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1 The relative importance of seed availability and the post-dispersal environment in causing the distribution and abundance patterns of five halophytic forbs and a shrub was investigated across a New England salt marsh tidal gradient. Seed traps and soil samples were used to assess the spatial pattern of seed availability across the marsh, and experimental seed additions were performed to examine the effects of tidal elevation and interspecific competition with dominant grasses and rushes on seedling emergence and survivorship. 2 Seed distributions strongly paralleled adult plant abundance patterns across the marsh, suggesting localized dispersal with limited movement out of parental environments. 3 Adding seeds typically increased seedling densities by at least an order of magnitude, thus lack of seed availability may be important in limiting plant abundance within marsh zones. 4 Post-dispersal factors were primarily responsible for determining species distribution patterns across zones. Lower limits to the distribution of species typically found at high-marsh elevations were determined by intolerance to abiotic conditions in the lower marsh zones. In contrast, species typically found at low-marsh elevations were precluded from the high marsh due to competitive suppression by dominant plants. Patterns of post-dispersal success were strongly reinforced by limited dispersal. 5 Seed dispersal patterns and post-dispersal factors may therefore interact to generate distribution and abundance patterns in salt marsh plant communities.

Seedling dynamics are crucial for forest regeneration and species coexistence. Understanding the factors influencing seedling survival rate provides insights into improving natural regeneration. This study evaluated the effects of biotic and habitat factors on Larix gmelinii var. principis-rupprechtii seedling survival rate over 2 years on Guandi Mountain, North China, using a generalized linear mixed-effects model. Our results did not provide strong evidence of negative density dependence at the community level during the seedling stage. For seedlings < 2 cm in diameter and < 8 years old, survival rate was primarily influenced by conspecific seedling density. As seedlings grew, the impact of biotic factors declined, whereas soil and light conditions became more important. Elevation strongly affected seedlings < 0.3 m in height. With increasing height, biotic factors became more influential, and conspecific seedling density positively impacted survival rate. Seedling regeneration was found to be increasing on Guandi Mountain, with no evidence of seed limitation. Biotic and habitat factors influenced survival rate, but conspecific seedling density negatively affected survival rate at all levels except for seedlings < 0.3 m in height. Overall, negative density dependence was negligible in the early-seedling stage. In conclusion, although habitat heterogeneity and density effects shape seedling survival rate, density effect plays a more critical role in L. gmelinii var. principis-rupprechtii forest regeneration.

1. Ecologists have long recognized that plant performance is affected by the density and composition of neighbouring individuals. With the advent of highly resolved species-level phylogenies, it has become possible to test whether such density-dependent neighbourhood interactions are also phylogenetically dependent. Most studies of density dependence have focused on a single life stage; however, the relative importance of different neighbourhood interactions may shift over the lifetime of an individual. 2. We examined effects of conspecific neighbour density, heterospecific neighbour density and average phylogenetic relatedness of heterospecific neighbours on the survival of seedlings, saplings, juveniles and adult trees of 29 focal tree species using long-term, spatially explicit forest dynamics data and a highly resolved DNA barcode phylogeny from the tropical forest of Barro Colorado Island (BCI), Panama. 3. Our results show a decline in the strength of conspecific negative density dependence across life stages: strong negative conspecific neighbour effects at early life stages gave way to weak positive conspecific neighbour effects for adult trees. In contrast, the effect of heterospecific neighbour density on survival showed no clear trend with life stage. 4. We found evidence of phylogenetic density dependence in the BCI forest, with a significant negative impact of neighbourhood relatedness on focal tree survival, but only for later life stages. In contrast to studies from other tropical forests, neighbourhood relatedness had a significant positive effect on seedling survival. 5. Furthermore, we found that focal species varied much more widely in their sensitivity to conspecific neighbour density than in their reactions to heterospecific neighbour density or phylogenetic relatedness. 6. Synthesis. Overall, our results demonstrate that both conspecific density dependence and phylogenetic density dependence influence tropical tree survival, but that their relative importance varies with life stage and among species. Our study highlights the need to incorporate multiple life stages and multiple species when assessing the factors contributing to individual survival and species coexistence for long-lived organisms.

Factors that control tree seedling dynamics are critical determinants of forest diversity. We examined the role of density-dependent mortality and abiotic factors in the differential establishment and survival of tree seedlings at three, large, mapped forest plots in Indiana, Virginia, and Wisconsin, USA. We tested whether seedling densities and seedling survival are related to local biotic and abiotic factors with generalized linear mixed models. Spatial point pattern analyses were utilized to determine if the distribution patterns of seedlings and saplings are consistent with a pattern generated by negative density-dependent mortality with respect to conspecific trees. Initial sampled seedling density for nearly a third of species showed a positive correlation with increasing conspecific basal area, indicating dispersal limitation, but few had any association with abiotic variables. By contrast, survival of seedlings over one year significantly declined with increasing conspecific basal area. Point pattern analyses indicated that nearly one-third of tree species had significantly over-dispersed point patterns of conspecific seedlings and saplings relative to adult densities; the majority of other species exhibited random spatial arrangements. Our results demonstrate-that negative conspecific density-dependent mortality of seedlings could generate the spatial patterns observed at later life stages. By differentially favoring seedlings of other species, this process may contribute to the maintenance of tree diversity in temperate forests, just as others have demonstrated for tropical forests.

