The Relative Importance of Fungal Infection,Conspecific Density and Environmental Heterogeneity for Seedling Survival in a Dominant Tropical Tree

A gap remains in our understanding of how host‐specific fungal pathogens impact negative density dependence (NDD). Here, we investigated survival of Cinnamomum subavenium Miq. seedlings, the dominant canopy species in a seasonal tropical evergreen forest, Thailand. It is infected by a host‐specific fungus that is easily identifiable in the field. We quantified the effects of conspecific seedling and adult density on fungal infection and seedling survival over a wide range of environmental heterogeneity in elevation, understory vegetation and presence of forest gaps. Generalized linear mixed models (GLMMs) for seedling survival revealed that fungal infection significantly reduced survival and had the strongest effect on seedling survival as compared with conspecific density and environmental heterogeneity. Adult conspecific density was not, however, significantly correlated with the probability of infection, and conspecific seedling density was positively associated with increased infection only at high elevations. In contrast to infection, we found a significant positive correlation between conspecific seedling density and the probability of seedling survival. Consequently, our results demonstrate that fungal infection can have major impacts on seedling survival, but not in a manner consistent with local NDD effects on seedlings, as assumed in the Janzen–Connell hypothesis. Our study provides an example of how quantifying the interaction between environmental heterogeneity and a host‐specific plant‐pathogen can yield unexpected insights into the dynamics of seedling populations. The combined effects of host‐specific pathogens and environmental heterogeneity on survival of dominant seedling species may ultimately provide a chance for rarer species to recruit.     

Altered climate leads to positive density‐dependent feedbacks in a tropical wet forest

Climate change is predicted to result in warmer and drier Neotropical forests relative to current conditions. Negative density‐dependent feedbacks, mediated by natural enemies, are key to maintaining the high diversity of tree species found in the tropics, yet we have little understanding of how projected changes in climate are likely to affect these critical controls. Over 3 years, we evaluated the effects of a natural drought and in situ experimental warming on density‐dependent feedbacks on seedling demography in a wet tropical forest in Puerto Rico. In the +4°C warming treatment, we found that seedling survival increased with increasing density of the same species (conspecific). These positive density‐dependent feedbacks were not associated with a decrease in aboveground natural enemy pressure. If positive density‐dependent feedbacks are not transient, the diversity of tropical wet forests, which may rely on negative density dependence to drive diversity, could decline in a future warmer, drier world.     

Density‐dependent Survival in Seedlings Differs among Woody Life‐forms in Tropical Wet Forests of a Caribbean Island

Negative density dependence contributes to seedling dynamics in forested ecosystems, but the relative importance of this factor for different woody plant life‐forms is not well‐understood. We used 1 yr of seedling survivorship data for woody seedlings in 17 different plots of lower to mid‐montane rain forests on the island of Dominica to examine how seedling height, abiotic factors, and biotic factors such as negative density dependence are related to seedling survival of five different life‐forms (canopy, midstory, and understory trees; shrubs; and lianas). Across 64 species, taller seedlings in seedling plots with higher canopy openness, greater seedling density, lower relative abundance of conspecific seedlings, and lower relative abundance of conspecific adults generally had a greater probability of surviving. Height was the strongest predictor of seedling survival for all life‐forms except lianas. Greater seedling density was positively related to survival for canopy and midstory trees but negatively related to survival for the other life‐forms. For trees, the relative abundance of conspecific seedling and adult neighbors had weak and strong negative effects on survival respectively. Neither shrub nor liana seedling survival was affected by the relative abundance of conspecific neighbors. Thus, negative density dependence is confirmed as an important structuring mechanism for tree seedling communities but does not seem to be important for lianas and shrubs in Dominican rain forests. These results represent the first direct assessment of controls on seedling survival of all woody life‐forms – an important step in understanding the dynamics and structure of the entire woody plant community.     

