotic house mice (Mus musculus) irrupt after periodic heavy beech (Nothofagus spp.) seedfall. However, in alpine grasslands, where snow tussock grasses (Chionochloa spp.) also flower and set seeds periodically, little is known about house mouse population dynamics. Aims. Our primary objective was to test for an increase in alpine mouse density following a summer when snow tussocks flowered profusely. We also estimated mouse density in adjacent montane forest over 2 years, and assessed mouse diet, to predict their potential impacts on native species. Methods. Flowering intensity of Chionochloa was assessed by counting flowering tillers on permanent transects (2003–06). Mouse density was estimated with capture–mark–recapture trapping in alpine (2003–07) and forest (2003–04) habitats. Mice were also collected and their stomach contents analysed. Flowering or fruiting of alpine shrubs and herbs, and beech seedfall at forest sites, were also measured. Key results. Chionochloa flowered profusely in austral summer 2005/06. Between autumn (May) and spring (November) 2006, mean alpine mouse density increased from 4 ha–1 to 39 ha–1, then declined to 8 ha–1 by autumn (May 2007). No mice were captured in 768 trap-nights during the following spring (November 2007). Prior to the mouse irruption, mouse density was consistently higher at alpine (0.4–4.0 mice ha–1) than at montane forest (0.02–1.8 mice ha–1) sites (in 2003–04). Alpine mouse diet was dominated by arthropods before mast flowering, and by seeds during it. Conclusions. The density and dynamics of alpine mice in relation to intensive snow-tussock flowering were similar to those in New Zealand beech forest in relation to beech masts. Implications. We predict the timing and duration of periods of heightened predation risk to native alpine fauna, as the result of pulses in mouse density and likely associated pulses in the density of stoats (Mustela erminea), a key exotic predator.

Masting mediated by summer drought reduces acorn predation in Mediterranean oak forests

Temporally variable production of seed crops by perennial plants (masting) has been hypothesized to be a valuable mechanism in the reduction of seed predation by satiating and starving seed consumers. To achieve these benefits, coexisting species subjected to the same predator would benefit from a similar pattern of seeding fluctuation over time that could lead to a reduction in predation at the within-species level. We tested for the existence of an environmental factor enforcing synchrony in acorn production in two sympatric Mediterranean oaks (Quercus ilex and Q. humilis) and the consequences on within-species and between-species acorn predation, by monitoring 15 mixed forests (450 trees) over seven years. Acorn production in Q. ilex and Q. humilis was highly variable among years, with high population variability (CVp) values. The two species exhibited a very different pattern across years in their initial acorn crop size (sum of aborted, depredated, and sound acorns). Nevertheless, interannual differences in summer water stress modified the likelihood of abortion during acorn ripening and enforced within- and, particularly, between-species synchrony and population variability in acorn production. The increase in CVp from initial to mature acorn crop (after summer) accounted for 33% in Q. ilex, 59% in Q. humilis, and 60% in the two species together. Mean yearly acorn pre-dispersal predation by invertebrates was considerably higher in Q. humilis than in Q. ilex. Satiation and starvation of predators was recorded for the two oaks, and this effect was increased by the year-to-year variability in the size of the acorn crop of the two species combined. Moreover, at a longer time scale (over seven years), we observed a significant reduction in the mean proportion of acorns depredated for each oak and the variability in both species' acorn production combined. Therefore, our results demonstrate that similar patterns of seeding fluctuation over time in coexisting species mediated by an environmental cue (summer drought) may contribute to the reduction of the impact of seed predation at a within-species level. Future research should be aimed at addressing whether this process could be a factor assisting in the coexistence of Q. ilex and Q. humilis.

Pollen and water limitation in Astragalus scaphoides, a plant that flowers in alternate years

Mast seeding is common in plant populations, but its causes have rarely been tested experimentally. We tested mechanisms of alternate-year flowering and fruit set in an iteroparous, bee-pollinated, herbaceous plant, Astragalus scaphoides, in semi-arid sagebrush steppe. Patterns of reproduction from 1986 to 1999 indicated that spring precipitation was a cue for synchronous flowering, and that increased pollination in high-flowering years was a fitness advantage of synchrony. We tested these patterns by adding supplemental water and pollen to plants in high- and low-flowering sites and years. Supplemental water had no effect on flowering or seed set, so water is not a proximate cue for reproduction, though it could be important over longer (>3 year) time scales. Supplemental pollination increased fruit set in low- but not high-flowering years, indicating that synchronous flowering increases pollination success. Many shorter-term studies also report increased fruit set after pollen supplementation, but not after resource addition. This pattern may reflect the fact that plants can store and reallocate resources, but not pollen, across multiple years. For animal-pollinated herbs such as these, uniting theories about pollination ecology and mast seeding may promote an understanding of the mechanisms that determine patterns of reproduction over time.

