Conspecific Negative Distance Research Articles

Roles of pathogens and mycorrhizae in conspecific negative distance dependency and replacement of tree species in a temperate forest

Maintaining and recovering species diversity is one of the main management goals of forest managers. It is thus important to understand the mechanisms that drive species diversity. The mechanism is strongly related to the harsh biological environment surrounding the tree, especially its interaction with natural enemies, and a well-known mechanism is the Janzen-Connell hypothesis. The hypothesis is best understood by evaluating not only conspecific negative distance-dependent (CNDD) seedling mortality but also replacement of conspecific seedlings by heterospecifics. However, it remains unclear how antagonistic pathogens and mutualistic mycorrhizae affect the strength of replacement. A reciprocal seed-sowing experiment was conducted for five tree species with different mycorrhizal types in forest understorey (FU) and gaps (Gap) in a temperate forest. We investigated seedling mass, mortality, mortality agents, and root infection with arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. In both FU and Gap sites, two AM species were attacked by pathogens causing damping-off and leaf diseases more severely beneath conspecific than heterospecific adults, and more severely among conspecifics than heterospecifics of focal adults, resulting in strong CNDD and replacement, whereas seedling mortality in two EM species due to damping-off pathogens and leaf diseases did not differ among treatments, resulting in weak CNDD and replacement. One AM species showed an intermediate trend between two AM species and two EM species. Thus, CNDD and replacement caused by pathogens were positively correlated in FU but not Gap sites. These traits reveal that replacement closely correlated with CNDD play an important role in maintaining species diversity, especially when gaps are absent. This study also suggest that replacement plays an important role in regulating the spatial distribution of individual trees and subsequent relative abundance of tree species, as well as CNDD effects. Forest managers need to consider differences in the strength of CNDD and replacement between mycorrhizal fungi types when attempting to achieve successful seedling establishment after harvesting without compromising species diversity.

Do natural enemies mediate conspecific negative distance‐ and density‐dependence of trees? A meta‐analysis of exclusion experiments

Conspecific negative distance‐ and density‐dependence is believed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta‐analysis of studies that involved robust manipulative experiments, using fungicides, insecticides or exclosures, to assess the contributions of different natural enemy types to distance‐ and density‐dependent effects at seed and seedling stages. We found that conspecific negative distance‐ and density‐dependence was more common at the seedling stage. Only 30% of studies showed significant conspecific negative distance‐ and density‐dependence. Among those studies, applying fungicides significantly reduced the effect of conspecific negative distance‐dependent mortality. Excluding large mammals reduced the effect of conspecifics, but with only marginal significance (p < 0.1). Our study confirms that plant pathogens have a significant impact on distance‐dependent mortality and suggests that the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, appear to weaken and enhance negative conspecific distance‐dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types, and assess how global change may affect natural enemies and thus influence the strength of conspecific distance‐ and density‐dependence.

Neighbourhood-based evidence of tree diversity promotion by beech in an old-growth deciduousconiferous mixed forest (Eastern Carpathians)

Neighbourhood models are useful tools for understanding the role of positive and negative interactions in maintaining the tree species diversity in mixed forests. Under such a presumption, we aimed at testing several hypotheses concerning the mechanisms of autogenic species coexistence in an old-growth, beech-fir-spruce stand, which is part of the Slătioara forest reserve (Eastern Carpathians). Univariate/bivariate spatial point pattern analyses, the individual tree species-area relationship, the species mingling analysis and generalised linear mixed models of neighbour interference were applied on data concerning the position and allometry of all saplings and trees occurring within a 0.24 ha plot. The monospecific distribution of either beech or spruce saplings did not support the spatial segregation hypothesis. There was no evidence of conspecific negative distance dependence, as no spatial segregation was detected between the saplings and trees of any species. Within 4 m-neighbourhood, the beech saplings appeared as diversity accumulators, which might be indicative of indirect facilitation (e.g., herd protection hypothesis). At tree stage, none of the three species showed either accumulator or repeller patterns in their neighbourhood with respect to sapling species richness. Signals of positive and negative interspecific association were found in tree-sized beech (at scales of 10 to 20 m) and spruce (at scales of 4 to 17 m), respectively. The former, highly interspersed pattern is in accordance with the hypothesis of positive complementary effects, whereas the latter, poorly intermingled pattern is probably linked to the unexpected, positive neighbouring effect of spruce trees on the stem growth of their conspecific saplings. Such self-favouring process might be due to a facilitative below-ground mechanism. Conversely, the beech saplings were suppressed through interference from the neighbouring conspecific trees. The beech appears to be the key promoter of tree species coexistence in the study forest stand, in contrast to the low interspersion of spruce in the overstorey leading to lower local tree diversity.