Virulence of oomycete pathogens from Phragmites australis‐invaded and noninvaded soils to seedlings of wetland plant species

Soil pathogens affect plant community structure and function through negative plant–soil feedbacks that may contribute to the invasiveness of non‐native plant species. Our understanding of these pathogen‐induced soil feedbacks has relied largely on observations of the collective impact of the soil biota on plant populations, with few observations of accompanying changes in populations of specific soil pathogens and their impacts on invasive and noninvasive species. As a result, the roles of specific soil pathogens in plant invasions remain unknown. In this study, we examine the diversity and virulence of soil oomycete pathogens in freshwater wetland soils invaded by non‐native Phragmites australis (European common reed) to better understand the potential for soil pathogen communities to impact a range of native and non‐native species and influence invasiveness. We isolated oomycetes from four sites over a 2‐year period, collecting nearly 500 isolates belonging to 36 different species. These sites were dominated by species of Pythium, many of which decreased seedling survival of a range of native and invasive plants. Despite any clear host specialization, many of the Pythium species were differentially virulent to the native and non‐native plant species tested. Isolates from invaded and noninvaded soils were equally virulent to given individual plant species, and no apparent differences in susceptibility were observed between the collective groups of native and non‐native plant species.     

Oomycetes associated with Prunus serotina persist in soil after tree harvest

Soil-borne pathogens can shape forest communities by lowering seedling survivorship. Many soil pathogens can persist long-term as survival spores, but how long pathogens outlive tree hosts in gap soils and whether they continue to affect seedling survival is uncertain. We studied the presence of oomycetes and evaluated seedling performance in soils near live Prunus serotina trees, and 0.5 and 1.5-y-old stumps. We isolated five species of oomycetes from soils, two of which were pathogenic (Pythium intermedium and Pythium irregulare) to Prunus serotina. There was a non-significant ~10.5% increase in conspecific seedling survival in stumps versus live trees, and pathogens were present in soils of all stump ages. The continued presence of pathogens of Prunus serotina in gap soils demonstrates the potential for impacts on conspecific regeneration after tree death, though the slight improvement in survival suggests that these effects may weaken with time.     

Distance‐dependent effects of soil‐derived biota on seedling survival of the tropical tree legume Ormosia semicastrata

Question: How do seed germination and subsequent seedling survival of O. semicastrata (Hance forma litchiifolia How) vary with respect to distance from parent trees and conspecific density in different types of tropical forest? Are there effects of soil biota on O. semicastrata that systematically depend on distance from parent trees and conspecific density? Do soil pathogens differently affect survival of O. semicastrata in different types of tropical forest? Location: Tropical lowland rain forest and tropical montane rain forest in Jianfengling National Nature Reserve, Hainan Island, China. Methods: Individual adult O. semicastrata trees were selected in lowland rain forest and montane rain forest. Soil was collected at a distance of 0‐5 m or 15‐20 m from the parent tree. Soil samples from each distance were combined into a bulk sample. Half of the soil sample was sterilized by autoclaving. Surface‐sterilized seeds were then added to the soil material in shade‐houses at both forests. Results: Germination of O. semicastrata seeds at low‐ or high‐seed density was barely affected by the sterilization procedure. In both forests, seedlings grown in non‐sterilized soil collected close to parent trees had significantly higher mortality compared to those in sterilized soil. In contrast, seedling survival with soil collected far from parent trees was not affected by the soil sterilization procedure. Conclusions: Host‐specific pathogens concentrated in the soil around parent trees may regulate community structure of tropical trees at the stage of seedling development.     

Species with greater seed mass are more tolerant of conspecific neighbours: a key driver of early survival and future abundances in a tropical forest

Multiple niche‐based processes including conspecific negative density dependence (CNDD) determine plant regeneration and community structure. We ask how interspecific and intraspecific density‐dependent interactions relate to plant life histories and associated functional traits. Using hierarchical models, we analysed how such interactions affected first‐year survival of seedling recruits of 175 species in a tropical forest, and how species abundances and functional traits are related to interspecific variation in density‐dependent effects. Conspecific seedling neighbour effects prevailed over the effects of larger conspecific and all heterospecific neighbours. Tolerance of seedling CNDD enhanced recruit survival and subsequent abundance, all of which were greater among larger seeded, slow‐growing and well‐defended species. Niche differentiation along the growth–survival trade‐off and tolerance of seedling CNDD strongly correlated with regeneration success, with manifest consequences for community structure. The ability of larger seeded species to better tolerate CNDD suggests a novel mechanism for CNDD to contribute to seed‐size variation and promote species coexistence through a tolerance–fecundity trade‐off.     

