Intra‐specific relatedness, spatial clustering and reduced demographic performance in tropical rainforest trees

Intraspecific variation in Janzen-Connell effect is mediated by stress and plant-soil feedbacks

Janzen-Connell (JC) effects, hypothesized to be partially driven by negative plant-soil feedbacks (PSFs), are considered to be a key mechanism that regulates tropical forest plant diversity and coexistence. However, intraspecific variation in JC effects may weaken this mechanism, with the strength of PSFs being a potentially key variable process. We conducted a manipulated experiment with seedlings from two populations of Pometia pinnata (Sapindaceae), a tropical tree species in southwest China. We aimed to measure the intraspecific difference in PSF magnitude caused by inoculating the soil from different P. pinnata source populations and growing seedlings under differing light intensity and water availability treatments, and at varying plant densities. We found negative PSFs for both populations with the inoculum soil originating from the same sites, but PSFs differed significantly with the inoculum soil from different sites. PSF strength responded differently to biotic and abiotic drivers; PSF strength was weaker in low moisture and high light treatments than in high moisture and low light treatments. Our study documents intraspecific variation in JC effects: specifically, P. pinnata have less defenses to their natively-sourced soil, but are more defensive to the soil feedbacks from soil sourced from other populations. Our results imply that drought and light intensity tended to weaken JC effects, which may result in loss of species diversity with climate change.

Differential impacts of adult trees on offspring and non-offspring recruits in a subtropical forest

An important mechanism promoting species coexistence is conspecific negative density dependence (CNDD), which inhibits conspecific neighbors by accumulating host-specific enemies near adult trees. Natural enemies may be genotype-specific and regulate offspring dynamics more strongly than non-offspring, which is often neglected due to the difficulty in ascertaining genetic relatedness. Here, we investigated whether offspring and non-offspring of a dominant species, Castanopsis eyrei, suffered from different strength of CNDD based on parentage assignment in a subtropical forest. We found decreased recruitment efficiency (proxy of survival probability) of offspring compared with non-offspring near adult trees during the seedling-sapling transition, suggesting genotype-dependent interactions drive tree demographic dynamics. Furthermore, the genetic similarity between individuals of same cohort decreased in late life history stages, indicating genetic-relatedness-dependent tree mortality throughout ontogeny. Our results demonstrate that within-species genetic relatedness significantly affects the strength of CNDD, implying genotype-specific natural enemies may contribute to population dynamics in natural forests.

Intraspecific variation mediates density dependence in a genetically diverse plant species

Interactions between neighboring plants are critical for biodiversity maintenance in plant populations and communities. Intraspecific trait variation and genome duplication are common in plant species and can drive eco-evolutionary dynamics through genotype-mediated plant-plant interactions. However, few studies have examined how species-wide intraspecific variation may alter interactions between neighboring plants. We investigate how subspecies and ploidy variation in a genetically diverse species, big sagebrush (Artemisia tridentata), can alter the demographic outcomes of plant interactions. Using a replicated, long-term common garden experiment that represents range-wide diversity of A. tridentata, we ask how intraspecific variation, environment, and stand age mediate neighbor effects on plant growth and survival. Spatially explicit models revealed that ploidy variation and subspecies identity can mediate plant-plant interactions but that the effect size varied in time and across experimental sites. We found that demographic impacts of neighbor effects were strongest during early stages of stand development and in sites with greater growth rates. Within subspecies, tetraploid populations showed greater tolerance to neighbor crowding compared to their diploid variants. Our findings provide evidence that intraspecific variation related to genome size and subspecies identity impacts spatial demography in a genetically diverse plant species. Accounting for intraspecific variation in studies of conspecific density dependence will improve our understanding of how local populations will respond to novel genotypes and biotic interaction regimes. As introduction of novel genotypes into local populations becomes more common, quantifying demographic processes in genetically diverse populations will help predict long-term consequences of plant-plant interactions.

