Proline concentrations in seedlings of woody plants change with drought stress duration and are mediated by seed characteristics: a meta-analysis

Proline accumulation represents one of mechanisms used by plants to prevent the adverse consequences of water stress. The effects of increased proline levels in response to drought differ among species. Trees are exposed to the long-term effects of climate change. The reproductive success of species in a specific environment depends on the functional trait of tree seeds. We conducted a meta-analysis to evaluate the effects of drought stress on the proline concentrations in seedling leaf tissues of woody plant species and their relationships to drought duration, seed mass, seed category and coniferous/deciduous classification. Drought duration exhibited a nonlinear effect on proline accumulations. The drought effect on proline accumulations is greater for deciduous than for coniferous species and is higher for orthodox seed species than for recalcitrant. The seedlings of large-seeded species showed greater effect sizes than those of small-seeded species. Our results suggest that there is an optimum level at which proline accumulations under the influence of drought are the highest. A link between seed functional traits, as well as the coniferous/deciduous classification, and proline concentrations in tree seedlings during water stress were determined for the first time. Proline may help to identify high-quality seeds of trees used for reforestation.

Seed size variation impacts local adaptation and life-history strategies in a perennial grass

Seed mass is an ecologically important trait that often differs considerably among ecotypes. Yet, because few studies examine the impacts of seed mass on adult life-history traits, its role in local adaptation is unclear. In this study, using accessions of Panicum hallii that spanned the two major ecotypes, we examined whether covariation between seed mass, seedling and reproductive traits impacts ecotypic divergence and local adaptation. The perennial grass P. hallii has two distinct ecotypes-a large-seeded upland ecotype adapted to xeric environments and a small-seeded lowland ecotype adapted to mesic environments. In the greenhouse, seed mass varied greatly across P. hallii genotypes in a manner consistent with ecotypic divergence. Seed mass covaried significantly with several seedling and reproductive traits. At field sites representing the habitats of the two ecotypes, seed mass had different impacts on seedling and adult recruitment: selection favoured large seeds in upland habitat and small seeds in lowland habitat, which was consistent with local adaptation. By demonstrating the central role of seed mass in ecotypic differences in P. hallii and its importance to seedling and adult recruitment under field conditions, these studies show that early life-history traits can promote local adaptation and potentially explain ecotype formation.

Tree functional composition, functional diversity, and aboveground biomass show dissimilar trajectories in a tropical secondary forest restored through assisted natural regeneration

The growing trend of agricultural abandonment requires an understanding of the development of secondary forests on old fields in the context of restoration. However, few studies examine the regeneration trajectories of functional composition and functional diversity in afrotropical secondary forests. We tested how functional composition, diversity, and aboveground biomass (AGB) change with age and determined restoration success for a secondary forest restored through assisted natural regeneration in Uganda. We assessed the influence of distance to forests on regeneration. We sampled trees in 63 plots (2000 m² each) in the secondary forest (16-22-year old) and five plots in an old-growth forest in 2011, 2014 and 2017. We computed functional composition (community-weighted means-CWM) and diversity using categorical (habitat type, dispersal mode, fruit size, and successional group) and continuous traits (wood density and maximum height) of the species and calculated AGB. The secondary forest showed dissimilar trajectories of functional composition, diversity, and AGB. After 16-22 years, the secondary forest had not yet reached equivalent values of most attributes of functional composition, diversity and AGB in the old-growth forest. The distance to forests had a negative effect on CWM of forest-dependent species, nonpioneer light demanders, and functional divergence and a positive effect on CWM of pioneer species. We show that assisted natural regeneration can enhance the functional composition, functional diversity, and AGB of degraded forests and that continued monitoring is needed to attain full recovery. In planning passive restoration, sites closer to existing forests should be prioritized in order to achieve faster recovery.

Soil seedbank: Importance for revegetation in the water level fluctuation zone of the reservoir area

Vegetation succession in the water level fluctuation zone (WLFZ) is driven by periodical water fluctuations, the mechanisms of response and synergistic evolution between aboveground vegetation (AGV) and soil seed bank (SSB) in the WLFZ remain unclear. To illustrate the response between AGV and SSB and the importance of SSB for revegetation in the WLFZ, the Three Gorges Reservoir (TGR) was taken as the target in the current study, and five sampling sites that lie in the center of reservoir and major tributaries of the TGR were selected. Plant community survey along the three water level gradients (i.e., 165-170 m, 170-175 m and 175-180 m) was conducted. Simultaneously, SSB samples on the topsoil (0-5 cm) were collected for germination experiments. A total of forty-nine species were observed from the SSB, belonging to 24 families and 47 genera, of which Asteraceae (8 species) and Poaceae (6 species) dominated. The number of total germinated seedlings species from soil samples from the sampling sites differed, i.e., was lower in Fengjie and Wushan compared to Yunyang, Gaoyang and Fengdu. The seed density in 165-170 m was significantly lower than that of 170-175 m and 175-180 m (p < 0.05). The Sørensen similarity coefficients between AGV and SSB tended to decrease with the increase of water level gradient, ranging from 0.04 to 0.42. SSB species composition was significantly associated with total carbon and total nitrogen contents of the soil (both p < 0.05). The SSB density was significantly negatively correlated with concentration of soil total nitrogen (p < 0.05), the species richness of SSB was significantly negatively correlated with soil pH value (p < 0.05). Hence, the relationship between the SSB and the soil habitat might be an important factor driving the construction of vegetation in the WLFZ. The correlation between dynamic of SSB and TGR hydrological regimes should be considered for revegetation in the WLFZ.