Seedling dynamics play a crucial role in determining species distributions and coexistence. Exploring causes of variation in seedling dynamics can therefore provide key insights into the factors affecting these phenomena. We examined the relative importance of biotic neighborhood processes and habitat heterogeneity using survival data for 5,827 seedlings in 39 tree and shrub species over 2 years from an old-growth temperate forest in northeastern China. We found significant negative density-dependence effects on survival of tree seedlings, and limited effects of habitat heterogeneity (edaphic and topographic variables) on survival of shrub seedlings. The importance of negative density dependence on young tree seedling survival was replaced by habitat in tree seedlings ≥4 years old. As expected, negative density dependence was more apparent in gravity-dispersed species compared to wind-dispersed and animal-dispersed species. Moreover, we found that a community compensatory trend existed for trees. Therefore, although negative density dependence was not as pervasive as in other forest communities, it is an important mechanism for the maintenance of community diversity in this temperate forest. We conclude that both negative density dependence and habitat heterogeneity drive seedling survival, but their relative importance varies with seedling age classes and species traits.

1. Many forests experience periodic, large-scale disturbances, such as hurricanes and cyclones, which open the forest canopy, causing dramatic changes in understorey light conditions and seedling densities. Thus, in hurricane-impacted forests, large variations in abiotic and biotic conditions likely shape seedling dynamics, which in turn will contribute to patterns of forest recovery. 2. We monitored 13 836 seedlings of 82 tree and shrub species over 10 years following Hurricane Georges in 1998 in a subtropical, montane forest in Puerto Rico. We quantified changes in the biotic and abiotic environment of the understorey and linked seedling dynamics to changes in canopy openness and seedling density, and to spatial variation in soil type, topography and tree density. 3. Canopy openness was highest when first measured after Hurricane Georges and dropped significantly within c. 3 years, while seedling densities remained high for c. 5 years post-hurricane. When all species and census intervals were analysed together, generalized linear mixed effects models revealed that canopy openness, seedling and adult tree densities were significant drivers of seedling survival. 4. The relative importance of abiotic and biotic factors changed over time. Separate analyses for each census interval revealed that canopy openness was a significant predictor of survival only for the first census interval, with lower survival at the highest levels of canopy openness. The effect of conspecific seedling density was significant in all intervals except the first, and soil type only in the final census interval. 5. When grouping species into life-history guilds based on adult tree susceptibility to hurricane damage, we found clear differences among guilds in the effects of biotic and abiotic factors on seedling survival. Seedlings of hurricane-susceptible and intermediate guilds were more strongly influenced by canopy openness, while seedlings of the hurricane-resistant group were less affected by conspecific seedling density. Individual species-level analyses for 12 common species, however, showed considerable variation among species within guilds. 6. Synthesis. Our results suggest that hurricanes shape species composition by altering understorey conditions that differentially influence the success of seedlings. Thus, predicted increases in the intensity and frequency of hurricanes in the Caribbean will likely alter seedling dynamics and ultimately the species composition in hurricane-impacted forests.

Negative density-dependent mortality can promote species coexistence through a spacing mechanism that prevents species from becoming too locally abundant. Negative density-dependent seedling mortality can be caused by interactions among seedlings or between seedlings and neighboring adults if the density of neighbors affects the strength of competition or facilitates the attack of natural enemies. We investigated the effects of seedling and adult neighborhoods on the survival of newly recruited seedlings for multiple cohorts of known age from 163 species in Yasuní National Park, Ecuador, an ever-wet, hyper-diverse lowland Amazonian rain forest. At local scales, we found a strong negative impact on first-year survival of conspecific seedling densities and adult abundance in multiple neighborhood sizes and a beneficial effect of a local tree neighborhood that is distantly related to the focal seedling. Once seedlings have survived their first year, they also benefit from a more phylogenetically dispersed seedling neighborhood. Across species, we did not find evidence that rare species have an advantage relative to more common species, or a community compensatory trend. These results suggest that the local biotic neighborhood is a strong influence on early seedling survival for species that range widely in their abundance and life history. These patterns in seedling survival demonstrate the role of density-dependent seedling dynamics in promoting and maintaining diversity in understory seedling assemblages. The assemblage-wide impacts of species abundance distributions may multiply with repeated cycles of recruitment and density-dependent seedling mortality and impact forest diversity or the abundance of individual species over longer time scales.

