The Janzen-Connell effect on the population dynamics of a Fagus engleriana- Cyclobalanopsis oxyodon community in a subtropical zone of China

The Janzen-Connell (J-C) hypothesis provides a mechanism explaining the high species diversity in tropical rainforests. It postulates that predation could cause greater mortality on seeds and seedlings near their parental trees. In this study, we tested the hypothesis in a subtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon. The study area was in the Shennongjia region, a key area of biodiversity conserva-tion in both China and the world. The recruitment probability index was used to detect the J-C effect on nine species of the community, which were more than 50 individuals. Six large adults of each species were selected, and the numbers of saplings and adults were counted at the distance intervals of 0-5, 5-10, 10-15, 15-20, and 20-25 m from each focal tree. Two species in saplings stage and six in adult stage supported the J-C hypothesis, but their χ2 was not significant. Three species, the F. engleri-ana, Rhododendron hypoglaucum, and Toona sinensis, showed a strong Hubbell pattern in the adult stage. Because of these results, we reject the J-C hypothesis and conclude that species could recruit near the conspecific trees in subtropical forest. The reasons why the J-C hypothesis fails to explain the species diversity in this community are the shortage of seed-consuming agents of subtropical forest and the influence of microsite topo-graphic variation.     

The interaction between propagule pressure,habitat suitability and density‐dependent reproduction in species invasion

Seedling recruitment limitations create a demographic bottleneck that largely determines the viability and structure of plant populations and communities, and pose a core restriction on the colonization of novel habitat. We use a shade‐tolerant, invasive grass, Microstegium vimineum, to examine the interplay between seed and establishment limitations – phenomena that together determine recruitment success but usually are investigated individually. We add increasing amounts of seed to microhabitats containing variable levels of leaf litter thickness – with reduced leaf litter simulating disturbance – to investigate whether reduced seed limitation overcomes the establishment limitation posed by litter cover. We do this across gradients in understory light, moisture and temperature, and quantify germination, survival, and then per capita adult biomass and reproduction in order to understand the implications for invasion across the landscape. We find that the combined effects of seed and establishment limitation influence recruitment; however, propagule pressure overwhelms the inhibitory effects of leaf litter thickness. Leaf litter reduces germination by 22–57% and seedling survival by 13–15% from that observed on bare soil. However, density‐dependent reproduction compensates as 1–3 plants can produce far more seeds (approx. 525) than are required for persistence. As such, just a few plants may establish in understory forest habitat and subsequently overwhelm establishment barriers with copious propagule production. These results, for a widespread, invasive plant, are consistent with the emerging perspective for native plants that seed and establishment limitation jointly influence recruitment. The ability for an exotic plant species to compensate for low population densities with high per capita seed production, that then overrides establishment limitations, makes its invasive potential daunting. Further work is required to test if this is a common mechanism underlying plant invasions.     

Janzen–Connell effects partially supported in reef-building corals: adult presence interacts with settler density to limit establishment

In many plant and sessile marine invertebrate (SMI) taxa, population and community dynamics are heavily influenced by processes occurring during the dispersal and establishment phases. The Janzen–Connell (J–C) hypothesis predicts increased survival of early life stages with decreasing conspecific density and increased distance from conspecific adults. Evidence of J–C effects in maintaining diversity is common in plant communities, but its importance in SMI communities remains unclear. Under controlled aquarium conditions, we examined the effect of density-dependence and adult conspecific water treatments (absent/present) on propagule settlement success and settler post-settlement survival, along with associated spatial patterns, for six broadcast-spawning, reef-building coral species from three families. We also tested if settlement success was linked to increasing propagule species diversity for three coral species from two families. We found that the probability of settlement was density independent and not influenced by adult present water treatments. Yet, adult present water treatments and settler density did have a synergistic negative effect on the probability of short-term settler survival for all species examined. Settlers also showed greater spatial aggregation as their numbers increased, but were less aggregated in adult present water treatments compared to those in adult absent water treatments. We further show evidence of significant species interactions among propagules, as settlement in single-species trials was four-fold higher compared to mixed-species trials. Our findings from controlled experimental arenas indicate that the early establishment of corals was predominantly limited by density-dependent settler–adult interactions among conspecifics and propagule–propagule interactions among heterospecifics. Thus, the proximity to established conspecific adults, settler density and species diversity of propagules are relevant drivers of local coral community diversity and structure. Based on these outcomes, we suggest that the J–C hypothesis, with demonstrated importance for plants, is partially upheld for reef corals.     