Seed size-seed number trade-offs: influence of seed size on the density of fire-stimulated persistent soil seed banks

PREMISE

Does the seed size-seed number allocation trade-off model apply to long-term persistent soil seed banks? This trade-off between seed size versus number of seeds produced is usually applied at a single population on an annual basis. Our question is how this model might apply to close relatives that produce dormant seed forming long-term persistent soil seed banks. These two criteria allow a focus on divergent evolution of conspecifics and permits us to isolate seed size in the spectrum of life history traits that may be influencing seed traits, and on how seed size influences accumulation and persistence in the soil.

METHODS

In California, Arctostaphylos species only produce physiologically dormant seed that are fire-stimulated and that vary in seed size permitting seed size-seed bank density relationship as a test of the seed size-seed number allocation model. Soil seed banks of 10 species of Arctostaphylos were sampled with fruit volumes ranging from 21-1063 mm³ . Seed bank density was determined by hand extraction from soil samples.

RESULTS

We found that seed bank densities were significantly negatively related to fruit or seed size.

CONCLUSIONS

Rather than an issue of allocational trade-off between size and number, we interpret these results as reflecting seed predation and postfire seedling establishment. Seed bank densities, even after decades, generally were less than one or two-year's seed production, suggesting intense seed predation. Burial by scatter-hoarding rodents provided sufficient seeds deep enough for survival of fire. Variation on seed size suggests seedling establishment constraints, but it needs further research.

Seed size affects rodent-seed interaction consistently across plant species but not within species: evidence from a seed tracking experiment of 41 tree species

Scatter-hoarding rodents play a crucial role in seed survival and seed dispersal. As one of the most important seed traits, seed size and its effect on rodent-seed interaction attract lots of attention. Current studies usually target one or a few species and show inconsistent patterns; however, few experiments include a large number of species although many plant species usually coexist in natural forest and overlap in fruiting time. Here, we tracked the dispersal and predation of 26,100 seeds belonging to 41 tree species in a subtropical forest for two years. Most species showed no relationships between seed size and rodent foraging preference, while the remaining species displayed diverse of patterns: monotonic decrease and increase trends, and hump-shaped and U-shaped patterns, indicating that a one-off study with a few species might give misleading information. However, the seed size effect across species was consistent in both years, indicating that including a large number of species that hold a sufficient range of seed size may avoid the aforementioned bias. Interestingly, seed size effect differed among rodent foraging processes: a negative effect on seed harvest, a hump-shaped effect on seed removal and removal distance, while a positive effect on overwinter survival of cached seeds, indicating that rodents may make trade-offs between large and small seeds both among foraging processes and within a single process, thus lead to a parabolic relationship between seed size and seed dispersal success, i.e., medium-sized seeds were more likely to be removed and cached, and transported with a further distance. This article is protected by copyright. All rights reserved.

Macroevolutionary consequences of mast seeding

Masting characterizes large, intermittent and highly synchronous seeding events among individual plants and is found throughout the plant Tree of Life (ToL). Although masting can increase plant fitness, little is known about whether it results in evolutionary changes across entire clades, such as by promoting speciation or enhanced trait selection. Here, we tested if masting has macroevolutionary consequences by combining the largest existing dataset of population-level reproductive time series and time-calibrated phylogenetic tree of vascular plants. We found that the coefficient of variation (CVp) of reproductive output for 307 species covaried with evolutionary history, and more so within clades than expected by random. Speciation rates estimated at the species level were highest at intermediate values of CVp and regional-scale synchrony (Sr) in seed production, that is, there were unimodal correlations. There was no support for monotonic correlations between either CVp or Sr and rates of speciation or seed size evolution. These results were robust to different sampling decisions, and we found little bias in our dataset compared with the wider plant ToL. While masting is often adaptive and encompasses a rich diversity of reproductive behaviours, we suggest it may have few consequences beyond the species level. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

The biology of seed discrimination and its role in shaping the foraging ecology of carabids: A review

