skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Friday, September 29 until 11:59 PM ET on Saturday, September 30 due to maintenance. We apologize for the inconvenience.

Title: Seed‐to‐seedling transitions exhibit distance‐dependent mortality but no strong spacing effects in a Neotropical forest

Patterns of seed dispersal and seed mortality influence the spatial structure of plant communities and the local coexistence of competing species. Most seeds are dispersed in proximity to the parent tree, where mortality is also expected to be the highest, because of competition with siblings or the attraction of natural enemies. Whereas distance‐dependent mortality in the seed‐to‐seedling transition was often observed in tropical forests, few studies have attempted to estimate the shape of the survival‐distance curves, which determines whether the peak of seedling establishment occurs away from the parent tree (Janzen–Connell pattern) or if the peak attenuates but remains at the parent location (Hubbell pattern). In this study, we inferred the probability density of seed dispersal and two stages of seedling establishment (new recruits, and seedlings 20 cm or taller) with distance for 24 tree species present in the 50‐ha Forest Dynamics Plot of Barro Colorado Island, Panama. Using data from seed traps, seedling survey quadrats, and tree‐census records spanning the 1988–2014 period, we fit hierarchical Bayesian models including parameters for tree fecundity, the shape of the dispersal kernel, and overdispersion of seed or seedling counts. We combined predictions from multiple dispersal kernels to obtain more robust inferences. We find that Hubbell patterns are the most common and Janzen–Connell patterns are very rare among those species; that distance‐dependent mortality may be stronger in the seed stage, in the early recruit stage, or comparable in both; and that species with larger seeds experience less overall mortality and less distance‐dependent mortality. Finally, we describe how this modeling approach could be extended at a community scale to include less abundant species.

more » « less
Award ID(s):
1754668 1754632
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. The Janzen–Connell hypothesis is a well-known explanation for why tropical forests have large numbers of tree species. A fundamental prediction of the hypothesis is that the probability of adult recruitment is less in regions of high conspecific adult density, a pattern mediated by density-dependent mortality in juvenile life stages. Although there is strong evidence in many tree species that seeds, seedlings, and saplings suffer conspecific density-dependent mortality, no study has shown that adult tree recruitment is negatively density dependent. Density-dependent adult recruitment is necessary for the Janzen–Connell mechanism to regulate tree populations. Here, we report density-dependent adult recruitment in the population ofHandroanthus guayacan, a wind-dispersed Neotropical canopy tree species. We use data from high-resolution remote sensing to track individual trees with proven capacity to flower in a lowland moist forest landscape in Panama and analyze these data in a Bayesian framework similar to capture–recapture analysis. We independently quantify probabilities of adult tree recruitment and detection and show that adult recruitment is negatively density dependent. The annualized probability of adult recruitment was 3.03% ⋅ year−1. Despite the detection of negative density dependence in adult recruitment, it was insufficient to stabilize the adult population ofH. guayacan, which increased significantly in size over the decade of observation.

    more » « less
  2. Abstract

    Seed dispersal and local filtering interactively govern community membership and scale up to shape regional vegetation patterns, but data revealing how and why particular species are excluded from specific communities in nature are scarce. This lack of data is a missing link between our theoretical understanding of how diversity patterns can form and how they actually form in nature, and it hampers our ability to predict community responses to climate change. Here, we compare seed, seedling, and adult plant communities at 12 grassland sites with different climates in southern Norway to examine how community membership is interactively shaped by seed dispersal and local filtering, and how this process varies with climate across sites. To do this, we divide species at each site into two groups:locally transientspecies, which occur as seeds but are rare or absent as adults (i.e., they arrive but are filtered out), andlocally persistentspecies, which occur consistently as adults in annual vegetation surveys. We then ask how and why locally transient species are disfavored during community assembly. Our results led to four main conclusions: (1) the total numbers of seeds and species that arrived, but failed to establish locally persistent populations, rose with temperature, indicating an increase in the realized effects of local filtering on community assembly, as well as an increase in the number of species poised to rapidly colonize those warmer sites if local conditions change in their favor, (2) locally transient species were selectively filtered out during seedling emergence, but not during seedling establishment, (3) selective filtering was partly driven by species climate preferences, exemplified by the poor performance of seeds dispersing outside of their realized climate niches into colder and drier foreign climates, and (4) locally transient species had traits that likely made them better dispersers (i.e., smaller seeds) but poorer competitors for light (i.e., shorter statures and less persistent clonal connections) than locally persistent species, potentially explaining why these species arrived to new sites but did not establish locally persistent adult populations. Our study is the first to combine seed, seedling, and adult survey data across sites to rigorously characterize how seed dispersal and local filtering govern community membership and shape climate‐associated vegetation patterns.

