Density-dependent Seedling Mortality Research Articles

Correction to: Microbial communities associated with distance- and density-dependent seedling mortality in a tropical rainforest

The article entitled “Microbial communities associated with distance- and density-dependent seedling mortality in a tropical rainforest”, which is part of the special issue on “Applying microbial communities to improve restoration and conservation outcomes” was published prematurely in Volume 221, Issue 1, January 2020.

Microbial communities associated with distance- and density-dependent seedling mortality in a tropical rainforest

The high levels of diversity within tropical rainforest communities has been linked to non-random patterns of seedling mortality with several studies implicating pathogenic plant–microbe interactions in driving mortality processes. Despite the proposed importance of microorganisms in maintaining rainforest diversity, few studies have investigated soil community dynamics in relation to non-random mortality processes. A mechanistic understanding of microbial processes that help create rainforest diversity is critical for the conservation of these ecosystems. This study investigated microbial community dynamics that may underpin distance- and density-dependent mortality in the long-term forest dynamics plot, Davies Creek, in tropical Far North Queensland using community fingerprinting. We hypothesized that: (1) microbial involvement in distance-dependent seedling mortality would result in an increase in community similarity or the presence of predictor OTUs in conspecific adult tree rhizospheres, relative to physically nearby heterospecifics; (2) on average, plant species identified as having a history of distance dependent seedling mortality would exhibit more similar microbial communities among their conspecific individuals, than those that did not; and (3) dense patches of conspecific seedlings would promote the assembly of distinct soil microbial communities, which may be involved in density-dependent seedling mortality. We found no evidence of rhizosphere community similarity amongst adult plant rhizospheres. However, the presence of densely germinating seedlings altered the soil communities relative to seedling-sparse soils, enriching different OTUs depending on the patch location.

Adult trees cause density-dependent mortality in conspecific seedlings by regulating the frequency of pathogenic soil fungi.

Negative density-dependent seedling mortality has been widely detected in tropical, subtropical and temperate forests, with soil pathogens as a major driver. Here we investigated how host density affects the composition of soil pathogen communities and consequently influences the strength of plant-soil feedbacks. In field censuses of six 1-ha permanent plots, we found that survival was much lower for newly germinated seedlings that were surrounded by more conspecific adults. The relative abundance of pathogenic fungi in soil increased with increasing conspecific tree density for five of nine tree species; more soil pathogens accumulated around roots where adult tree density was higher, and this greater pathogen frequency was associated with lower seedling survival. Our findings show how tree density influences populations of soil pathogens, which creates plant-soil feedbacks that contribute to community-level and population-level compensatory trends in seedling survival.

Elevational variation in density dependence in a subtropical forest.

Density-dependent mortality has been recognized as an important mechanism that underpins tree species diversity, especially in tropical forests. However, few studies have attempted to explore how density dependence varies with spatial scale and even fewer have attempted to identify why there is scale-dependent differentiation. In this study, we explore the elevational variation in density dependence. Three 1-ha permanent plots were established at low and high elevations in the Heishiding subtropical forest, southern China. Using data from 1200 1 m2 seedling quadrats, comprising of 200 1 m2 quadrats located in each 1-ha plot, we examined the variation in density dependence between elevations using a generalized linear mixed model with crossed random effects. A greenhouse experiment also investigated the potential effects of the soil biota on density-dependent differentiation. Our results demonstrated that density-dependent seedling mortality can vary between elevations in subtropical forests. Species found at a lower elevation suffered stronger negative density dependence than those found at a higher elevation. The greenhouse experiment indicated that two species that commonly occur at both elevations suffered more from soilborne pathogens during seed germination and seedling growth when they grew at the lower elevation, which implied that soil pathogens may play a crucial role in density-dependent spatial variation.

Density dependence across multiple life stages in a temperate old-growth forest of northeast China.