Species of carabid (ground) beetles are among the most important postdispersal weed seed predators in temperate arable lands. Field studies have shown that carabid beetles can remove upwards of 65%-90% of specific weed seeds shed in arable fields each year. Such data do not explain how and why carabid predators go after weed seeds, however. It remains to be proven that weed seed predation by carabids is a genuine ecological interaction driven by certain ecological factors or functional traits that determine interaction strength and power predation dynamics, bringing about therefore a natural regulation of weed populations. Along these lines, this review ties together the lines of evidence around weed seed predation by carabid predators. Chemoperception rather than vision seems to be the primary sensory mechanism guiding seed detection and seed selection decisions in carabid weed seed predators. Selection of weed seeds by carabid seed predators appears directed rather than random. Yet, the nature of the chemical cues mediating detection of different seed species and identification of the suitable seed type among them remains unknown. Selection of certain types of weed seeds cannot be predicted based on seed chemistry per se in all cases, however. Rather, seed selection decisions are ruled by sophisticated behavioral mechanisms comprising the assessment of both chemical and physical characteristics of the seed. The ultimate selection of certain weed seed types is determined by how the chemical and physical properties of the seed match with the functional traits of the predator in terms of seed handling ability. Seed density, in addition to chemical and physical seed traits, is also an important factor that is likely to shape seed selection decisions in carabid weed seed predators. Carabid responses to seed density are rather complex as they are influenced not only by seed numbers but also by trait-based suitability ranks of the different seed types available in the environment.

Exposure time is an important variable in quantifying post-dispersal seed removal

A literature synthesis concluded that small mammals have the greatest impact on post-dispersal removal of intermediate-sized seeds (Dylewski et al. 2020). However, this study failed to consider the duration of seed exposure to predators. Re-analyses of the corrected dataset revealed only a weak effect of seed mass on seed removal.

Pre-dispersal seed predators boost seed production in a short-lived plant

Pre-dispersal seed predation diminishes fitness and population growth rate of many plant species. Therefore, plants have developed multiple strategies to reduce the harmful effects of this type of herbivory. The present study aims to determine the effect of pre-dispersal seed predators (PSPs) on the fitness of a short-lived herb, and to discern the mechanisms allowing the plants to reduce the impact of pre-dispersal seed predation. Knowing that the interplay between pre-dispersal seed predators and plants is strongly shaped by the presence of other co-occurring organisms, we tested whether detritivores modulate plant responses towards pre-dispersal seed predators. To do so, we experimentally manipulated in the field pre-dispersal seed predators and detritivores interacting with the short-lived herb Moricandia moricandioides. We found that detritivores did not alter the response of plants to PSPs. Strikingly, the plant overcompensated for pre-dispersal seed predation, almost doubling the number of seeds produced. Plant response to PSPs led to substantial changes in shoot architecture, reproductive traits, chemical defences in leaves and seeds and in seed nutrient content. The overcompensating mechanism seems to be meristem activation, which allowed plants to produce more reproductive tissue, and increasing the proportion of ovules that became seeds, a response which specifically compensates for pre-dispersal seed predation. As far as we know, this is the first experimental evidence of a positive effect of PSPs on plant lifetime fitness as a consequence of plant overcompensation.

Enemies mediate distance- and density-dependent mortality of tree seeds and seedlings: a meta-analysis of fungicide, insecticide and exclosure studies

Conspecific negative distance- and density-dependence is often assumed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta-analysis of studies that involved robust manipulative experiments, using fungicides, insecticides and exclosures, to assess the contributions of different natural enemy types to distance- and density-dependent effects at seed and seedling stages. We found that distance- and density-dependent mortality caused by natural enemies was most likely at the seedling stage and was greater at higher mean annual temperatures. Conspecific negative distance- and density-dependence at the seedling stage is significantly weakened when fungicides were applied. By contrast, negative conspecific distance- and density-dependence is not a general pattern at the seed stage. High seed mass reduced distance- and density-dependent mortality at the seed stage. Seed studies excluding only large mammals found significant negative conspecific distance-dependent mortality, but exclusion of all mammals resulted in a non-significant effect of conspecifics. Our study suggests that plant pathogens are a major cause of distance- and density-dependent mortality at the seedling stage, while the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, weaken and enhance negative conspecific distance-dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types and assess how global change may affect natural enemies and thus influence the strength of conspecific distance- and density-dependence.

Which Seed Properties Determine the Preferences of Carabid Beetle Seed Predators?

Simple Summary

The carabid beetles are well known for the consumption of weed seeds in arable land, but how they choose the seeds is poorly known. In this work, we try to explain the patterns in preferences of 37 species of carabids based on eight seed properties of 28 species of seeds. Surprisingly, chemical properties of the seeds did not affect the preferences. Instead, preferences were driven mainly by seed structural properties. The importance of particular seed properties was also affected by the degree of predator specialization.

