Safe sites, seed supply, and the recruitment function in plant populations

Density-dependent species interactions modulate alpine treeline shifts

Species interactions such as facilitation and competition play a crucial role in driving species range shifts. However, density dependence as a key feature of these processes has received little attention in both empirical and modelling studies. Herein, we used a novel, individual-based treeline model informed by rich in situ observations to quantify the contribution of density-dependent species interactions to alpine treeline dynamics, an iconic biome boundary recognized as an indicator of global warming. We found that competition and facilitation dominate in dense versus sparse vegetation scenarios respectively. The optimal balance between these two effects was identified at an intermediate vegetation thickness where the treeline elevation was the highest. Furthermore, treeline shift rates decreased sharply with vegetation thickness and the associated transition from positive to negative species interactions. We thus postulate that vegetation density must be considered when modelling species range dynamics to avoid inadequate predictions of its responses to climate warming.

The Similarity between Species Composition of Vegetation and Soil Seed Bank of Grasslands in Inner Mongolia, China: Implications for the Asymmetric Response to Precipitation

The asymmetric response of productivity to precipitation was recently proposed as an early warning signal for the shifts in temperate grassland function in China. It was hypothesized that the asymmetry was influenced by the increased growth of the newly emerged seedlings from the soil seed bank. Therefore, the seed density in the soil seed bank and the similarity between species composition of the vegetation and the soil seed bank should be maximized where asymmetry was maximized. However, this knowledge was still limited and unconfirmed. In this study, the desert steppe, typical steppe and the transition zone between them (with the highest asymmetry) were selected for studying the similarity index in both 2018 (dry year) and 2019 (wet year). Plant species composition was monitored in situ using an unmanned aerial vehicle. Soil seed bank samples were collected, and the seed bank density and species composition were then examined and identified in the laboratory. Results showed that: (1) The variation in vegetation species richness between the two years was the highest (41%) in the transition zone (p < 0.05), while it was only 7% and 13% for the desert steppe and typical steppe, respectively. The presence of herbaceous species mainly caused the differences in variation among three grassland types. (2) Seed density was the highest in the transition zone (114 seeds/m² and 68 seeds/m² in the transient and persistent soil seed bank, respectively) (p < 0.05). Additionally, herbaceous species were the main components of the soil seed bank. (3) The similarity index was the highest in the transition zone (p < 0.05), with 38%/44% and 33%/44% for the transient/persistent soil seed bank in 2018 and 2019, respectively. Our study demonstrated that variation in vegetation species composition was very similar to the composition of the seeds accumulated in the soil seed bank. These results warrant further investigation for the mechanism of asymmetric response of productivity to precipitation.

Seedling recruitment correlates with seed input across seed sizes: implications for coexistence

Understanding controls on recruitment is critical to predicting community assembly, diversity, and coexistence. Theory posits that at mean fecundity, recruitment of highly fecund small-seeded plants should be primarily microsite limited, which is indicated by a saturating recruitment function. In contrast, species that produce fewer large seeds are more likely to be seed-limited, which is characterized by a linear recruitment function. If these patterns hold in nature, seed predation that disproportionately affects larger-seeded species can limit their establishment. We tested these predictions by comparing recruitment functions among 16 co-occurring perennial forb species that vary by over two orders of magnitude in seed size. We also assessed how postdispersal seed predation by mice influenced recruitment. We added seeds at densities from zero to three times natural fecundity of each species to undisturbed plots and examined spatial variation in recruitment by conducting experiments across 10 grassland sites that varied in productivity and resource availability. Consistent across two replicated years, most species had linear recruitment functions across the range of added seed densities, indicative of seed-limited recruitment. Depending on year, the recruitment functions of only 19-37% of target species saturated near their average fecundity, and this was not associated with seed size. Recruitment was strongly inhibited by rodent seed predation for large-seeded species but not for smaller-seeded species. Proportional recruitment was more sensitive to spatial variation in recruitment conditions across sites for some small-seeded species than for large-seeded species. These results contradict the common belief that highly fecund small-seeded species suffer from microsite-limited recruitment. Rather, they imply that, at least episodically, recruitment can be strongly correlated to plant fecundity. However, proportional recruitment of small-seeded species was inhibited at productive sites to a greater extent than large-seeded species. Results also show that in a system where the dominant granivore prefers larger seeds, low-fecundity large-seeded species can suffer from even greater seed-limited recruitment than would occur in the absence of predators.