Recent studies on species coexistence suggest that density dependence is an important mechanism regulating plant populations. However, there have been few studies of density dependence conducted for more than one life-history stage or that control for habitat heterogeneity, which may influence spatial patterns of survival and mask density dependence. We explored the prevalence of density dependence across multiple life stages, and the effects of controlling for habitat heterogeneity, in a temperate forest in northeast China. We used generalized linear mixed-effects models to test for density-dependent mortality of seedlings and spatial point pattern analysis to detect density dependence for sapling-to-juvenile transitions. Conspecific neighbors had a negative effect on survival of plants in both life stages. At the seedling stage, we found a negative effect of conspecific seedling neighbors on survival when analyzing all species combined. However, in species-level analyses, only 2 of 11 focal species were negatively impacted by conspecific neighbors, indicating wide variation among species in the strength of density dependence. Controlling for habitat heterogeneity did not alter our findings of density dependence at the seedling stage. For the sapling-to-juvenile transition stage, 11 of 15 focal species showed patterns of local scale (<10 m) conspecific thinning, consistent with negative density dependence. The results varied depending on whether we controlled for habitat heterogeneity, indicating that a failure to account for habitat heterogeneity can obscure patterns of density dependence. We conclude that density dependence may promote tree species coexistence by acting across multiple life-history stages in this temperate forest.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2481-y) contains supplementary material, which is available to authorized users.

Widespread density‐dependent seedling mortality promotes species coexistence in a highly diverse Amazonian rain forest

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 Yasuni 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.

Widespread density-dependent seedling mortality promotes species coexistence in a highly diverse Amazonian rainforest

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.

Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar

Herbivores and pathogens with acute host specificity may promote high tree diversity in tropical forests by causing distance- and density-dependent mortality of seedlings, but evidence is scarce. Although Lepidoptera larvae are the most abundant and host-specific guild of herbivores in these forests, their impact upon seedling distributions remains largely unknown. A firm test of the mechanism underpinning the Janzen-Connell hypothesis is difficult, even for a single tree species, because it requires more than just manipulating seeds and seedlings and recording their fates. Experimental tests require: (1) an insect herbivore that is identified and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3) evaluation and confirmation among many conspecific adult trees across years. Here we present experimental evidence for a spatially explicit interaction between newly germinating seedlings of a Neotropical emergent tree, big-leaf mahogany (Swietenia macrophylla, Meliaceae), and caterpillars of a noctuid moth (Steniscadia poliophaea). In the understory of a southeastern Amazon forest, the proportion of attacks, leaf area lost, and seedling mortality due to this specialised herbivore peaked near Swietenia trees, but declined significantly with increasing distance from mature fruiting trees, as predicted by the Janzen-Connell hypothesis. We conclude that long-distance dispersal events (>50 m) provided an early survival advantage for Swietenia seedlings, and propose that the role of larval Lepidoptera as Janzen-Connell vectors may be underappreciated in tropical forests.

Distance- and density-dependent seedling mortality caused by several diseases in eight tree species co-occurring in a temperate forest

To examine whether the Janzen–Connell mechanism applies to temperate forests, seedling survival and causes of mortality were investigated at two distances (beneath, far) from conspecific adults and at two densities (high, low) at each distance for seedlings (n = 7935) of eight tree species co-occurring in a hardwood forest. Six of the eight species showed distance- and/or density-dependent seedling mortality mainly caused by diseases and rodents. In four of the five species primarily killed by disease (i.e. damping-off, blight, rot, powdery mildew), the infectivity (probability of infection by the disease) and/or the virulence (proportion of seedlings killed to those infected by the disease) were higher beneath than far from conspecific adults. These findings suggest that host specificity and/or spatially heterogeneous activity of natural enemies play an important role in the reciprocal replacement of tree species, maintaining species diversity in temperate forests.