The mint genus Dicerandra is the rarest of any plants in the southeastern USA, and the narrow endemic D. thinicola H.A. Mill. is restricted to one wild population on public land and a few unprotected populations on private lands. From 2001 to 2017, we studied ∼9,000 plants and 90,000 annual transitions in permanent plots in Florida scrub and roadsides to assess the health of this population and potential drivers of demographic change. Plant numbers have fluctuated widely, largely due to variably large pulses of winter seedling recruitment, but the overall trend has been upward, especially along sandy roadsides, in scrub gaps, and in chopped/burned scrub. Across the data set, annual survival (mean 67%) varied among habitats (being highest along roadsides) and among years (with decreased survival in some recent years). Nearly half of surviving vegetative plants advanced from vegetative to reproductive each year and most plants, once flowering, continued to flower each year. Growth in number of branches was consistently positive; only one-quarter of plants had reduced size each year. Relative growth rate was higher along roadsides than in other habitats. Reproductive output was lowest in scrub gaps and varied among years. Seedling recruitment was concentrated in winter months, varied widely among years, and was lower in the scrub matrix than in scrub gaps or roadsides. About half of seedlings died before their second year, the maximum observed life span was 13 yr, and fewer than 6% of plants survived 10 yr. Flowering began as early as age 2 (rarely as seedlings) and by age 4–7, 90% of surviving plants were reproductive. Land management (roller chopping and fire) in 2007 had profound effects on the population. Chopping killed 91% of plants and chopping followed by burning killed 100%. However, recruitment in these treated plots was 2.5–5.5 times higher in the 6 yr after treatments than in the 8 yr before treatments; no such differences were seen in untreated plots. Posttreatment plants grew faster and flowered earlier than other plants. Consequently, after several years, plant numbers in the treated areas had increased 4–8 times, whereas plant numbers in untreated areas changed little. This state-endangered plant is short-lived and depends heavily on disturbance (ideally fire) for recruitment and population growth. It is threatened not only by its narrow distribution but by insufficiently aggressive land management.

1. Differences in seedling survival in trees have a lasting imprint on seedling, juvenile and adult community structure. Identifying the drivers of these differences, therefore, is a critical research objective that ultimately requires knowledge regarding how organismal function interacts with the local environment to influence survival rates. 2. In tree communities, differences in light use strategies are frequently invoked to explain differences in seedling demographic performance through growth and survival trade-offs. For example, shade-tolerant species grow slowly and have higher survival rates, whereas shade-intolerant species grow quickly but have lower survival rates. Thus, functional traits related to photosynthesis should be strong predictors of demographic rates, but results in the literature are mixed indicating that additional or alternative information regarding organismal function should be considered. 3. Here, we provide a community-wide inventory of transcriptomes in a subtropical tree community. This information is utilized to determine the degree to which species share homologous genes related to gene ontologies for light use and harvesting. These species similarities are used in neighbourhood generalized linear mixed-effects models of seedling survival that evaluated seedling survival as a function of the transcriptomic, functional trait and phylogenetic composition of the local neighbourhood. The results show neighbourhood similarity in three of the 15 gene ontologies evaluated are significantly related to survival rates based on neighbourhood composition. For two of these ontologies, survival rates increase when neighbours are similar in their gene tree composition indicating the importance of abiotic filtering and performance hierarchies. 4. Synthesis. The present work takes a novel approach by sequencing the transcriptomes of naturally co-occurring tree species in a subtropical forest in China. The results show that the transcriptomic similarity of species is a significant predictor of differential survival. The study demonstrates that exploring the functional genomic similarity of non-model species in nature has the potential to increase the breadth and depth of our understanding of how gene function influences species co-occurrence and population dynamics in communities.

Evaluating the mechanisms that drive plant invasions in grassland ecosystems can provide insight into subtle, yet critical, drivers of ecosystem function. Common hypotheses for invader success are that (1) an invader’s physiology may allow better use of resources and competitively exclude resident natives; (2) the lack of invader‐specific herbivores and diseases may result in increased growth and competitive advantage over natives; (3) invaders may bring pathogens or herbivores that negatively impact native plants directly; (4) the invader may be able to establish earlier or later than natives, giving it a priority effect or phenological niche separation; (5) the invader may produce more seeds, and propagule pressure may allow it to eventually dominate; and (6) once the invader occurs on a site, its plant detritus or litter can offer a legacy effect advantage. Yet, a widely neglected aspect of invasion is that invaders may prime the conditions in places other than where they are currently established, giving them a pioneer effect advantage. These pioneer effects can come from plant materials that move from invaded to uninvaded areas through three pathways—pollen, litter, seeds, and their associated microbiomes—each of which can favor the invader directly or indirectly. In turn, unsuccessful cohorts of seedlings can change the biotic community through root exudates as well as any biotic load they bring with them, along with taking up nutrients and potentially subverting the litter decay cycle. When seedlings of invaders occur at times when the native species have evolved not to germinate, site priming impacts may increase or accelerate. We propose that monitoring of uninvaded areas adjacent to invaded areas is needed to assess the emerging significance of pioneer effects and site priming in advancing plant invasions, especially in grassland systems worldwide.