Conspecific density dependence and community structure: Insights from 11 years of monitoring in an old‐growth temperate forest in Northeast China

Forest community structure may be influenced by seedling density dependence, however, the effect is loosely coupled with population dynamics and diversity in the short term. In the long term the strength of conspecific density dependence may fluctuate over time because of seedling abundance, yet few long‐term studies exist. Based on 11 years of seedling census data and tree census data from a 25‐ha temperate forest plot in Northeast China, we used generalized linear mixed models to test the relative effects of local neighborhood density and abiotic factors on seedling density and seedling survival. Spatial point pattern analysis was used to determine if spatial patterns of saplings and juveniles, in relation to conspecific adults, were in accordance with patterns uncovered by conspecific negative density dependence at the seedling stage. Our long‐term results showed that seedling density was mainly positively affected by conspecific density, suggesting dispersal limitation of seedling development. The probability of seedling survival significantly decreased over 1 year with increasing conspecific density, indicating conspecific negative density dependence in seedling establishment. Although there was variation in conspecific negative density dependence at the seedling stage among species and across years, a dispersed pattern of conspecific saplings relative to conspecific adults at the local scale (<10 m) was observed in four of the 11 species examined. Overall, sapling spatial patterns were consistent with the impacts of conspecific density on seedling dynamics, which suggests that conspecific negative density dependence is persistent over the long term. From the long‐term perspective, conspecific density dependence is an important driver of species coexistence in temperate forests.     

Environmental heterogeneity and biotic interactions mediate climate impacts on tropical forest regeneration

Predicting the fate of tropical forests under a changing climate requires understanding species responses to climatic variability and extremes. Seedlings may be particularly vulnerable to climatic stress given low stored resources and undeveloped roots; they also portend the potential effects of climate change on future forest composition. Here we use data for ca. 50,000 tropical seedlings representing 25 woody species to assess (i) the effects of interannual variation in rainfall and solar radiation between 2007 and 2016 on seedling survival over 9 years in a subtropical forest; and (ii) how spatial heterogeneity in three environmental factors—soil moisture, understory light, and conspecific neighborhood density—modulate these responses. Community‐wide seedling survival was not sensitive to interannual rainfall variability but interspecific variation in these responses was large, overwhelming the average community response. In contrast, community‐wide responses to solar radiation were predominantly positive. Spatial heterogeneity in soil moisture and conspecific density were the predominant and most consistent drivers of seedling survival, with the majority of species exhibiting greater survival at low conspecific densities and positive or nonlinear responses to soil moisture. This environmental heterogeneity modulated impacts of rainfall and solar radiation. Negative conspecific effects were amplified during rainy years and at dry sites, whereas the positive effects of radiation on survival were more pronounced for seedlings existing at high understory light levels. These results demonstrate that environmental heterogeneity is not only the main driver of seedling survival in this forest but also plays a central role in buffering or exacerbating impacts of climate fluctuations on forest regeneration. Since seedlings represent a key bottleneck in the demographic cycle of trees, efforts to predict the long‐term effects of a changing climate on tropical forests must take into account this environmental heterogeneity and how its effects on regeneration dynamics play out in long‐term stand dynamics.     

Density dependence is not very prevalent in a heterogeneous subtropical forest

There is a growing body of evidence demonstrating that tree survival is influenced by negative density‐dependence, but it is still controversial how the effect may vary with life‐stage, and to what extent it plays a role in regulating tree survival in heterogeneous subtropical forests. In this study, we investigated density‐dependent effects on tree survival of six tree species in a 5‐ha subtropical forest in eastern China. The roughly 45 000 individuals in the forest were fully censused in 2003 and 2008. For each of these species, we used an inhomogeneous pair‐correlation function to quantify the change in spatial distribution for different size classes, and a case‐control design to study seedling–adult associations in 2003. Autologistic regression was used to determine the influence of neighborhood factors on individual survival from 2003 to 2008. We found that seedlings of five species were repulsed by distance to nearest conspecific adults in terms of their survival, consistent with predictions of the Janzen–Connell mechanism. By contrast, only the least shade‐tolerant Schima superba had a negative relationship with individual survival and conspecific distance‐weighted basal area. This suggests that the Janzen–Connell effect is only prevalent at the early seedling stage or seed‐to‐seedling phase. The strength of clustering significantly declined at sapling–pole and pole–adult transitions for Sycopsis sinensis and at seedling–sapling transition for Cleyera pachyphylla. Correlations between individual survival and conspecific abundance for these species were consistent with trends in the strength of clustering. These results suggest that density dependence plays a limited role in individual survival and species spatial structure beyond the early seedling stage (i.e. after true leaves growing) in this forest. In addition, this study indicates that including individuals from early life‐stages and factoring out potential confounding factors such as habitat preference are important in studies that seek evidence for density dependence in forest trees.     