Propagule pressure and environmental conditions interact to determine establishment success of an invasive plant species,glossy buckthorn (<Emphasis Type="Italic">Frangula alnus</Emphasis>), across five different wetland habitat types

Many invasive plant species are able to establish within a wide range of community types. This establishment success depends on high propagule pressure and successful recruitment of seedlings following propagule dispersal into receptive environments. This study aimed to investigate interactions between propagule pressure and environmental resistance to seedling recruitment of the invasive shrub, glossy buckthorn (Frangula alnus Mill.), over a range of wetland habitat types. We measured propagule deposition using seed traps and recruitment success using sown plots, while characterizing vegetation and abiotic environmental conditions in five adjacent wetland habitat types. Drier habitats, which included Cedar Swamp, Shrub Carr, and String, had lower resistance to buckthorn establishment than the wetter Flark and Cattail Marsh. The drier habitats supported more woody species and provided more raised hummock surfaces essential for successful buckthorn recruitment and establishment. Propagule pressure was also higher in dry habitats that supported higher densities of adult glossy buckthorn, while long-distance dispersal into areas with low adult density was uncommon. Natural recruitment was highest in sites with intense propagule pressure, but experimental sowing of seeds demonstrated that buckthorn establishes in wet sites with higher resistance if propagule pressure is increased and seeds are deposited on hummocks. This study demonstrates the affinity of glossy buckthorn for drier wetland sites, and provides empirical evidence that environmental resistance can be overcome by higher propagule pressure.     

Plants as reef fish: fitting the functional form of seedling recruitment

The life histories of many species depend first on dispersal to local sites and then on establishment. After dispersal, density-independent and density-dependent mortalities modify propagule supply, determining the number of individuals that establish. Because multiple factors influence recruitment, the dichotomy of propagule versus establishment limitation is best viewed as a continuum along which the strength of propagule or establishment limitation changes with propagule input. To evaluate the relative importance of seed and establishment limitation for plants, we (1) describe the shape of the recruitment function and (2) use limitation and elasticity analyses to quantify the sensitivity of recruitment to perturbations in seed limitation and density-independent and density-dependent mortality. Using 36 seed augmentation studies for 18 species, we tested four recruitment functions against one another. Although the linear model (accounting for seed limitation and density-independent mortality) fitted the largest number of studies, the nonlinear Beverton-Holt model (accounting for density-dependent mortality) performed better at high densities of seed augmentation. For the 18 species, seed limitation constrained population size more than other sources of limitation at ambient conditions. Seedling density reached saturation with increasing seed density in many studies, but at such high densities that seedling density was primarily limited by seed availability rather than microsite availability or density dependence.     

Impacts of logging on density-dependent predation of dipterocarp seeds in a South East Asian rainforest

Much of the forest remaining in South East Asia has been selectively logged. The processes promoting species coexistence may be the key to the recovery and maintenance of diversity in these forests. One such process is the Janzen-Connell mechanism, where specialized natural enemies such as seed predators maintain diversity by inhibiting regeneration near conspecifics. In Neotropical forests, anthropogenic disturbance can disrupt the Janzen-Connell mechanism, but similar data are unavailable for South East Asia. We investigated the effects of conspecific density (two spatial scales) and distance from fruiting trees on seed and seedling survival of the canopy tree Parashorea malaanonan in unlogged and logged forests in Sabah, Malaysia. The production of mature seeds was higher in unlogged forest, perhaps because high adult densities facilitate pollination or satiate pre-dispersal predators. In both forest types, post-dispersal survival was reduced by small-scale (1 m(2)) conspecific density, but not by proximity to the nearest fruiting tree. Large-scale conspecific density (seeds per fruiting tree) reduced predation, probably by satiating predators. Higher seed production in unlogged forest, in combination with slightly higher survival, meant that recruitment was almost entirely limited to unlogged forest. Thus, while logging might not affect the Janzen-Connell mechanism at this site, it may influence the recruitment of particular species.     

