Minhua Zhang

Seedling establishment is a critical demographic bottleneck for species regeneration and diversity maintenance in forests, yet the temporal contributions of biotic and abiotic factors remain poorly understood. This is because most previous studies focused disproportionately on biotic factors or limited abiotic factors and used observational data lacking standardized baselines for seedling age. We conducted a two-year transplant experiment in the 25-ha Baishanzu subtropical forest dynamics plot, monitoring 2,886 seedlings from six species spanning a range of abundance across conspecific adult density gradients. We analyzed the temporal effects of neighborhood conspecifics, heterospecifics, light, soil nutrients, and topography on seedling establishment both in aggregate and for individual species. Our results revealed that negative effects from conspecific adults intensified as seedlings grow. Conversely, heterospecific neighbors consistently exerted positive effects throughout the establishment. Environmental factors exhibited temporal variation in their effects. While light availability negatively impacted early establishment, soil nutrients emerged as strong predictors for older seedlings. For individual species, responses were highly species-specific. Negative coefficients for conspecific adult density were observed across species over time, yet statistical significance was detected only in species Daphniphyllum macropodum and Photinia davidiana. Heterospecific effects remained neutral or positive across all six species. Notably, abiotic factors showed species-specific directional effects, benefiting certain species while inhibiting others, suggesting that niche differentiation across heterogeneous microhabitats drives regeneration dynamics. Our results highlight that both abiotic and biotic factors are indispensable drivers during seedling establishment, and their shifting temporal dynamics combined with species-specific responses fundamentally shape seedling dynamics in heterogeneous forests.

The global decrease in species diversity from low to high latitudes is among the most robust biogeographic patterns ^1,2 . There is continuing debate on the contribution of conspecific negative density dependence (CNDD) to the latitudinal diversity gradient evident for trees ^3,4 . Theory suggests that CNDD based on pairwise interactions alone is not sufficient to explain the intricacies of diverse communities, because higher-order interactions (HOIs) may greatly modify these interactions ^5,6 . However, there has been a lack of empirical studies investigating how HOIs intertwine with pairwise interactions and how they may contribute to the latitudinal tree diversity gradient. Here we examined both pairwise interactions and HOIs across 32 large permanent forest plots, most in the northern hemisphere. We detected evidence of HOIs in 40% of the 1,543 species–plot combinations for tree growth, and 23% of the 1,340 such combinations for tree survival, with the strength of these interactions declining with latitude. HOIs were found to benefit rare species but disadvantage common species, suggesting a potential mechanism promoting species diversity. This stabilizing effect weakened towards higher latitudes, consistent with the latitudinal tree diversity gradient. Our findings reveal an important interplay between pairwise interactions and HOIs in promoting the latitudinal tree diversity gradient and help to clarify the contribution of CNDD to this biogeographic pattern.

Plant hosts play a crucial role in structuring rhizosphere fungal symbioses which in turn influence the composition of tree species in forest ecosystems. However, there is little understanding of how changes in host species abundance (e.g. rare vs. common host species) affect the rhizosphere fungal community. By sampling rhizosphere soils from 272 adult individuals representing 68 tree species in a 25‐ha subtropical forest plot, we investigated the relationship between tree species abundance and rhizosphere fungal diversity. We also conducted a greenhouse experiment by planting seedlings of five test tree species in pots filled with the same heterospecific soils to assess this relationship under the same soil (fungal) condition. The results of the in situ rhizosphere soil sampling revealed that the diversity of total rhizosphere fungi decreased with tree species abundance. Similarly, the greenhouse experiment showed that the least abundant species, Callicarpa membranacea (CM), exhibited the weakest host selection effect, as indicated by its highest total rhizosphere fungal richness. After controlling for differences in total rhizosphere fungal richness, the greenhouse experiment showed that CM hosted a high proportion of moderate‐specialization fungi compared to other more abundant tree species except the most abundant species Schima superba which appeared to host the highest proportion of high‐specialization fungi. Our results further showed that both the above‐ground biomass and the biomass allocation ratio of CM seedlings were positively correlated with total rhizosphere fungal diversity. These findings suggest a potential role for a high total rhizosphere fungal richness in sustaining this least abundant species in the field. Synthesis . Our study demonstrates that rare tree species harbour higher total rhizosphere fungal diversity than common ones, reflecting their weaker selection on soil fungi. Tree species with varying abundance may exhibit differential selection preferences for fungi with different specialization levels. This variation in host selection in both strength and preferences could offer new insights into understanding rarity and commonness of forest tree species.