Neighbourhood-based evidence of tree diversity promotion by beech in an old-growth deciduous-coniferous mixed forest (Eastern Carpathians)

Neighbourhood models are useful tools for understanding the role of positive and negative interactions in maintaining the tree species diversity in mixed forests. Under such a presumption, we aimed at testing several hypotheses concerning the mechanisms of autogenic species coexistence in an old-growth, beech-fir-spruce stand, which is part of the Slatioara forest reserve (eastern Carpathians). Univariate/bivariate spatial point pattern analyses, the individual tree species-area relationship, the species mingling analysis and generalised linear mixed models of neighbour interference were applied on data concerning the position and allometry of all saplings and trees occurring within a 0.24 ha plot. The monospecific distribution of either beech or spruce saplings did not support the spatial segregation hypothesis. There was no evidence of conspecific negative distance dependence, as no spatial segregation was detected between the saplings and trees of any species. Within 4 m-neighbourhood, the beech saplings appeared as diversity accumulators, which might be indicative of indirect facilitation (e.g., herd protection hypothesis). At tree stage, none of the three species showed either accumulator or repeller patterns in their neighbourhood with respect to sapling species richness. Signals of positive and negative interspecific association were found in tree-sized beech (at scales of 10 to 20 m) and spruce (at scales of 4 to 17 m), respectively. The former, highly interspersed pattern is in accordance with the hypothesis of positive complementary effects, whereas the latter, poorly intermingled pattern is probably linked to the unexpected, positive neighbouring effect of spruce trees on the stem growth of their conspecific saplings. Such self-favouring process might be due to a facilitative below-ground mechanism. Conversely, the beech saplings were suppressed through interference from the neighbouring conspecific trees. The beech appears to be the key promoter of tree species coexistence in the study forest stand, in contrast to the low interspersion of spruce in the overstorey leading to lower local tree diversity.

Enemies mediate distance- and density-dependent mortality of tree seeds and seedlings: a meta-analysis of fungicide, insecticide and exclosure studies.

Conspecific negative distance- and density-dependence is often assumed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta-analysis of studies that involved robust manipulative experiments, using fungicides, insecticides and exclosures, to assess the contributions of different natural enemy types to distance- and density-dependent effects at seed and seedling stages. We found that distance- and density-dependent mortality caused by natural enemies was most likely at the seedling stage and was greater at higher mean annual temperatures. Conspecific negative distance- and density-dependence at the seedling stage is significantly weakened when fungicides were applied. By contrast, negative conspecific distance- and density-dependence is not a general pattern at the seed stage. High seed mass reduced distance- and density-dependent mortality at the seed stage. Seed studies excluding only large mammals found significant negative conspecific distance-dependent mortality, but exclusion of all mammals resulted in a non-significant effect of conspecifics. Our study suggests that plant pathogens are a major cause of distance- and density-dependent mortality at the seedling stage, while the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, weaken and enhance negative conspecific distance-dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types and assess how global change may affect natural enemies and thus influence the strength of conspecific distance- and density-dependence.

When do Janzen-Connell effects matter? A phylogenetic meta-analysis of conspecific negative distance and density dependence experiments.

The Janzen-Connell (J-C) hypothesis suggests that specialised natural enemies cause distance- or density-dependent mortality among host plants and is regarded as an important mechanism for species coexistence. However, there remains debate about whether this phenomenon is widespread and how variation is structured across taxa and life stages. We performed the largest meta-analysis of experimental studies conducted under natural settings to date. We found little evidence of distance-dependent or density-dependent mortality when grouping all types of manipulations. Our analysis also reveals very large variation in response among species, with 38.5% of species even showing positive responses to manipulations. However, we found a strong signal of distance-dependent mortality among seedlings but not seed experiments, which we attribute to (a) seedlings sharing susceptible tissues with adults (leaves, wood, roots), (b) seedling enemies having worse dispersal than seed enemies and (c) seedlings having fewer physical and chemical defences than seeds. Both density- and distance-dependent mortality showed large variation within genera and families, suggesting that J-C effects are not strongly phylogenetically conserved. There were no clear trends with latitude, rainfall or study duration. We conclude that J-C effects may not be as pervasive as widely thought. Understanding the variation in J-C effects provides opportunities for new discoveries that will refine our understanding of J-C effects and its role in species coexistence.

Role of seed size and relative abundance in conspecific negative distance-dependent seedling mortality for eight tree species in a temperate forest

Conspecific negative distance dependent seedling mortality and negative density-dependent seedling survival (CNDD) are among the most important mechanisms of species diversity maintenance. Recent research has demonstrated that CNDD is greater for species with abundant seeds, due mainly to attack by density-dependent pathogens. If seed number were correlated negatively with seed size, species with smaller seeds would exhibit much greater negative CNDD. We explored whether CNDD is affected by seed size. We evaluated the strength of CNDD as a log response ratio of the probability of mortality among seedlings at conspecific and heterospecific sites and at high and low densities as functions of distance and density, respectively, in a seed-sowing experiment using eight hardwood species in a temperate forest in northern Japan. As a function of distance, CNDD attributable to pathogens was correlated positively with seed mass at low density, but not at high density, whereas seed mass was not correlated with CNDD attributable to invertebrate or vertebrate herbivores. As a function of density, seed mass was not correlated with CNDD attributable to pathogens or invertebrate or vertebrate herbivores. The ability of large-seeded species to better tolerate CNDD suggests a novel mechanism through which seed size may contribute to CNDD variation and promote species coexistence due to distance-dependent attack by pathogens, particularly those bearing leaf diseases.