Seed supply, recruitment, and assembly: quantifying relative seed and establishment limitation in a plant community context

There is a growing consensus that the relative constraints of seed limitation and establishment limitation in recruitment strongly influence abundance patterns in plant communities. Although these constraints have direct relevance to coexistence, most investigations utilize a seed addition approach that offers limited insight into these dynamics. Here we report the results of an assembly experiment with annual plant species from California grasslands to examine how propagule pool characteristics (dominant species abundance, functional diversity) influence establishment and seed limitation (density independence and density dependence across a gradient of seed supply) for each species, as well as how these constraints affect community diversity. Species were predominantly colimited by seed and establishment constraints, exhibiting saturating recruitment functions with increased seed supply. Consistent with competition-colonization trade-off predictions, recruitment constraints often depended on the degree of seed limitation of the competitive dominant, Brassica nigra; diversity was greatest in communities where Brassica was seed limited. Functional similarity within the propagule pool did not affect recruitment across a range of seed supply; likewise, functional diversity of the propagule pool was not related to community diversity. We conclude that seed limitation of the dominant species rather than niche similarity influences interspecific competition for safe sites and scales up to affect community-level diversity.     

Testing for Janzen–Connell Effects in a West African Montane Forest

The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the parent tree, together promote regular spacing of species and thus α diversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense, Deinbollia pinnata and Sterculia setigera), and also for naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima, N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the African continent and in particular montane tropical forest and suggest that such effects may be pantropical.     

Propagule pressure governs establishment of an invasive herb

The success of plant invasions may be limited by the availability of propagules and/or of suitable microsites, with microsite availability being affected by, for example, disturbance and interspecific competition. A mechanistic understanding of the contributions of propagule pressure and microsite limitation to plant invasions is therefore required to minimise future invasions. Here, we investigated the relative roles of propagule pressure, the availability of microsites, and their interaction on the establishment of an invasive herb, Lupinus polyphyllus, in two geographic regions representing different climate and growth conditions in Finland (a more productive southern region and a harsher central region). We carried out a field experiment in 14 L. polyphyllus populations, in which we manipulated both propagule pressure and disturbance. In a complementary greenhouse experiment, we manipulated propagule pressure and interspecific competition. Seedling establishment of L. polyphyllus was higher in the more productive southern region than in the harsher central region. The number of L. polyphyllus seedlings increased with increasing propagule pressure regardless of disturbance or interspecific competition. However, the number of L. polyphyllus seedlings per sown seed (relative establishment) tended to decrease with increasing propagule pressure, indicating that the positive effect of propagule pressure on early invasion is partially counteracted by density-dependent mortality at high seed densities. Our results highlight the dominant role of propagule pressure over disturbance and interspecific competition in the establishment of L. polyphyllus, suggesting that the early stage of invasion is limited by the availability of propagules rather than the availability of suitable microsites.     

Spatial and temporal variability in propagule limitation of California native grasses