Abstract

Ground beetles are important invertebrate seed predators in temperate agro-ecosystems. However, there is a lack of information regarding which seed properties are important to carabids when they select seeds for consumption. Therefore, seed properties, such as size, shape, morphological defence, and chemical composition, were measured, and in addition to seed taxonomy and ecology, these data were used to explain carabid preferences. Carabid preferences were assessed using a multi-choice experiment with 28 species of weed seeds presented to 37 species of Carabidae. Multiple regression on distance matrices (MRM) was used to determine the importance of particular sets of seed properties for carabids. The analysis was conducted for the full set of carabids (37 species) as well as for subsets of species belonging to the tribes of Harpalini or Zabrini. For the complete set of species, seed dimensions, seed mass, taxonomy, plant strategy, and seed coat properties significantly explained carabid preferences (proportion of explained variance, R² = 0.465). The model for Harpalini fit the data comparably well (R² = 0.477), and seed dimensions, seed mass and seed coat properties were significant. In comparison to that for Harpalini, the model for Zabrini had much lower explanatory power (R² = 0.248), and the properties that significantly affected the preferences were seed dimensions, seed mass, taxonomy, plant strategy, and seed coat properties. This result suggests that the seed traits that carabids respond to may be specific to taxonomic and likely relate to the degree of specialisation for seeds. This study contributes to understanding the mechanisms that determine the preferences of carabid beetles for seeds.

Extending Plant Defense Theory to Seeds

Plant defense theory explores how plants invest in defenses against natural enemies but has focused primarily on the traits expressed by juvenile and mature plants. Here we describe the diverse ways in which seeds are chemically and physically defended. We suggest that through associations with other traits, seeds are likely to exhibit defense syndromes that reflect constraints or trade-offs imposed by selection to attract dispersers, enable effective dispersal, ensure appropriate timing of seed germination, and enhance seedling performance. We draw attention to seed and reproductive traits that are analogous to defense traits in mature plants and describe how the effectiveness of defenses is likely to differ at pre- and postdispersal stages. We also highlight recent insights into the mutualistic and antagonistic interactions between seeds and microbial communities, including fungi and endohyphal bacteria, that can influence seed survival in the soil and subsequent seedling vigor.

Macroevolutionary patterns in seed component mass and different evolutionary trajectories across seed desiccation responses

Seed coat and seed reserve show substantial mass variation, play different roles in plant life strategies and are shaped by different selective forces. However, remarkably little is known about the macroevolution of the relative allocation in seed components and its influence on important ecophysiological processes. Using phylogenetic comparative methods and evolutionary modelling approaches, we modelled mass changes in seed components along individual lineages for 940 species and compared the patterns across seed desiccation responses. Seed component allocation was driven primarily by changes in reserve mass rather than coat mass, as evolutionary rates in reserve mass significantly outpaced those in coat mass. Although the scaling patterns between reserve mass and coat mass were similar across desiccation responses, desiccation-sensitive seeds allocated more and evolved faster in reserve compared to desiccation-tolerant seeds. The findings emphasize the relative importance of reserve to coat in the evolution of plant reproductive strategies, revealing potential ecological advantages gained by enlarged reserve. As the first quantification of the evolutionary tempo and mode of seed component mass, our study allows a detailed interpretation of evolutionary pathways underlying seed storage behaviours and advances the understanding of the evolution of desiccation sensitivity in seeds.

Large-scale patterns of seed removal by small mammals differ between areas of low- versus high-wolf occupancy

Because most tree species recruit from seeds, seed predation by small-mammal granivores may be important for determining plant distribution and regeneration in forests. Despite the importance of seed predation, large-scale patterns of small-mammal granivory are often highly variable and thus difficult to predict. We hypothesize distributions of apex predators can create large-scale variation in the distribution and abundance of mesopredators that consume small mammals, creating predictable areas of high and low granivory. For example, because gray wolf (Canis lupus) territories are characterized by relatively less use by coyotes (C. latrans) and greater use by foxes (Vulpes vulpes, Urocyon cinereoargentus) that consume a greater proportion of small mammals, wolf territories may be areas of reduced small-mammal granivory. Using large-scale, multiyear field trials at 22 sites with high- and low-wolf occupancy in northern Wisconsin, we evaluated whether removal of seeds of four tree species was lower in wolf territories. Consistent with the hypothesized consequences of wolf occupancy, seed removal of three species was more than 25% lower in high-wolf-occupancy areas across 2 years and small-mammal abundance was more than 40% lower in high-wolf areas during one of two study years. These significant results, in conjunction with evidence of seed consumption in situ and the absence of significant habitat differences between high- and low-wolf areas, suggest that top-down effects of wolves on small-mammal granivory and seed survival may occur. Understanding how interactions among carnivores create spatial patterns in interactions among lower trophic levels may allow for more accurate predictions of large-scale patterns in seed survival and forest composition.