Integrated management of itchgrass in a corn cropping system: modeling the effect of control tactics

Abstract A population model of itchgrass was developed for a typical corn–based cropping system in the Pacific coastal region of Costa Rica. Field experiments were conducted to quantify density-dependent seedling mortality and fecundity. Additional information required for the model was obtained from the literature. Effect of control methods on itchgrass density—including a leguminous cover crop (velvetbean), a preemergence herbicide (pendimethalin), and classical biocontrol with the head smut—alone and in combination, were investigated using the model. According to model results, the cover crop planted at high and low densities between corn rows was highly efficient, reducing the initial itchgrass density from 54 plants m−2 to 4 and 17 plants m−2, respectively. Associating velvetbean with corn solely in the first crop each year resulted in predicted itchgrass densities of 33 and 36 plants m−2 (at high and low cover crop planting densities, respectively). The improvement in corn yield from preemergence he...

Consumer pressure, seed versus safe-site limitation, and plant population dynamics.

Plants often suffer reductions in fecundity due to insect herbivory. Whether this loss of seeds has population-level consequences is much debated and often unknown. For many plants, particularly those with long-lived seedbanks, it is frequently asserted that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number determines the magnitude of future plant recruitment and hence population abundance. However, empirical tests of this assertion are generally lacking and the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank dynamics is likely to be complex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal frequency of seed-limited recruitment, the strength of density-dependent seedling survival, and longevity of seeds in the soil influence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank, herbivores can have substantial effects on mean population abundance even if recruitment is primarily safe-site-limited in either time or space. Second, increasing seedbank longevity increases the population effects of herbivory, because annual reductions in seed input due to herbivory are accumulated in the seedbank. Third, population impacts of herbivory are robust even in the face of moderately strong density-dependent seedling mortality. These results imply that the conditions under which herbivores influence plant population dynamics may be more widespread than heretofore expected. Experiments are now needed to test these predictions.

Why is mast seeding in Chionochloa rubra (Poaceae) most extreme where seed predation is lowest?

Mast seeding (highly variable population seed crops among years) is common in the New Zealand flora, and is especially evident in high elevation populations of Chionochloa (snow tussocks). However, mast seeding is less pronounced at low elevations in New Zealand plants, including C. rubra. Past studies of high elevation Chionochloa populations showed that the main benefit of mast seeding is the satiation of pre‐dispersal insect seed predators. We therefore predicted that, to be able to set seed regularly, either C. rubra should suffer less predation than other Chionochloa species, or low elevation populations of C. rubra should experience lower seed predation than higher elevation populations. To test these predictions we surveyed seed predation in C. rubra and neighbouring C. pallens and C. flavescens at 22 New Zealand sites spanning over 1000 m in altitude. Total seed predation was not significantly lower in C. rubra (48%) than in C. pallens (22%) and C. flavescens (54%). Seed predation significantly increased with decreasing altitude, in direct contradiction to our prediction. This reveals a reproductive paradox: why do low elevation populations of C. rubra not show more pronounced mast seeding? We discuss three possible answers: (1) C. rubra has not formed altitudinal ecotypes with different critical temperatures for heavy floral induction; (2) the seed predators at low elevation are harder to satiate; or (3) low elevation populations suffer stronger disadvantages from masting through high intraspecific competition and or density dependent seedling mortality. We suggest that this paradox may also occur in many other masting plants with wide altitudinal ranges, that predator satiation will often break down at the lower altitudinal limit of the plant, and that seed predation may limit the lower elevational range of some masting plant species.

The impact of some environmental factors on the establishment of Plantago maritima seedlings and juveniles along a distributional gradient

The establishment of Plantago maritima seedlings and juveniles along a distributional gradient was investigated with pantograph technique. Mortality was closely dependent on vegetational characters such as cover, height of vegetation and amount of litter. Superimposed on these factors was mortality due to flooding. The density dependent mortality of seedlings was also investigated. The number of surrounding seedlings seemed to be of vital importance for survival, i.e. self thinning occurred most likely as a result of competition for light. The establishment and survival of seedlings cannot fully explain the distribution of adults.