Community composition and diversity arise from limitations in propagule supply (i.e. propagule or seed limitation) and propagule establishment after arriving at a site (i.e. establishment or microsite limitation). Recent meta‐analyses suggest that the degree of propagule limitation depends on local abiotic and biotic conditions, which in turn are likely to vary spatially and temporally. Nevertheless, seed addition studies testing propagule limitation are rarely replicated in multiple locations and years and often lack experimental manipulations of critical determinants of propagule limitation, such as the density and species richness of the propagule pool and disturbance and resource supply of recipient site. The invasion of California (USA) grasslands by exotic annual species from the Mediterranean region is unique in its scope (over 9 million ha) and persistence (more than 150 years). This invasion provides an exciting context in which to test the role of spatial and temporal variability in mediating propagule limitation and ultimately the potential for restoration. Here I present the results of native‐grass, seed‐addition trials conducted along a 500 km gradient in California in three consecutive years spanning a wide range of environmental conditions and initial conditions (e.g. rainfall, seeding density and disturbance). While native grasses were able to establish at many locations, per capita seed survival was low suggesting that the fate of these species is governed by interplay between propagule and establishment limitation. Establishment primarily varied spatially, with the most successful establishment occurring at fertile locations with low resident species richness. While recruitment was highly variable among years initially, there was no difference among seedling trials after three years.     

Negative density dependence of seed dispersal and seedling recruitment in a Neotropical palm

Negative density dependence (NDD) of recruitment is pervasive in tropical tree species. We tested the hypotheses that seed dispersal is NDD, due to intraspecific competition for dispersers, and that this contributes to NDD of recruitment. We compared dispersal in the palm Attalea butyracea across a wide range of population density on Barro Colorado Island in Panama and assessed its consequences for seed distributions. We found that frugivore visitation, seed removal and dispersal distance all declined with population density of A. butyracea, demonstrating NDD of seed dispersal due to competition for dispersers. Furthermore, as population density increased, the distances of seeds from the nearest adult decreased, conspecific seed crowding increased and seedling recruitment success decreased, all patterns expected under poorer dispersal. Unexpectedly, however, our analyses showed that NDD of dispersal did not contribute substantially to these changes in the quality of the seed distribution; patterns with population density were dominated by effects due solely to increasing adult and seed density.     

Movement strategies of seed predators as determinants of plant recruitment patterns

Plant recruitment in nature exhibits several distinctive patterns ranging from hump shaped to monotonically decreasing with distance from the seed source. We investigate the role of postdispersal seed predation in shaping these patterns, introducing a new mechanistic model that explicitly accounts for the movement strategy used by seed eaters. The model consists of two partial differential equations describing the spatiotemporal dynamics of both seed and predator densities. The movement strategy is defined by how predators move in response to the different cues they can use to search for seeds. These cues may be seed density, seed intake, distance from the plant, density of conspecific foragers, or a mixture of these four. The model is able to reproduce all the basic plant recruitment patterns found in the field. We compare the results to those of the ideal free distribution (IFD) theory and show that hump-shaped plant recruitment patterns cannot be generated by IFD predators and, in general, by foragers that respond exclusively to seed density. These foragers can produce only nonincreasing patterns, the shapes of which are determined by the foragers' navigation capacities. In contrast, hump-shaped patterns can be produced by distance-responsive predators or by foragers that use conspecifics as a cue for seed abundance.     

The Janzen-Connell effect on the population dynamics of a Fagus engleriana- Cyclobalanopsis oxyodon community in a subtropical zone of China

The Janzen-Connell (J-C) hypothesis provides a mechanism explaining the high species diversity in tropical rainforests. It postulates that predation could cause greater mortality on seeds and seedlings near their parental trees. In this study, we tested the hypothesis in a subtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon. The study area was in the Shennongjia region, a key area of biodiversity conservation in both China and the world. The recruitment probability index was used to detect the J-C effect on nine species of the community, which were more than 50 individuals. Six large adults of each species were selected, and the numbers of saplings and adults were counted at the distance intervals of 0–5, 5–10, 10–15, 15–20, and 20–25 m from each focal tree. Two species in saplings stage and six in adult stage supported the J-C hypothesis, but their χ² was not significant. Three species, the F. engleriana, Rhododendron hypoglaucum, and Toona sinensis, showed a strong Hubbell pattern in the adult stage. Because of these results, we reject the J-C hypothesis and conclude that species could recruit near the conspecific trees in subtropical forest. The reasons why the J-C hypothesis fails to explain the species diversity in this community are the shortage of seed-consuming agents of subtropical forest and the influence of microsite topographic variation.     