Correlated evolution of diaspore traits and potential frugivore-mediated selection in a fleshy-fruited tropical lineage (Artabotrys, Annonaceae)

Dispersal syndromes are often defined by reference to fruit traits that are associated with distinct frugivore guilds. Studies rarely examine the relationship between seed traits and frugivores or test the alternative hypothesis that traits are shaped by climatic variables. We assess whether the evolution of seed size and physical defense are correlated with dispersal-related traits and climatic variables in Artabotrys, a fleshy-fruited tropical lineage. Diaspore traits and WorldClim bioclimatic variables were compiled for 43 species. Correlated evolution was evaluated using phylogenetic regression and model-fitting approaches. The best-fitting multioptima Ornstein-Uhlenbeck model suggests that lineages with smooth testa and thin pericarp (SP) have evolved toward smaller seeds with a thinner testa, whereas lineages with rough testa and/or thick pericarp have evolved toward larger seeds with a thicker testa. A smooth testa facilitates spitting and/or swallowing of intact seeds while fruits with thin pericarp may be preferentially consumed by frugivores with less destructive oral processing, enabling lower investment in seed physical defense in SP lineages. Moreover, small seeds are more likely to be swallowed intact with a food bolus. The effect of climate on seed size and physical defense is equivocal and warrants further investigation.

An allometry between seed kernel and seed coat shows greater investment in physical defense in small seeds

PREMISE OF THE STUDY

Numerous studies have treated the mass of a whole seed as an integrated unit, although the components seed kernel and seed coat play different roles and are subject to different evolutionary selection pressures. In this study, we provided the first global-scale quantification of the relative biomass investments in seed coats and seed kernels. We tested the following hypotheses: there is a negative allometry between seed kernel mass and seed coat mass, and therefore, seed coat ratio (SCR) is negatively correlated with seed mass.

METHODS

We compiled a global-scale data set from the published literature, including 680 plant species from 420 genera and 108 families. The relationships between seed components were quantified using standardized major axis regression, ordinary least squares regression, and phylogenetic independent analyses.

KEY RESULTS

We found a weak but significantly negative allometry between seed kernel mass and seed coat mass, which resulted in a negative relationship between seed mass and SCR. Similar results were found after accounting for the phylogeny.

CONCLUSIONS

The finding that smaller seeds invest more in protective tissues but less in stored reserves may explain the general prediction that larger seeds suffer greater predation than smaller seeds. Furthermore, this weak allometry may also explain, at least in part, why so many studies failed to identify a clear pattern of the effect of seed mass on many ecological processes. Our study suggests that the allometry between the two seed components must be considered when evaluating the ecological significance and evolutionary history of seed mass.

The role of herbivorous insects and pathogens in the regeneration dynamics of Guazuma ulmifolia in Panama

A significant proportion of the mortality of rainforest trees occurs during early life stages (seeds and seedlings), but mortality agents are often elusive. Our study investigated the role of herbivorous insects and pathogens in the early regeneration dynamics of Guazumaulmifolia (Malvaceae), an important tree species in agroforestry in Central America. We reared pre-dispersal insect seed predators from G.ulmifolia seeds in Panama. We also carried out an experiment, controlling insects and pathogens using insecticide and/or fungicide treatments, as well as seed density, and compared survivorship of G.ulmifolia seeds and seedlings among treatments and relative to untreated control plots. We observed (1) high pre-dispersal attack (92%) of the fruits of G.ulmifolia, mostly by anobiine and bruchine beetles; (2) negligible post-dispersal attack of isolated seeds by insects and pathogens; (3) slow growth and high mortality (&gt; 95%) of seedlings after 14 weeks; (4) low insect damage on seedlings; and (5) a strong positive correlation between seedling mortality and rainfall. We conclude that for G.ulmifolia at our study site the pre-dispersal seed stage is by far the most sensitive stage to insects and that their influence on seedling mortality appears to be slight as compared to that of inclement weather. Thus, the regeneration of this important tree species may depend on effective primary dispersal of seeds by vertebrates (before most of the seed crop is lost to insects), conditioned by suitable conditions in which the seedlings can grow.

Transgenerational effects of ungulates and pre-dispersal seed predators on offspring success and resistance to herbivory

Herbivorous mammals and insect pre-dispersal seed predators are two types of herbivores that, despite their functional and morphological differences, tend to severely impact many plant species, highly decreasing their seed production and even imperiling the performance of their offspring through transgenerational effects. However, how they influence offspring resistance to herbivory remains largely unknown. In this study we experimentally examined the effects of ungulates and pre-dispersal seed predators on seed quality as well as on the emergence, survival and resistance to herbivory of the seedlings of a semiarid herb. We found that ungulates reduced seedling recruitment but increased seedling resistance to leaf miners. These effects were probably a consequence of insufficient carbon provisioning in seeds that reduced seed viability and provoked carbon limitation in seedlings. Pre-dispersal seed predators did not influence seedling recruitment, but seedlings from mothers damaged by ungulates and by pre-dispersal seed predators suffered less herbivory by grasshoppers. Remarkably, intra-individual differences in damage by pre-dispersal seed predators affected the rate of damage underwent by seedlings. That is, seedlings derived from fruits attacked by seed predators were more resistant to herbivores than siblings derived from un-attacked fruits in plant populations exposed to ungulates. To our knowledge, this is the first study reporting variation in transgenerational-induced resistance of seedlings from the same maternal plant. This study is a valuable contribution to the understanding of transgenerational effects of multiple herbivores and their implications for a deeper comprehension of the natural systems in which they co-occur.

