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

Masting ontogeny: the largest masting benefits accrue to the largest trees

BACKGROUND AND AIMS

Both plants and animals display considerable variation in their phenotypic traits as they grow. This variation helps organisms to adapt to specific challenges at different stages of development. Masting, the variable and synchronized seed production across years by a population of plants, is a common reproductive strategy in perennial plants that can enhance reproductive efficiency through increasing pollination efficiency and decreasing seed predation. Masting represents a population-level phenomenon generated from individual plant behaviours. While the developmental trajectory of individual plants influences their masting behaviour, the translation of such changes into benefits derived from masting remains unexplored.

METHODS AND KEY RESULTS

We used 43 years of seed production monitoring in European beech (Fagus sylvatica) to address that gap. The largest improvements in reproductive efficiency from masting happen in the largest trees. Masting leads to a 48-fold reduction in seed predation in large trees compared with 28-fold in small trees. Masting yields a 6-fold increase in pollination efficiency in large trees compared with 2.5-fold in small trees. Paradoxically, although the largest trees show the biggest reproductive efficiency benefits from masting, large trees mast less strongly than small trees.

CONCLUSIONS

Apparently suboptimal allocation of effort across years by large plants may be a consequence of anatomical constraints or bet-hedging. Ontogenetic shifts in individual masting behaviour and associated variable benefits have implications for the reproductive potential of plant populations as their age distribution changes, with applications in plant conservation and management.

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.

Community Synchrony in Seed Production is Associated With Trait Similarity and Climate Across North America

Mast seeding, the synchronous and highly variable production of seed crops by perennial plants, is a population-level phenomenon and has cascading effects in ecosystems. Mast seeding studies are typically conducted at the population/species level. Much less is known about synchrony in mast seeding between species because the necessary long-term data are rarely available. To investigate synchrony between species within communities, we used long-term data from seven forest communities in the U.S. Long-Term Ecological Research (LTER) network, ranging from tropical rainforest to boreal forest. We focus on cross-species synchrony and (i) quantify synchrony in reproduction overall and within LTER sites, (ii) test for relationships between synchrony with trait and phylogenetic similarity and (iii) investigate how climate conditions at sites are related to levels of synchrony. Overall, reproductive synchrony between woody plant species was greater than expected by chance, but spanned a wide range of values between species. Based on 11 functional and reproductive traits for 103 species (plus phylogenetic relatedness), cross-species synchrony in reproduction was driven primarily by trait similarity with phylogeny being largely unimportant, and synchrony was higher in sites with greater climatic water deficit. Community-level synchrony in masting has consequences for understanding forest regeneration dynamics and consumer-resource interactions.

The Relationship Between Maturation Size and Maximum Tree Size From Tropical to Boreal Climates

The fundamental trade-off between current and future reproduction has long been considered to result in a tendency for species that can grow large to begin reproduction at a larger size. Due to the prolonged time required to reach maturity, estimates of tree maturation size remain very rare and we lack a global view on the generality and the shape of this trade-off. Using seed production from five continents, we estimate tree maturation sizes for 486 tree species spanning tropical to boreal climates. Results show that a species' maturation size increases with maximum size, but in a non-proportional way: the largest species begin reproduction at smaller sizes than would be expected if maturation were simply proportional to maximum size. Furthermore, the decrease in relative maturation size is steepest in cold climates. These findings on maturation size drivers are key to accurately represent forests' responses to disturbance and climate change.

Masting and Efficient Production of Seedlings: Balancing Costs of Variation Through Synchronised Fruiting

The efficient conversion of tissues into reproductive success is a crucial aspect affecting the evolution of life histories. Masting, the interannually variable and synchronous seed production in perennial plants, is a strategy that can enhance reproductive efficiency by mitigating seed predation and pollen limitation. However, evaluating benefits is insufficient to establish whether efficiency has improved, as such assessments neglect the associated costs of masting, particularly during the critical seed-to-seedling stage. We conducted a parentage analysis of seedlings and adults in a population of 209 Sorbus aucuparia trees, monitored over 23 years, providing pioneering documentation of the effects of masting on the fitness of individual trees beyond the seed stage. Our results show high costs of interannual variation that can be mitigated by high synchrony and reveal the existence of phenotypes that appear to reap the benefits of masting while avoiding its costs through regular reproduction.

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.

Remotely sensed crown nutrient concentrations modulate forest reproduction across the contiguous United States

Global forests are increasingly lost to climate change, disturbance, and human management. Evaluating forests' capacities to regenerate and colonize new habitats has to start with the seed production of individual trees and how it depends on nutrient access. Studies on the linkage between reproduction and foliar nutrients are limited to a few locations and few species, due to the large investment needed for field measurements on both variables. We synthesized tree fecundity estimates from the Masting Inference and Forecasting (MASTIF) network with foliar nutrient concentrations from hyperspectral remote sensing at the National Ecological Observatory Network (NEON) across the contiguous United States. We evaluated the relationships between seed production and foliar nutrients for 56,544 tree-years from 26 species at individual and community scales. We found a prevalent association between high foliar phosphorous (P) concentration and low individual seed production (ISP) across the continent. Within-species coefficients to nitrogen (N), potassium (K), calcium (Ca), and magnesium (Mg) are related to species differences in nutrient demand, with distinct biogeographic patterns. Community seed production (CSP) decreased four orders of magnitude from the lowest to the highest foliar P. This first continental-scale study sheds light on the relationship between seed production and foliar nutrients, highlighting the potential of using combined Light Detection And Ranging (LiDAR) and hyperspectral remote sensing to evaluate forest regeneration. The fact that both ISP and CSP decline in the presence of high foliar P levels has immediate application in improving forest demographic and regeneration models by providing more realistic nutrient effects at multiple scales.

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.

Evolution of masting in plants is linked to investment in low tissue mortality

Masting, a variable and synchronized variation in reproductive effort is a prevalent strategy among perennial plants, but the factors leading to interspecific differences in masting remain unclear. Here, we investigate interannual patterns of reproductive investment in 517 species of terrestrial perennial plants, including herbs, graminoids, shrubs, and trees. We place these patterns in the context of the plants' phylogeny, habitat, form and function. Our findings reveal that masting is widespread across the plant phylogeny. Nonetheless, reversion from masting to regular seed production is also common. While interannual variation in seed production is highest in temperate and boreal zones, our analysis controlling for environment and phylogeny indicates that masting is more frequent in species that invest in tissue longevity. Our modeling exposes masting-trait relationships that would otherwise remain hidden and provides large-scale evidence that the costs of delayed reproduction play a significant role in the evolution of variable reproduction in plants.