Empirical Models of Pollen Limitation, Resource Acquisition, and Mast Seeding by a Bee‐Pollinated Wildflower

Synchronous mast seeding is increasingly recognized as common in plant populations. Recent theoretical models show that synchronous mast seeding could be a consequence of resource allocation and storage within individual plants, coupled by pollen limitation in low-flowering years. We used long-term population and weather data to parameterize models of flowering based on stored resources and pollen limitation in Astragalus scaphoides, a bee-pollinated plant that flowers in alternate years. We used these models to test whether internal resource dynamics could explain mast seeding in A. scaphoides and, if so, whether synchrony was caused by pollen limitation and/or fluctuations in precipitation. We compared predictions of models that included all combinations of three factors: constant versus precipitation-dependent resource gain, uniform versus heterogeneous resource gain (among individual plants), and resource-dependent versus resource- and pollen-limited fruit set. Pollen limitation and heterogeneous resource gain were necessary and sufficient to explain alternate-year flowering, but precipitation increased the quantitative match between model predictions and flowering dynamics. Together, our results support the importance of density-dependent pollen limitation as an ultimate and proximate cause of mast seeding in A. scaphoides. Precipitation does not act as a direct cue for synchrony in this species but might affect long-term resource gain and fruiting dynamics.

Spatial dynamics of specialist seed predators on synchronized and intermittent seed production of host plants

Masting, the synchronized and intermittent seed production by plant populations, provides highly variable food resources for specialist seed predators. Such a reproductive mode helps minimize seed losses through predator satiation and extinction of seed predator populations. The seed predators can buffer the resource variation through dispersal or extended diapause. We developed a spatially explicit resource-consumer model to understand the effect of masting on specialist seed predators. The masting dynamics were assumed to follow a resource-based model for plant reproduction, and the population dynamics of the predator were represented by a spatially extended Nicholson-Bailey model. The resultant model demonstrated that when host plants reproduce intermittently, seed predator populations go locally extinct, but global persistence of the predator is facilitated by dispersal or extended diapause. Global extinction of the predator resulted when the intermittent reproduction is highly synchronized among plants. An approximate invasion criterion for the predators showed that negative lag-1 autocorrelation in seeding reduces invasibility, and positive lag-1 cross-correlation enhances invasibility. Spatial synchronization in seeding at local scale caused by pollen coupling (or climate forcing) further prevented invasion of the predators. If the predators employed extended diapause, extremely high temporal variability in reproduction was required for plants to evade the predators.

Resource use of insect seed predators during general flowering and seeding events in a Bornean dipterocarp rain forest

Insect seed predators of 24 dipterocarp species (including the genera ot Dipterocarpus, Dryobalanops and Shorea) and five species belonging to the Moraceae, Myrtaceae, Celastraceae and Sapotaceae were investigated. In a tropical lowland dipterocarp forest in Sarawak, Malaysia, these trees produces seeds irregularly by intensely during general flowering and seeding events in 1996 and/or 1998. Dipterocarp seeds were preyed on by 51 insect species (11 families), which were roughly classified into three taxonomic groups: smaller moths (Trotricidae, Pyralidae, Crambidae, Immidae, Sesiidae, and Cosmopterigidae), scolytids (Scolydae) and weevils (Curdulionidae, Apionidae, Anthribidae, and Attelabidae). Although the host-specificity of invertebrate seed predators has been assumed to be high in tropical forests, it was found that the diet ranges of some insect predators were relatively wide and overlapped one another. Most seed predators that were collected in both study years changes their diets between general flowering and seeding events. The results of cluster analyses based on the number of adult of each predator species that emerged from 100 seeds of each tree species, suggested that the dominant species was not consistent, alternating between the two years.

Measuring mast seeding behavior: relationships among population variation, individual variation and synchrony

Mast seeding, or masting, is the variable production of flowers, seeds, or fruit across years more or less synchronously by individuals within a population. A critical issue is the extent to which temporal variation in seed production over a collection of individuals can be viewed as arising from a combination of individual variation and synchrony among individuals. Studies of masting typically quantify such variation in terms of the coefficient of variation (CV). In this paper we examine mathematically how the population CV relates to the mean individual CV and synchrony, concluding that the relationship is a complex one which cannot isolate an overall measure of synchrony, and involves additional factors, principally the number of plants sampled and the mean productivity per plant. Our development suggests some simple approximate relationships of population CV to individual variability, synchrony and the number of individuals. These were found to fit quite well when applied to data from 59 studies which included seed production at the individual level.