Functional trade-offs and the phylogenetic dispersion of seed traits in a biodiversity hotspot of the Mountains of Southwest China

The diversity of traits associated with plant regeneration is often shaped by functional trade-offs where plants typically do not excel at every function because resources allocated to one function cannot be allocated to another. By analyzing correlations among seed traits, empirical studies have shown that there is a trade-off between seedling development and the occupation of new habitats, although only a small range of taxa have been tested; whether such trade-off exists in a biodiverse and complex landscape remains unclear. Here, we amassed seed trait data of 1,119 species from a biodiversity hotspot of the Mountains of Southwest China and analyzed the relationship between seed mass and the number of seeds and between seed mass and time to germination. Our results showed that seed mass was negatively correlated with seed number but positively correlated with time to germination. The same trend was found regardless of variation in life-form and phylogenetic conservatism. Furthermore, the relation between seed mass and other seed traits was randomly dispersed across the phylogeny at both the order and family levels. Collectively, results suggest that there is a functional trade-off between seedling development and new habitat occupation for seed plants in this region. Larger seeds tend to produce fewer seedlings but with greater fitness compared to those produced by smaller seeds, whereas smaller seeds tend to have a larger number of seeds that germinate faster compared to large-seeded species. Apart from genetic constraints, species that produce large seeds will succeed in sites where resource availability is low, whereas species with high colonization ability (those that produce a high number of seeds per fruit) will succeed in new niches. This study provides a mechanistic explanation for the relatively high levels of plant diversity currently found in a heterogeneous region of the Mountains of Southwest China.

Termites and large herbivores influence seed removal rates in an African savanna

Seed removal can influence plant community dynamics, composition, and resulting vegetation characteristics. In the African savanna, termites and large herbivores influence vegetation in various ways, likely including indirect effects on seed predators and secondary dispersers. However, the intensity and variation of seed removal rates in African savannas has seldom been studied. We experimentally investigated whether termites and large herbivores were important factors in the mechanisms contributing to observed patterns in tree species composition on and off mounds, in Lake Mburo National Park, Uganda. Within fenced (excluding large herbivores) and unfenced termite mound and adjacent savanna plots, we placed seeds of nine native tree species within small open "cages," accessed by all animals, roofed cages that only allowed access to small vertebrates and invertebrates, and closed cages that permitted access by smaller invertebrates only (5 mm wire mesh). We found that mean seed removal rate was high (up to 87.3% per 3 d). Mound habitats experienced significantly higher removal rates than off-mound habitats. The mean removal rate of native seeds from closed cages was 11.1% per 3 d compared with 19.4% and 23.3% removed per 3 d in the roofed and open cages, respectively. Smaller seeds experienced higher removal rates than larger seeds. Large herbivore exclusion on mounds reduced native seed removal rates by a mean of 8.8% in the open cages, but increased removal rates by 1.7% in the open cages when off-mound habitats were fenced. While removal rates from open cages were higher on active mounds (30.9%) than on inactive mounds (26.7%), the removal rates from closed cages were lower on active vs. inactive mounds (6.1% vs. 11.6%, respectively). Thus, we conclude that large herbivores and Macrotermes mounds influence seed removal rates, though these effects appear indirect.

Acorn size and tolerance to seed predators: the multiple roles of acorns as food for seed predators, fruit for dispersal and fuel for growth

Fitness of parents and offspring is affected by offspring size. In oaks (Quercus spp.), acorns vary considerably in size across, and within, species. Seed size influences dispersal and establishment of oaks, but it is not known whether size imparts tolerance to seed predators. Here, we examine the relative extent to which cotyledon size serves as both a means for sustaining partial consumption and energy reserves for developing seedlings during early stages of establishment. Acorns of 6 oak species were damaged to simulate acorn predation by vertebrate and invertebrate seed predators. Seedling germination/emergence and growth rates were used to assess seedling performance. We predicted that if cotyledons are important for dispersal, acorns should show tolerance to partial seed consumption. Alternatively, if the cotyledon functions primarily as an energy reserve, damage should significantly influence seedling performance. Acorns of each species germinated and produced seedlings even after removing >50% of the cotyledon. Seed mass explained only some of the variation in performance. Within species, larger acorns performed better than smaller acorns when damaged. Undamaged acorns performed as well or better than damaged acorns. There was no pattern among individual species with increasing amounts of damage. In some species, simulated invertebrate damage resulted in the poorest performance, suggesting alternative strategies of oaks to sustain damage. Large cotyledons in acorns may be important for attracting seed dispersers and sustaining partial damage, while also providing energy to young seedlings. Success of oak establishment may follow from the resilience of acorns to sustain damage at an early stage.