Seed limitation in an Amazonian floodplain forest

We monitored a close-spaced grid of 289 seed traps in 1.44 ha for 8.4 yr in an Amazonian floodplain forest. In a tree community containing hundreds of species, a median of just three to four species of tree seeds falls annually into each 0.5-m² establishment site. The number of seed species reaching a given site increased linearly with time for the duration of the monitoring period, indicating a roughly random arrival of seed species in a given site-year. The number of seed species captured each year over the entire grid ranged from one-third to one-half of the total captured over the 8.4 yr of monitoring, revealing a substantial temporal component of variation in the seed rain. Seed rain at the 0.5-m² scale displayed extreme spatial variability when all potentially viable seeds were tallied, whereas the rain of dispersed seeds was scant, more nearly uniform, and better mixed. Dispersal limitation, defined as failure of seeds to reach establishment sites, is ≥99% per year for a majority of species, explaining why seed augmentation experiments are often successful. Dispersal limitation has been evoked as an explanation for distance-dependent species turnover in tropical tree communities, but that interpretation contrasts with the fact that many Amazonian tree species possess large geographical ranges that extend for hundreds or thousands of kilometers. A better understanding of the processes that bridge the gap between the scales of seedling establishment and the regulation of forest composition will require new methodologies for studying dispersal on scales larger than those yet achieved.

Traits linked with species invasiveness and community invasibility vary with time, stage and indicator of invasion in a long-term grassland experiment

Much uncertainty remains about traits linked with successful invasion - the establishment and spread of non-resident species into existing communities. Using a 20-year experiment, where 50 non-resident (but mostly native) grassland plant species were sown into savannah plots, we ask how traits linked with invasion depend on invasion stage (establishment, spread), indicator of invasion success (occupancy, relative abundance), time, environmental conditions, propagule rain, and traits of invaders and invaded communities. Trait data for 164 taxa showed that invader occupancy was primarily associated with traits of invaders, traits of recipient communities, and invader-community interactions. Invader abundance was more strongly associated with community traits (e.g. proportion legume) and trait differences between invaders and the most similar resident species. Annuals and invaders with high-specific leaf area were only successful early in stand development, whereas invaders with conservative carbon capture strategies persisted long-term. Our results indicate that invasion is context-dependent and long-term experiments are required to comprehensively understand invasions.

Factors influencing recruitment and appearance of bull kelp, Nereocystis luetkeana (phylum Ochrophyta)

The dynamics of annual species are strongly tied to their capacity for recruitment each year. We examined how competition and propagule availability influence recruitment and appearance and tracked survivorship of an annual species of marine macroalgae, the bull kelp (Nereocystis luetkeana), which serves as major biogenic habitat in the Salish Sea of Washington State. We hypothesized that (i) juvenile N. luetkeana would exhibit a seasonal appearance as a cohort in the spring and (ii) competition for space would be more limiting than propagules (spores) to recruitment at sites adjacent to established N. luetkeana beds. We tagged N. luetkeana recruits in the field to track appearance and survivorship across seasons (spring, summer, fall, and winter), using a two-factor crossed design to assess effects of competition and propagule availability on appearance of new N. luetkeana sporophytes. Survivorship of N. luetkeana recruits was low and, whereas most new individuals arose in the spring, some appeared in every season. New N. luetkeana recruits also appeared the earliest (median 8 weeks vs. >20 weeks) after experimental "seeding" in the spring as compared to other seasons. Eliminating macroalgal competitors ("clearing") influenced the appearance of recruits more than enhancement of propagules in the spring. An improved understanding of factors regulating the seasonal appearance of new N. luetkeana sporophytes furthers our understanding of this crucial foundation species' appearance and persistence across seasons, which is increasingly important as global ocean conditions change, and highlights the importance of studying organisms with complex life histories across multiple stages and geographical regions.