The distance dependence prediction of the Janzen-Connell hypothesis: a meta-analysis

The Janzen-Connell hypothesis explains the maintenance of tropical diversity through the interacting effects of parent-centered dispersal patterns and distance- and density-dependent propagule survival. These effects were thought to support regular spacing of species within tropical forest, enhancing diversity. One of the predictions of the hypothesis is that seed and seedling survival should improve with increased parental distance. Although there are many independent tests of this hypothesis for individual species, there are few synthetic studies that have brought these data together to test its validity across species. This paper reports the results of a meta-analysis of the effect of distance on enhancing propagule survival, employing an odds-ratio effect size metric. We found no general support for the distance-dependent prediction of the hypothesis, and conclude that further testing to explore this hypothesis as a diversity-maintaining mechanism is unnecessary. However, we did find that distance from parent slightly reduces survivorship in the temperate zone, as contrasted with the tropics, and we saw stronger evidence in support of the hypothesis for seedlings than for seeds. The phenomenon of enhanced propagule survival with distance from the parent may be important for the population biology of particular species, but it is not a general phenomenon across communities, life history stages or life forms.     

Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Experimental field test of spatial variation in rodent predation of nuts relative to distance and seed density

The spatial context in which seed predation occurs may modify the spatial structure of recruitment generated by seed dispersal. The Janzen–Connell (J-C) model predicts that granivores will exert greater pressure on the parent plant or at those sites where the density of dispersed seeds is higher. We have investigated how the probability of post-dispersal survival of Juglans australis varies with nut density across a hierarchy of spatial scales. We experimentally evaluated the survival of 3,120 nuts at three spatial scales: meso-scale (≤1.5 ha), as forest sites with two densities of fruiting J. australis individuals; intermediate scale (<0.2 ha), as individual trees with two experimental crop sizes; small scale (<0.1 m²), as microsites with two factors (number of nuts and distance from source). Nut removal coincided with seed predation, a condition that allowed us to test the density-dependent seed predation hypothesis. We found that the probability of nut survival was greater at forest sites with higher J. australis density. Nut survival was not affected by nut density in the seed shadow of individual specimens: at sites where J. australis density was greater, the proportion of surviving nuts did not differ between microsites located at different distances from the parent plant, but it was greater at microsites with greater initial nut density. Nut survival depended on the scale at which rodents responded to nut density, being negatively density dependent at the meso-scale and spatially random at intermediate and small scales. At the meso-scale, excess nut supply increased the probability of nut survival, which is in agreement with a model of granivore satiation near the seed source. Rodent satiation at the meso-scale may favour maintenance of sites with high J. australis density, where individual trees may have greater probabilities of passing their genes onto the next stage of the dispersal cycle.     

Factors affecting post-dispersal seed survival in a tropical forest

Summary Using the subcanopy tree Faramea occidentalis in Panama, I studied post-dispersal seed survival as a function of five characteristics describing seed locations. By simultaneously considering distance from a conspecific adult, size of the nearest conspecific adult, leaf litter quantity, proximity to logs or tree trunks, and whether or not the seed was in a gap, I was able to analyze the influences of individual factors, as well as the interactions among factors. Seed survival was significantly less in treefall gaps than in the forest understory. Seed survival was also influenced by the size of the nearest adult but in a complex interaction with distance to an adult. For seeds beneath adults, survival decreased with increasing tree size, while for seeds away from adults, survival was independent of the size of the nearest conspecific adult. Distance did not directly affect seed survival, nor did the quantity of leaf litter or the proximity to a tree trunk or a log. In a separate analysis, the relationship between distance and seed survival was consistent over four years, suggesting that single cohort studies may provide accurate insights into the consequences of dispersal. In contrast, the spatial locations of surviving seeds were not consistent over the four-year period. Transects with high survival one year did not tend to have high survival in other years, and the locations of surviving seeds in any particular year could not be predicted from the knowledge of where seeds survived in other years. While survival is patchy within a year, the locations of patches shift from year to year.     