Pathogen mortality of tropical tree seedlings: experimental studies of the effects of dispersal distance,seedling density,and light conditions

Summary We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.     

Conspecific plant-soil feedbacks of temperate tree species in the southern Appalachians, USA

Many tree species have seedling recruitment patterns suggesting that they are affected by non-competitive distance-dependent sources of mortality. We conducted an experiment, with landscape-level replication, to identify cases of negative distance-dependent effects and whether variation in these effects corresponded with tree recruitment patterns in the southern Appalachian Mountains region. Specifically, soil was collected from 14 sites and used as inocula in a 62 day growth chamber experiment determining whether tree seedling growth was less when interacting with soil from conspecific (like) than heterospecific (other) tree species. Tests were performed on six tree species. Three of the tree species had been previously described as having greater recruitment around conspecifics (i.e. facilitator species group) compared to the other half (i.e. inhibitor species group). We were then able to determine whether variation in negative distance-dependent effects corresponded with recruitment patterns in the field. Across the six species, none were negatively affected by soil inocula from conspecific relative to heterospecific sources. Most species (four of six) were unaffected by soil source. Two species (Prunus serotina and Tsuga canadensis) had enhanced growth in pots inoculated with soil from conspecific trees vs. heterospecifics. Species varied in their susceptibility to soil pathogens, but trends across all species revealed that species classified as inhibitors were not more negatively affected by conspecific than heterospecific soil inocula or more susceptible to pathogenic effects than facilitators. Although plant-soil biota interactions may be important for individual species and sites, it may be difficult to scale these interactions over space or levels of ecological organization. Generalizing the importance of plant-soil feedbacks or other factors across regional scales may be especially problematic for hyperdiverse temperate forests where interactions may be spatially variable.     

Testing for enemy‐mediated density‐dependence in the mortality of seedlings: field experiments with five Neotropical tree species

The coexistence of plant species in species‐rich tropical forests can be promoted by specialised enemies acting in a negatively density‐dependent manner. While survival of tropical tree seedlings is often negatively density‐dependent, the causes have rarely been identified. We tested whether insects and plant pathogens cause density‐dependent seedling recruitment and survival in five forest tree species in Belize, Central America. We manipulated densities of seeds or newly germinated seedlings in small (1 m² or 0.25 m²) plots close to fruiting conspecific trees. Using a factorial design, we excluded enemies from subsets of the plots with fungicides and insecticides. Seed germination (for two species) and early seedling survival (for all species) were monitored at approximately weekly intervals for up to eight weeks, during the period when plants are likely to be most susceptible to natural enemies. In Terminalia amazonia, seed germination was negatively density‐dependent and the proportion of seeds germinating increased when insects were excluded. However, the magnitude of the insecticide effect was independent of density. The only significant density effect for survival of young seedlings was in Acacia polyphylla; counter to expectation, seedling survival was higher at high densities. In a few cases pesticide application had a significant effect on seedling survival, but in only one case (Terminalia amazonia) was a significant pesticide × density interaction detected. Our results caution against generalising from studies conducted on a single species at a single time and place and illustrate the challenges of experimentally testing for enemy‐mediated negative density‐dependence. Experimental outcomes are likely to depend on the spatial scale at which the principal enemies disperse and respond to plant density, and the timescales over which they act. Gathering information on these variables will improve our understanding of the natural histories of tropical forest species and help inform the design of future experiments.     

Plant‐driven changes in soil microbial communities influence seed germination through negative feedbacks

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Plant‐soil feedbacks from 30‐year family‐specific soil cultures: phylogeny,soil chemistry and plant life stage