The global significance of biodiversity science in China: an overview

Biodiversity science in China has seen rapid growth over recent decades, ranging from baseline biodiversity studies to understanding the processes behind evolution across dynamic regions such as the Qinghai-Tibetan Plateau. We review research, including species catalogues; biodiversity monitoring; the origins, distributions, maintenance and threats to biodiversity; biodiversity-related ecosystem function and services; and species and ecosystems' responses to global change. Next, we identify priority topics and offer suggestions and priorities for future biodiversity research in China. These priorities include (i) the ecology and biogeography of the Qinghai-Tibetan Plateau and surrounding mountains, and that of subtropical and tropical forests across China; (ii) marine and inland aquatic biodiversity; and (iii) effective conservation and management to identify and maintain synergies between biodiversity and socio-economic development to fulfil China's vision for becoming an ecological civilization. In addition, we propose three future strategies: (i) translate advanced biodiversity science into practice for biodiversity conservation; (ii) strengthen capacity building and application of advanced technologies, including high-throughput sequencing, genomics and remote sensing; and (iii) strengthen and expand international collaborations. Based on the recent rapid progress of biodiversity research, China is well positioned to become a global leader in biodiversity research in the near future.

Resistance Genes Affect How Pathogens Maintain Plant Abundance and Diversity

AbstractSpecialized pathogens are thought to maintain plant community diversity; however, most ecological studies treat pathogens as a black box. Here we develop a theoretical model to test how the impact of specialized pathogens changes when plant resistance genes (R-genes) mediate susceptibility. This work synthesizes two major hypotheses: the gene-for-gene model of pathogen resistance and the Janzen-Connell hypothesis of pathogen-mediated coexistence. We examine three scenarios. First, R-genes do not affect seedling survival; in this case, pathogens promote diversity. Second, seedlings are protected from pathogens when their R-gene alleles and susceptibility differ from those of nearby conspecific adults, thereby reducing transmission. If resistance is not costly, pathogens are less able to promote diversity because populations with low R-gene diversity suffer higher mortality, putting those populations at a disadvantage and potentially causing their exclusion. R-gene diversity may also be reduced during population bottlenecks, creating a priority effect. Third, when R-genes affect survival but resistance is costly, populations can avoid extinction by losing resistance alleles, as they cease paying a cost that is unneeded. Thus, the impact pathogens can have on tree diversity depends on the mechanism of plant-pathogen interactions. Future empirical studies should examine which of these scenarios most closely reflects the real world.

Evidence of within-species specialization by soil microbes and the implications for plant community diversity

Microbes are thought to maintain diversity in plant communities by specializing on particular species, but it is not known whether microbes that specialize within species (i.e., on genotypes) affect diversity or dynamics in plant communities. Here we show that soil microbes can specialize at the within-population level in a wild plant species, and that such specialization could promote species diversity and seed dispersal in plant communities. In a shadehouse experiment in Panama, we found that seedlings of the native tree species, Virola surinamensis (Myristicaceae), had reduced performance in the soil microbial community of their maternal tree compared with in the soil microbial community of a nonmaternal tree from the same population. Performance differences were unrelated to soil nutrients or to colonization by mycorrhizal fungi, suggesting that highly specialized pathogens were the mechanism reducing seedling performance in maternal soils. We then constructed a simulation model to explore the ecological and evolutionary consequences of genotype-specific pathogens in multispecies plant communities. Model results indicated that genotype-specific pathogens promote plant species coexistence-albeit less strongly than species-specific pathogens-and are most effective at maintaining species richness when genetic diversity is relatively low. Simulations also revealed that genotype-specific pathogens select for increased seed dispersal relative to species-specific pathogens, potentially helping to create seed dispersal landscapes that allow pathogens to more effectively promote diversity. Combined, our results reveal that soil microbes can specialize within wild plant populations, affecting seedling performance near conspecific adults and influencing plant community dynamics on ecological and evolutionary time scales.

Direct and indirect effects of climate on richness drive the latitudinal diversity gradient in forest trees

Climate is widely recognised as an important determinant of the latitudinal diversity gradient. However, most existing studies make no distinction between direct and indirect effects of climate, which substantially hinders our understanding of how climate constrains biodiversity globally. Using data from 35 large forest plots, we test hypothesised relationships amongst climate, topography, forest structural attributes (stem abundance, tree size variation and stand basal area) and tree species richness to better understand drivers of latitudinal tree diversity patterns. Climate influences tree richness both directly, with more species in warm, moist, aseasonal climates and indirectly, with more species at higher stem abundance. These results imply direct limitation of species diversity by climatic stress and more rapid (co-)evolution and narrower niche partitioning in warm climates. They also support the idea that increased numbers of individuals associated with high primary productivity are partitioned to support a greater number of species.