Life history trade-off moderates model predictions of diversity loss from climate change

Climate change can trigger species range shifts, local extinctions and changes in diversity. Species interactions and dispersal capacity are important mediators of community responses to climate change. The interaction between multispecies competition and variation in dispersal capacity has recently been shown to exacerbate the effects of climate change on diversity and to increase predictions of extinction risk dramatically. Dispersal capacity, however, is part of a species’ overall ecological strategy and are likely to trade off with other aspects of its life history that influence population growth and persistence. In plants, a well-known example is the trade-off between seed mass and seed number. The presence of such a trade-off might buffer the diversity loss predicted by models with random but neutral (i.e. not impacting fitness otherwise) differences in dispersal capacity. Using a trait-based metacommunity model along a warming climatic gradient the effect of three different dispersal scenarios on model predictions of diversity change were compared. Adding random variation in species dispersal capacity caused extinctions by the introduction of strong fitness differences due an inherent property of the dispersal kernel. Simulations including a fitness-equalising trade-off based on empirical relationships between seed mass (here affecting dispersal distance, establishment probability, and seedling biomass) and seed number (fecundity) maintained higher initial species diversity and predicted lower extinction risk and diversity loss during climate change than simulations with variable dispersal capacity. Large seeded species persisted during climate change, but developed lags behind their climate niche that may cause extinction debts. Small seeded species were more extinction-prone during climate change but tracked their niches through dispersal and colonisation, despite competitive resistance from residents. Life history trade-offs involved in coexistence mechanisms may increase community resilience to future climate change and are useful guides for model development.

Effects of seed density and proximity to refuge habitat on seed predation rates for a rare and a common Lupinus species

PREMISE OF THE STUDY

Biotic interactions such as seed predation can play a role in explaining patterns of abundance among plant species. The effect of seed predation will depend on how the strength of predation differs across species and environments, and on the degree to which seed loss at one life-cycle phase increases fitness at another phase. Few studies have simultaneously quantified predispersal and postdispersal predation in co-occurring rare and common congeners, despite the value of estimating both for understanding causes of rarity.

METHODS

We quantified predispersal seed predation on the rare, herbaceous species Lupinus tidestromii (Fabaceae) and its common, shrubby congener L. chamissonis across multiple years in the same community. We experimentally measured postdispersal seed predation at two seed densities and locations near or far from an exotic grass housing high densities of deer mice (Peromyscus maniculatus), their primary, native seed predator.

KEY RESULTS

The common L. chamissonis had the lowest predispersal seed predation of the two lupine species, potentially because of its height: its high racemes received less predation than those low to the ground. By contrast, the same species experienced higher postdispersal seed predation, and at predators traveled long distances away from refuge habitat to consume their seeds. Across both plant species, mice preferentially predated high-density seed sources.

CONCLUSIONS

Our results show differences in the magnitude and direction of seed predation between the species across different life-cycle phases. We demonstrated possible roles of proximity to refuge habitat, seed density, and seed size in these patterns. Congeneric comparisons would benefit from a comprehensive framework that considers seed predation across different life-cycle phases and the environmental context of predation.

The mechanical defence advantage of small seeds

Seed size and toughness affect seed predators, and size-dependent investment in mechanical defence could affect relationships between seed size and predation. We tested how seed toughness and mechanical defence traits (tissue density and protective tissue content) are related to seed size among tropical forest species. Absolute toughness increased with seed size. However, smaller seeds had higher specific toughness both within and among species, with the smallest seeds requiring over 2000 times more energy per gram to break than the largest seeds. Investment in mechanical defence traits varied widely but independently of the toughness-mass allometry. Instead, a physical scaling relationship confers a toughness advantage on small seeds independent of selection on defence traits and without a direct cost. This scaling relationship may contribute to seed size diversity by decreasing fitness differences among large and small seeds. Allometric scaling of toughness reconciles predictions and conflicting empirical relationships between seed size and predation.