Intraspecific negative feedback effects, where performance is reduced on soils conditioned by conspecifics, are widely documented in plant communities. However, interspecific feedbacks are less well studied, and their direction, strength, causes, and consequences are poorly understood. If more closely related species share pathogens, or have similar soil resource requirements, plants may perform better on soils conditioned by more distant phylogenetic relatives. There have been few empirical tests of this prediction across plant life stages, and none of which attempt to account for soil chemistry. Here, we test the utility of phylogeny for predicting soil feedback effects on plant survival and performance (germination, seedling survival, growth rate, biomass). We implement a full factorial experiment growing species representing five families on five plant family‐specific soil sources. Our experiments exploit soils that have been cultured for over 30 years in plant family‐specific beds at Oxford University Botanic Gardens. Plant responses to soil source were idiosyncratic, and species did not perform better on soils cultured by phylogenetically more distant relatives. The magnitude and sign of feedback effects could, however, be explained by differences in the chemical properties of “home” and “away” soils. Furthermore, the direction of soil chemistry‐related plant‐soil feedbacks was dependent on plant life stage, with the effects of soil chemistry on germination success and accumulation of biomass inversely related. Our results (1) suggest that the phylogenetic distance between plant families cannot predict plant–soil feedbacks across multiple life stages, and (2) highlight the need to consider changes in soil chemistry as an important driver of population responses. The contrasting responses at plant life stages suggest that studies focusing on brief phases in plant demography (e.g., germination success) may not give a full picture of plant–soil feedback effects.     

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.     

Temporal carry‐over effects in sequential plant–soil feedbacks

Plant–soil feedbacks (PSF) strongly influence plant performance. However, to what extent these PSF effects are persistent in the soil and how they are altered by species that subsequently condition the soil is unclear. Here we test how conspecific and heterospecific soil‐conditioning effects interact across different soil‐conditioning phases. We conducted a fully factorial glasshouse experiment where six plant species conditioned soils in two consecutive phases and measured the performance of Jacobaea vulgaris. The species that conditioned the soil during the second conditioning phase strongly determined the performance of J. vulgaris, but also the order and combination of species that conditioned the soil in the two phases accounted for a large part of the variance. For shoot biomass this interaction was the dominant variance component. We show that soil conditioning legacies carry‐over and interact with the conditioning effects of succeeding plants. In the field, species replacements at the patch level often appear to be unpredictable and we suggest that sequential feedbacks may explain these apparently unpredictable transitions.     

Integrating spatial structure and interspecific and intraspecific plant–soil feedback effects and responses into community structure

Plant–soil feedbacks have important effects on plant communities, but most theory has been derived from experiments on intraspecific plant–soil feedbacks. Much less is known about how interspecific plant–soil feedbacks affect coexistence and plant communities, due in part to experimental and analytical challenges. Here, we propose a framework for evaluating plant–soil feedbacks among multiple interacting species that incorporates 1) the average effect each species has on conspecific and heterospecific neighbors via how they modify soil biota, 2) the average response of each species to the soil modifications made by neighboring species, and 3) intraspecific feedback. We refer to this as the ‘effect–response–intraspecific’ (ERI) model. We used individual‐based models to evaluate the relative importance of intraspecific and interspecific soil feedback in determining species abundance ranks in simulated plant communities. To compare the heuristic value of the ERI model to that of an established model in which effects and responses to soil feedback are not explicitly recognized, we evaluated a ‘full‐factorial’ model in which soil feedbacks among five plant species were measured and then explicitly modeled. The ERI model indicated that the response to interspecific plant–soil feedbacks was the key factor for species’ abundance rank without spatial structure. In contrast, interspecific plant–soil feedback had no impact on species abundance with spatial structure, and intraspecific feedback became dominant. Thus, our models predict that the relative importance of intraspecific and interspecific feedbacks changes as a function of the degree of spatial structure in a system. Overall, the ERI model provides a novel and tractable framework for evaluating complex multi‐species plant–soil feedbacks.     

长白山次生针阔混交林乔木幼苗存活的影响因素分析

基于长白山次生针阔混交林样地, 以520个1 m × 1 m幼苗样方中胸径小于1 cm的乔木幼苗为研究对象, 选取2013年和2014年的幼苗调查数据, 运用广义线性混合模型(GLMM)分析了生物邻体和生境异质性对幼苗存活的影响, 探讨了次生针阔混交林幼苗存活影响因素及物种共存机制。结果表明: (1)适宜大树生长的局域生境同样也适宜幼苗的存活, 幼苗存活率与土壤含水量和有效氮等土壤养分显著正相关。(2)幼苗个体之间存在明显的竞争, 较多的幼苗邻体显著降低幼苗的存活率。同种大树邻体和同种幼苗邻体与幼苗存活显著负相关, 表明存在负密度制约效应。(3)随着幼苗年龄的增加, 生境异质性对幼苗存活的影响逐渐增大。该研究证实了密度制约效应和生境异质性对幼苗存活有着重要影响, 其相对重要性随着幼苗年龄级、功能群以及物种种类而变化。