Introduced species that overcome life history tradeoffs can cause native extinctions

Introduced species threaten native biodiversity, but whether exotic species can competitively displace native species remains contested. Building on theory that predicts multi-species coexistence based on a competition-colonisation tradeoff, we derive a mechanistic basis by which human-mediated species invasions could cause extinctions through competitive displacement. In contrast to past invasions, humans principally introduce modern invaders, repeatedly and in large quantities, and in ways that can facilitate release from enemies and competitors. Associated increases in exotic species’ propagule rain, survival and competitive ability could enable some introduced species to overcome the tradeoffs that constrain all other species. Using evidence from metacommunity models, we show how species introductions could disrupt species coexistence, generating extinction debts, especially when combined with other forms of anthropogenic environmental change. Even though competing species have typically coexisted following past biogeographic migrations, the multiplicity and interactive impacts of today’s threats could change some exotic species into agents of extinction.

Introduced species may displace ecologically similar native species, but mechanisms are still to be established. Here, Catford et al. provide theoretical evidence of how human-mediated species invasions may overcome competition-colonisation tradeoffs, leading to the local extinction of native species.

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.

Plant density can increase invertebrate postdispersal seed predation in an experimental grassland community

Janzen–Connell effects are negative effects on the survival of a plant's progeny at high conspecific densities or close to its conspecifics. Although the role of Janzen–Connell effects on the maintenance of plant diversity was frequently studied, only few studies targeted Janzen–Connell effects via postdispersal seed predation in temperate grassland systems. We examined effects of conspecific density (abundance of conspecific adult plants) on postdispersal seed predation by invertebrates of three grassland species (Centaurea jacea, Geranium pratense, and Knautia arvensis) in experimental plant communities. Additionally, we examined the impact of plant species richness and different seed predator communities on total and relative seed predation (= seed predation of one plant species relative to others). We offered seeds in an exclusion experiment, where treatments allowed access for (1) arthropods and slugs, (2) arthropods only, (3) small arthropods only, and (4) slugs only. Treatments were placed in plots covering a gradient of abundance of conspecific adults at different levels of plant species richness (1, 2, 3, 4, 8 species). Two of the plant species (C. jacea and K. arvensis) experienced higher rates of seed predation and relative predation with increasing abundance of conspecific adults. For C. jacea, this effect was mitigated with increasing plant species richness. Differences in seed predator communities shifted seed predation between the plant species and changed the magnitude of seed predation of one plant species relative to the others. We exemplify density‐dependent increase in seed predation via invertebrates in grassland communities shaping both the total magnitude of species‐specific seed predation and seed predation of one species relative to others. Further differences in seed predator groups shift the magnitude of seed predation between different plant species. This highlights the importance of invertebrate seed predation to structure grasslands via density‐dependent effects and differing preferences of consumer groups.

Propagule pressure-invasibility relationships: testing the influence of soil fertility and disturbance with Lespedeza cuneata

Although invasion risk is expected to increase with propagule pressure (PP), it is unclear whether PP-invasibility relationships follow an asymptotic or some other non-linear form and whether such relationships vary with underlying environmental conditions. Using manipulations of PP, soil fertility and disturbance, we tested how each influence PP-invasibility relationships for Lespedeza cuneata in a Kansas grassland and use recruitment curve models to determine how safe sites may contribute to plant invasions. After three growing seasons, we found that the PP-invasibility relationships best fit an asymptotic model of invasion reflecting a combination of density-independent and density-dependent processes and that seeds were aggregated within the plant community despite efforts to uniformly sow seeds. Consistent with some models, community invasibility decreased with enhanced soil fertility or reduced levels of disturbance in response to changes in the fraction of safe sites. Our results illustrate that disturbance and soil fertility can be a useful organizing principle for predicting community invasibility, asymptotic models are a reasonable starting point for modeling invasion, and new modeling techniques—coupled with classic experimental approaches—can enhance our understanding of the invasion process.

Use of spatial analysis to test hypotheses on plant recruitment in a hyper-arid ecosystem

Mounds originating from wind-blown sediment accumulation beneath vegetation (nebkhas) often indicate land degradation in dry areas. Thus far, most nebkha research has focused on individual plants. Here, we aimed to explore population-scale processes (up to scales of about 100 m) that might explain an observed nebkha landscape pattern. We mapped the Rhazya stricta Decne. population in a 3 ha study site in a hyper-arid region of Saudi Arabia. We compared the spatial patterns of five different cohorts (age classes) of observed nebkha host plants to those expected under several hypothesized drivers of recruitment and intraspecific interaction. We found that all R. stricta cohorts had a limited fractional vegetation cover and established in large-scale clusters. This clustering weakened with cohort age, possibly indicating merging of neighboring vegetation patches. Different cohort clusters did not spatially overlap in most cases, indicating that recruitment patterns changed position over time. Strong indications were found that the main drivers underlying R. stricta spatial configurations were allogenic (i.e. not driven by vegetation) and dynamic. Most likely these drivers were aeolian-driven sand movement or human disturbance which forced offspring recruitment in spatially dynamic clusters. Competition and facilitation were likely active on the field site too, but apparently had a limited effect on the overall landscape structure.