Propagule banks, herbivory and nutrient supply control population development and dominance patterns in macroalgal blooms

Destructive macroalgal mass blooms threaten estuarine and coastal ecosystems worldwide. We asked which factors regulate macroalgal bloom intensity, distribution and species composition. In field experiments in the Baltic Sea, we analyzed the relative effects of nutrients, herbivores and algal propagule banks on population development and dominance patterns in two co-occurring bloom-forming macroalgae, Enteromorpha intestinalis and Pilayella littoralis . Both species were highly affected by the combined effects of a propagule bank, herbivory and nutrients. The magnitude of effects varied with season. The propagule bank was an important overwintering mechanism for both algae, and allowed for recruitment two months earlier than recruitment via freshly dispersed propagules. This provided a seasonal escape from intense herbivory and nutrient limitation later in the year. Favored by massive recruitment from the propagule bank, Enteromorpha was the superior space occupier in early spring, thereby reducing recruitment of Pilayella . Elimination of the propagule bank and recruitment via freshly dispersed propagules favored Pilayella . Strong and selective herbivory on Enteromorpha supported Pilayella in the presence, but not in the absence of the propagule bank. Nutrient enrichment in summer counteracted herbivore pressure on Enteromorpha , thereby negatively affecting Pilayella . Herbivore and nutrient effects were more pronounced for early life stages than adult algae. These results show that recruitment processes and forces affecting early life stages at the beginning of the vegetation period determine development and dominance patterns of macroalgal blooms. Herbivores naturally suppress blooms but increasing nutrient enrichment can override this important control mechanism. The propagule bank plays a previously unrecognized role for population and community dynamics.     

Recruitment Distance from the Nearest Reproductive Conspecific Increases with Tree Size in Tropical Premontane Wet Forests

Studies have shown that median distances of plants from the nearest reproductive conspecific (recruitment distance) shifts outward with increasing age or size class, and that plant spatial distribution changes over time in a predictable manner. However, observations and empirical evidence for such predictable changes are limited, and underlying mechanisms explaining such patterns for a wide range of individual sizes are not fully explored. In Costa Rican premontane wet forests, I empirically tested whether recruitment distance changes in a predictable manner with increasing size for five animal‐dispersed tree species by considering all post‐germination sizes. Specifically, I tested the Janzen–Connell hypothesis and the colonization hypothesis by considering distance, density, size, herbivory, biotic infection, and light availability simultaneously. Recruitment distance increased with increasing size (16–22 m) for the four non‐pioneer species, suggesting eventual regeneration success for seeds dispersed away from reproductive conspecifics. During the 2 years of this study, I found positive distance‐dependent survivorship and light availability were important for post‐seedling survivorship, in agreement with the Janzen–Connell hypothesis and the colonization hypothesis, respectively, but only for seedlings. However, seedlings did not escape aboveground herbivory or biotic infection better when the seeds were dispersed greater distances. Results highlight the importance of seed dispersal for successful regeneration, and suggest that changes in spatial distribution over time may be predictable in the vicinity of maternal trees for some non‐pioneer tree species.     

Are all seeds equal? Spatially explicit comparisons of seed fall and sapling recruitment in a tropical forest

Understanding demographic transitions may provide the key to explain the high diversity of tropical tree communities. In a faunally intact Amazonian forest, we compared the spatial distribution of saplings of 15 common tree species with patterns of conspecific seed fall, and examined the seed-to-sapling transition in relation to locations of conspecific trees. In all species, the spatial pattern of sapling recruitment bore no resemblance to predicted distributions based on the density of seed fall. Seed efficiency (the probability of a seed producing a sapling) is strongly correlated with distance from large conspecific trees, with a >30-fold multiplicative increase between recruitment zones that are most distant vs. proximal to conspecific adults. The striking decoupling of sapling recruitment and conspecific seed density patterns indicates near-complete recruitment failure in areas of high seed density located around reproductive adults. Our results provide strong support for the spatially explicit predictions of the Janzen-Connell hypothesis.