Congeneric species often have similar ecological characteristics and use similar resources. These similarities may make it easier for them to co‐occur in a similar habitat but may also lead to strong competitions that limit their coexistence. Hence, how do similarities in congeneric species affect their coexistence exactly? This study mainly used spatial point pattern analysis in two 1 hm2 plots in the Baotianman National Nature Reserve, Henan, China, to compare the similarities in spatial distributions and interspecific associations of Quercus species. Results revealed that Quercus species were all aggregated under the complete spatial randomness null model, and aggregations were weaker under the heterogeneous Poisson process null model in each plot. The interspecific associations of Quercus species to non‐Quercus species were very similar in Plot 1. However, they can be either positive or negative in different plots between the co‐occurring Quercus species. The spatial distributions of congeneric species, interspecific associations with non‐Quercus species, neighborhood richness around species, and species diversity were all different between the two plots. We found that congeneric species did have some similarities, and the closely related congeneric species can positive or negative associate with each other in different plots. The co‐occurring congeneric species may have different survival strategies in different habitats. On the one hand, competition among congenerics may lead to differentiation in resource utilization. On the other hand, their similar interspecific associations can strengthen their competitive ability and promote local exclusion to noncongeneric species to obtain more living space. Our results provide new knowledge for us to better understand the coexistence mechanisms of species.

Aims With the increase of land-use change, habitat fragmentation has become a major factor affecting plant diversity. Generally, when the adult density is high, the survival rate of conspecific saplings may decline and provide more resources and space for other species to maintain a high local species diversity. Therefore, the spatial relationship of conspecific saplings and adults (SRCSA) can regulate plant diversity. However, very few studies have assessed SRCSA within fragmented forests, and we still largely unknown the relationship between SRCSA and species diversity in fragmented landscapes. Methods We calculated the mean strength of SRCSA using the spatial distribution data of saplings and adults with a log-transformed hierarchical offset-power model on 27 islands in a land-bridge island system. The higher strength of SRCSA reflects a more negative relationship between the density of conspecific saplings and adults on an island (i.e. the looser pattern of SRCSA). We tested the relationships among island attributes (i.e. island area, the distance to the mainland—MD, and the distance to the nearest island—ND), SRCSA and species diversity. Important Findings The strength of SRCSA increased with ND. Meanwhile, the species diversity significantly increased with the strength of SRCSA, and island area and the strength of SRCSA independently explained 26% and 6% of variation of species diversity, respectively, and their interactions explained 8%. Shade-intolerant and low-abundant species showed looser patterns of SRCSA. Our study suggests that SRCSA may have the effect of excluding species with a relatively low abundance in isolated island forests, which illustrates the importance of biotic interactions in maintaining plant diversity in fragmented forests. Our results also emphasize that we should consider forest connectivity when testing the conspecific negative density dependence.

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Plant species identity influences soil microbial communities directly by host specificity and root exudates, and indirectly by changing soil properties. As a native pioneer species common in early successional communities, Masson pine (Pinus massoniana) forests are widely distributed in subtropical China, and play a key role in improving ecosystem productivity. However, how pine forest composition, especially the dominance of plant functional groups, affects soil microbial diversity remains unclear. Here, we investigated linkages among woody plant composition, soil physicochemical properties, and microbial diversity in forests along a dominance gradient of Masson pine. Soil bacterial and fungal communities were mainly explained by woody plant community composition rather than by woody species alpha diversity, with the dominance of tree (without including shrub) species and ectomycorrhizal woody plant species accounting for more of the variation among microbial communities than pine dominance alone. Structural equation modeling revealed that bacterial diversity was associated with woody plant compositional variation via altered soil physicochemical properties, whereas fungal diversity was directly driven by woody plant composition. Bacterial functional groups involved in carbohydrate and amino acid metabolism were negatively correlated with the availability of soil nitrogen and phosphorus, whereas saprotrophic and pathogenic fungal groups showed negative correlations with the dominance of tree species. These findings indicate strong linkages between woody plant composition than soil microbial diversity; meanwhile, the high proportion of unexplained variability indicates great necessity of further definitive demonstration for better understanding of forest–microbe interactions and associated ecosystem processes.