The role of propagule pressure, genetic diversity and microsite availability for Senecio vernalis invasion

Genetic diversity is supposed to support the colonization success of expanding species, in particular in situations where microsite availability is constrained. Addressing the role of genetic diversity in plant invasion experimentally requires its manipulation independent of propagule pressure. To assess the relative importance of these components for the invasion of Senecio vernalis, we created propagule mixtures of four levels of genotype diversity by combining seeds across remote populations, across proximate populations, within single populations and within seed families. In a first container experiment with constant Festuca rupicola density as matrix, genotype diversity was crossed with three levels of seed density. In a second experiment, we tested for effects of establishment limitation and genotype diversity by manipulating Festuca densities. Increasing genetic diversity had no effects on abundance and biomass of S. vernalis but positively affected the proportion of large individuals to small individuals. Mixtures composed from proximate populations had a significantly higher proportion of large individuals than mixtures composed from within seed families only. High propagule pressure increased emergence and establishment of S. vernalis but had no effect on individual growth performance. Establishment was favoured in containers with Festuca, but performance of surviving seedlings was higher in open soil treatments. For S. vernalis invasion, we found a shift in driving factors from density dependence to effects of genetic diversity across life stages. While initial abundance was mostly linked to the amount of seed input, genetic diversity, in contrast, affected later stages of colonization probably via sampling effects and seemed to contribute to filtering the genotypes that finally grew up. In consequence, when disentangling the mechanistic relationships of genetic diversity, seed density and microsite limitation in colonization of invasive plants, a clear differentiation between initial emergence and subsequent survival to juvenile and adult stages is required.

Strong microsite control of seedling recruitment in tundra

The inclusion of environmental variation in studies of recruitment is a prerequisite for realistic predictions of the responses of vegetation to a changing environment. We investigated how seedling recruitment is affected by seed availability and microsite quality along a steep environmental gradient in dry tundra. A survey of natural seed rain and seedling density in vegetation was combined with observations of the establishment of 14 species after sowing into intact or disturbed vegetation. Although seed rain density was closely correlated with natural seedling establishment, the experimental seed addition showed that the microsite environment was even more important. For all species, seedling emergence peaked at the productive end of the gradient, irrespective of the adult niches realized. Disturbance promoted recruitment at all positions along the environmental gradient, not just at high productivity. Early seedling emergence constituted the main temporal bottleneck in recruitment for all species. Surprisingly, winter mortality was highest at what appeared to be the most benign end of the gradient. The results highlight that seedling recruitment patterns are largely determined by the earliest stages in seedling emergence, which again are closely linked to microsite quality. A fuller understanding of microsite effects on recruitment with implications for plant community assembly and vegetation change is provided.

Seedling dynamics at elevation limits: Complex interactions beyond seed and microsite limitations

PREMISE OF THE STUDY

Limited availability of seeds and microsites are important constraints for seedling emergence and survival. However, little is known about population-level feedbacks between seed and microsite limitation and how these relationships vary in contrasting conditions. Focusing on Armeria caespitosa, a high-mountain endemic, we asked whether seedling establishment was simultaneously limited by seed and microsite availability, whether the balance between seed and microsite limitations varied with contrasting environments, and whether seed and microsite limitations interacted with each other. •

METHODS

We studied seedling emergence and survival at the edges of the elevation range of A. caespitosa in central Spain over 4 years. We used mixed linear models and structural equation modeling. •

KEY RESULTS

Our results showed that A. caespitosa recruits similar numbers of new seedlings per plot in contrasting environments. Seedling emergence and survival in A. caespitosa were both limited by seed and microsite availability. Habitat suitability (the surrogate of microsite limitation) varied between populations and indirectly affected fertility (the surrogate of seed limitation). •

CONCLUSIONS

Seedling emergence and survival are both seed and microsite limited, there are interactions between seed and microsite limitations, and both factors, which control seedling establishment, and their interconnections, vary among local populations.