    more » « less
  3. Abstract

    The distribution of wind‐dispersed seeds around a parent tree depends on diaspore and tree traits, as well as wind conditions and surrounding vegetation. This study of a neotropical canopy tree,Platypodium elegans,explored the extent to which parental variation in diaspore and tree traits explained (1) rate of diaspore descent in still air, (2) distributions of diaspores dispersed from a 40‐m tower in the forest, and (3) natural diaspore distributions around the parent tree. The geometric mean rate of descent in still air among 20 parents was highly correlated with geometric mean wing loading1/2(r = 0.84). However, diaspore traits and rate of descent predicted less variation in dispersal distance from the tower, although descent rate−1consistently correlated with dispersal distance. Measured seed shadows, particularly their distribution edges, differed significantly among six parents (DBHrange 62–181 cm) and were best fit by six separate anisotropic dispersal kernels and surveyed fecundities. Measured rate of descent and tree traits, combined in a mechanistic seed dispersal model, did not significantly explain variation among parents in natural seed dispersal distances, perhaps due to the limited power to detect effects with only six trees. Seedling and sapling distributions were at a greater mean distance from the parents than seed distributions; saplings were heavily concentrated at far distances. Variation among parents in the distribution tails so critical for recruitment could not be explained by measured diaspore or tree traits with this sample size, and may be determined more by wind patterns and the timing of abscission in relation to wind conditions. Studies of wind dispersal need to devote greater field efforts at recording the “rare” dispersal events that contribute to far dispersal distances, following their consequences, and in understanding the mechanisms that generate them.

    more » « less
  4. Seed distribution and deposition patterns around parent trees are strongly affected by functional traits and therefore influence the development of plant communities. To assess the limitations of seed dispersal and the extent to which diaspore and neighbouring parental traits explain seed rain, we used a 9-year seed data set based on 150 seed traps in a 25-ha area of a temperate forest in the Changbai Mountain. Among 480,598 seeds belonging to 12 families, 17 genera, and 26 species were identified, only 54% of the species with mature trees in the community were represented in seeds collected over the 9 years, indicating a limitation in seed dispersal. Understory species were most limited; overstory species were least limited. Species with wind-dispersed seed had the least limitation, while the lowest similarity in species richness was for animal-dispersed species followed by gravity-dispersed species; fleshy-fruited species had stronger dispersal limitations than dry-fruited species. Generalized linear mixed models showed that relative basal area had a significant positive effect on seed abundance in traps, while the contribution of diaspore traits was low for nearly all groups. These results suggest that tree traits had the strongest contribution to seed dispersal and deposition for all functional groups examined here. These findings strengthen the knowledge that tree traits are key in explaining seed deposition patterns, at least at the primary dispersal stage. This improved knowledge of sources of seeds that are dispersed could facilitate greater understanding of seedling and community dynamics in temperate forests. 
    more » « less
  5. Abstract

    Successful forest expansion into grassland can be limited by seed dispersal and adverse conditions for tree seedlings in the grassland environment. In the high‐elevation Andes, human‐induced fragmentation has exacerbated the patchy distribution ofPolylepisforests, threatening their unique biological communities and spurring restoration interest. Studies ofPolylepisforest extent in Peru suggest that forest borders have remained stable over the past century despite decreasing anthropogenic disturbance, suggesting that tree seedling recruitment is being limited in the open grassland habitat. We studied natural seedling dispersion patterns ofPolylepis sericeaandPolylepis weberbaueri(Rosaceae) at forest–grassland edges across a range of environmental conditions to examine seedling recruitment and colonization of grasslands in Huascaran National Park (Peru). Using data from 2367 seedlings found in 48 forest–grassland edge plots (15 m × 15 m) at forest patches between 3900–4500 masl, we employed generalized mixed modelling to identify the significant associations of seedling densities with environmental covariates. In addition, we compared these associations to patterns of adult presence on the landscape. Seedling densities were associated with a combination of variables varying within (distance to forest edge) and among (elevation and dry season solar irradiation) plots across the landscape. For both species, seedling densities decreased with increasing distance away from the forest in a manner consistent with short‐distance seed dispersal by wind. Our results suggest that such short‐distance dispersal may slow forest expansion, but that there also appear to be substantial post‐dispersal limitations to seedling establishment in the grassland.Polylepis sericeadensities decreased with elevation, whileP. weberbaueriincreased with elevation and decreased with solar irradiation. Associations of adult presence with elevation and solar irradiation mirrored those of seedling densities. Management of areas with forest patches dominated by these species should consider these differences in their environmental tolerances, particularly during species selection and zonation for reforestation.

    more » « less