The size, number, and distribution of forest canopy gaps vary significantly across spatial scales, yet their relationships with underlying drivers, such as large trees, topography, and soil properties, remain insufficiently comprehended. We utilized an unmanned aerial vehicle to measure forest gap patterns (size, number, and aggregation) at seven spatial scales (20–400 m) in four subtropical forests and quantified the effect of large trees, topography, and soil in shaping gap patterns using ground inventory data. Gap size and aggregation showed significant variation across scales. Large trees and topographic complexity were major factors of gap patterns, with their effects depending on scale. Large trees in two forests had a significantly negative effect on gap size and density at the 20‐m scale, but this effect was weaker and positive at the 100‐m scale. Topography had the strongest effect on gap aggregation at a small scale (20 m) and on gap size and density at a larger scale (100 m). These results underscore the importance of spatial scale in understanding forest dynamics and highlight how community‐level factors shape canopy structure. Identifying scale‐dependent drivers of gap patterns can inform gap‐based restoration and conservation strategies. Forest managers can reduce gap clustering by protecting large trees, especially in wind‐prone areas, helping to maintain canopy structure, promote species diversity, and enhance ecosystem resilience.

Much evidence has shown that adaptive behavior can greatly modulate the dynamics of food webs, but little is known about how adaptive behaviors of invasive plant species affect community composition in multilayered networks. Following a proven network model, we constructed networks of native communities that are invaded by exotic plant species based on three linkage rules. We examined the effects of both adaptive behavior and network connectance of invasive plant species on the persistence of native species and diversity-invasion success relationship. Results showed that community persistence was mainly affected by the connectance of invasive plant species regardless of adaptive behavior. Given a fixed proportion (F₁) of native mutualist species linked to the invasive plant species, community persistence displayed an inverse hump-shaped relationship with the increasing proportion (F₂) of native plant species linked to the invasive plant species. Compared to the results without adaptive behavior, the adaptive behavior made most negative diversity-invasion relationship become a nonlinear U-shape at fixed proportion (F₁). In addition, the adaptive behavior made most negative diversity-invasion relationship insignificant for some proportion (F₁) when proportion (F₂) was fixed. It could even reverse this relationship if the invading species was more likely to link to native species already having fewer links than those having higher links. Our results underline the importance of considering adaptive behavior and the network degree of invasive plant species for understanding the effect of invasive species on the structure and composition of ecological networks.

Host specialization plays a critical role in the ecology and evolution of plant–microbe symbiosis. Theory predicts that host specialization is associated with microbial genome streamlining and is influenced by the abundance of host species, both of which can vary across latitudes, leading to a latitudinal gradient in host specificity. Here, we quantified the host specificity and composition of plant–bacteria symbioses on leaves across 329 tree species spanning a latitudinal gradient. Our analysis revealed a predominance of host‐specialized leaf bacteria. The degree of host specificity was negatively correlated with bacterial genome size and the local abundance of host plants. Additionally, we found an increased host specificity at lower latitudes, aligning with the high prevalence of small bacterial genomes and rare host species in the tropics. These findings underscore the importance of genome streamlining and host abundance in the evolution of host specificity in plant‐associated bacteria along the latitudinal gradient.

Understanding the evolution and maintenance of plant sexual diversity needs to incorporate both regional processes and local climate factors across large geographic scales. Using data of woody flowering plants from a global set of large-scale forest plots and multinomial logistic regression, we quantified regional effects on the proportions of dioecious, monoecious, and hermaphroditic species and their abundance while incorporating evolutionary history and local climate factors. Our results showed that plants were more likely to be dioecious than hermaphroditic in Oceania and tropical Asia but were more likely to be monoecious than dioecious in Europe and North America compared with tropical Africa. We further found that plants were more likely to be monoecious than dioecious in island communities. Plants were more likely to be monoecious than dioecious in areas with high precipitation but were more likely to be dioecious than monoecious in areas with high precipitation of coldest quarter. Our results suggest that both regional processes and local climate factors play important roles in shaping the geographic distribution of plant sexual systems, providing a baseline for predicting future changes in forest communities in the context of global change.

Keystone species are more important than others for community dynamics and stability. Keystone species can be identified and evaluated by their centrality (i.e., a relative ranking of the topological positional importance of a species) in ecological networks. Studies of node centrality of plant–fungus bipartite networks, for example, have identified the keystone species that are important for maintaining network structure and stability. However, the underlying drivers of the importance of species in a network have rarely been examined. We assessed the centrality (degree, closeness, and betweenness) of plant and fungal species in a plant–ectomycorrhizal fungus network in a subtropical forest in southern China. Based on the phylogenies of plants and fungi and plant traits, we explored ecological factors that led to a species taking a central position or not. We found one plant species (Ternstroemia gymnanthera) and four species of ectomycorrhizal fungi (Russula citrina, Scleroderma sp., and two Cenococcum sp.) were characterized by the highest centrality of degree, closeness, and betweenness among the bipartite network nodes and thus played key roles in maintaining network structure. Centrality for fungi (not for plants) was phylogenetically constrained. Plant traits and abundance together explained 46.36%, 46.0%, and 43.7% of variation in the centrality of degree, closeness, and betweenness of plant species in the bipartite network, respectively. When plant or fungal species were sequentially removed on the order of higher to lower centrality, network was less stable than randomly removed. We suggest that abundance and traits determine the positional importance of plant species in a network. This work helps understand how plant–fungus association networks will respond to species extinction and changes in species abundance and functional traits due to habitat fragmentation and human activities.