Past agricultural land use and present-day fire regimes can interact to determine the nature of seed predation

Historical agriculture and present-day fire regimes can have significant effects on contemporary ecosystems. Although past agricultural land use can lead to long-term changes in plant communities, it remains unclear whether these persistent land-use legacies alter plant-consumer interactions, such as seed predation, and whether contemporary disturbance (e.g., fire) alters the effects of historical agriculture on these interactions. We conducted a study at 27 sites distributed across 80,300 ha in post-agricultural and non-agricultural longleaf pine woodlands with different degrees of fire frequency to test the hypothesis that past and present-day disturbances that alter plant communities can subsequently alter seed predation. We quantified seed removal by arthropods and rodents for Tephrosia virginiana and Vernonia angustifolia, species of conservation interest. We found that the effects of land-use history and fire frequency on seed removal were contingent on granivore guild and microhabitat characteristics. Tephrosia virginiana removal was greater in low fire frequency sites, due to greater seed removal by rodents. Although overall removal of V. angustifolia did not differ among habitats, rodents removed more seeds than arthropods at post-agricultural sites and non-agricultural sites with low fire frequencies, but not at non-agricultural sites with high fire frequencies. Land-use history and fire frequency also affected the relationship between microhabitat characteristics and removal of V. angustifolia. Our results suggest that historical agriculture and present-day fire regimes may alter seed predation by shifting the impact of rodent and arthropod seed predators among habitats, with potential consequences for the establishment of rare plant species consumed by one or both predators.

Pre-Dispersal Seed Predation in a Species-Rich Forest Community: Patterns and the Interplay with Determinants

Pre-dispersal seed predation (PDSP) is commonly observed in woody plants, and recognized as a driver of seed production variability that is critical for successful regeneration. Earlier studies on PDSP and its determinants were mostly species specific, with community-level PDSP rarely estimated; and the interactions between the temporal variability of seed production and PDSP remain elusive. In this study, the community seed rain of woody plants in a mixed evergreen-deciduous broadleaf forest was monitored for seven years. We examined predation on collected seeds and analyzed the determinants of PDSP. PDSP was recorded in 17 out of 44 woody plant species, and three-quarters of PDSP was due to insect predators. Annual seed production varied substantially at community level, reversely linked with the temporal variation of PDSP rate. The PDSP rate was biased regarding fruit types, and being significantly correlated with seed mass when using phylogenetic independent contrasts (PICs) or without taking into account phylogenetic relations, especially for nuts. PDSP rate was also negatively correlated with seed density, showing a threshold-related predator satiation effect. The community-level PDSP rate was primarily determined by tree height, fruit type, and interannual variation of seed production and seed mass. Our analysis revealed a causal link between seed production and the dynamics of PDSP rate at the community level. The predator satiation effect was primarily contributed by the dominant species, whereas the rare species seemed to apply a distinct “hide-and-seek” strategy to control the risk of PDSP. The mechanistic difference of seed production between the common and rare species can shed new light on species coexistence and community assembly. Long-term monitoring of both seed rain and seed predation is required for understanding the ecological and evolutionary implications of species regeneration strategies in a species-rich forest community.

Leaf litter affects the survival and predation rates for large and small Pinus seeds in the Qinling Mountains, China

It is generally accepted that leaf litter covers seeds, which may be beneficial for their survival, but it is unclear whether leaf litter contributes in different ways to the survival of plant seeds with contrasting sizes. In this study, we examined the effect of different levels of leaf litter coverage on seed predation and foraging/caching for small-seeded Pinus tabulaeformis and large-seeded Pinus armandii in the Qinling Mountains, China. Our results showed that leaf litter failed to protect P. armandii seeds from being removed by small rodents, whereas it improved seed survival for P. tabulaeformis. P. tabulaeformis were eaten in situ, whereas more P. armandii seeds were eaten in the high leaf litter coverage plots. Therefore, we propose that leaf litter affects the survival and predation rates for large and small Pinus seeds in the Qinling Mountains, China.

Trait-mediated seed predation, dispersal and survival among frugivore-dispersed plants in a fragmented subtropical forest, Southwest China

By tracking the fate of individual seeds from 6 frugivore-dispersed plants with contrasting seed traits in a fragmented subtropical forest in Southwest China, we explored how rodent seed predation and hoarding were influenced by seed traits such as seed size, seed coat hardness and seed profitability. Post-dispersal seed fates varied significantly among the 6 seed species and 3 patterns were witnessed: large-seeded species with a hard seed coat (i.e. Choerospoadias axillaries and Diospyros kaki var. silvestris) had more seeds removed, cached and then surviving at caches, and they also had fewer seeds predated but a higher proportion of seeds surviving at the source; medium-sized species with higher profitability and thinner seed coat (i.e. Phoebe zhennan and Padus braohypoda) were first harvested and had the lowest probability of seeds surviving either at the source or at caches due to higher predation before or after removal; and small-seeded species with lower profitability (i.e. Elaeocarpus japonicas and Cornus controversa) had the highest probability of seeds surviving at the source but the lowest probability of seeds surviving at caches due to lower predation at the source and lower hoarding at caches. Our study indicates that patterns of seed predation, dispersal and survival among frugivore-dispersed plants are highly determined by seed traits such as seed size, seed defense and seed profitability due to selective predation and hoarding by seed-eating rodents. Therefore, trait-mediated seed predation, dispersal and survival via seed-eating rodents can largely affect population and community dynamics of frugivore-dispersed plants in fragmented forests.