The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.

1. Forest ecosystems are critical for the global regulation of carbon (C), a substantial portion of which is stored in above-ground biomass (AGB). While it is well understood that taxonomic and functional composition, stand structure and environmental gradients influence spatial variation in AGB, the relative strengths of these drivers at landscape scales have not been investigated in temperate forests. Furthermore, when biodiversity enhances C storage, it is unclear whether it is through mass-ratio effects (i.e. the dominant trait in communities regulates AGB) or through niche complementarity (i.e. increased AGB due to interspecific resource partitioning).2. To address these mechanisms, we analysed data from a census of 28,262 adult trees sampled across 900 ha of temperate deciduous forest in southwestern Pennsylvania. We used data on four key plant functional traits to determine if (1) there is a positive relationship between species diversity and AGB and (2) whether this is due to mass-ratio effects or niche complementarity. We also sought to (3) identify the physical stand structural attributes and topographic variables that influence AGB across this landscape.3. We found AGB was positively related to species richness and negatively related to species evenness, albeit weakly, while functional diversity indices had neutral effects. Above-ground biomass was enhanced in communities dominated by traits related to greater maximum tree height, deeper minimum rooting depths and larger seeds. Most importantly, areas with high AGB were dominated by Acer saccharum and Liriodendron tulipifera. Overall, these results support mass-ratio effects, with little evidence for niche complementarity. Synthesis.Stand structure, topography, and species and functional composition, but not taxonomic or functional diversity, were found to be key drivers of aboveground biomass at landscape scales (<900 ha) in this temperate deciduous forest.Our findings suggest that simultaneously managing for both high diversity andfor above-ground carbon storage may prove challenging in some forest systems.Our results further indicate that the impact of tree biodiversity loss on aboveground carbon stocks will depend greatly on the identity of the species that are lost. | Journal of EcologyFOTIS eT al. SUPPORTING INFORMATIONAdditional Supporting Information may be found online in the supporting information tab for this article.How to cite this article: Fotis AT, Murphy SJ, Ricart RD, et al.Above-ground biomass is driven by mass-ratio effects and stand structural attributes in a temperate deciduous forest.

650Patterns of diversity and community composition in forests are controlled by a combination of 651 environmental factors, historical events, and stochastic or neutral mechanisms. Each of these 652 processes has been linked to forest community assembly, but their combined contributions to 653 alpha and beta-diversity in forests has not been well explored. Here we use variance partitioning 654 to analyze ~40,000 individual trees of 49 species, collected within 137 ha of sampling area 655 spread across a 900 ha temperate deciduous forest reserve in Pennsylvania to ask: 1) To what 656 extent is site-to-site variation in species richness and community composition of a temperate 657 forest explained by measured environmental gradients and by spatial descriptors (used here to 658 estimate dispersal-assembly or unmeasured, spatially-structured processes)? 2) How does the 659 incorporation of land-use history information increase the importance attributed to deterministic 660 community assembly? And 3) How do the distributions and abundances of individual species 661 within the community correlate with these factors? Environmental variables (i.e., topography, 662 soils, and distance to stream), spatial descriptors (i.e. spatial eigenvectors derived from Cartesian 663 coordinates), and land-use history variables (i.e., land-use type and intensity, forest age, and 664 distance to road), explained about half of the variation in both species richness and community 665 composition. Spatial descriptors explained the most variation, followed by measured 666 environmental variables and then by land-use history. Individual species revealed variable 667 responses to each of these sets of predictor variables. Several species were associated with 668 stream habitats, and others were strictly delimited across opposing north and south-facing slopes. 669Several species were also associated with areas that experienced recent (i.e. < 100 years) human 670 land-use impacts. These results indicate that deterministic factors, including environmental and 671 land-use history variables, are important drivers of community response. The large amount of 672 'unexplained' variation seen here (about 50%) is commonly observed in other such studies 673 Murphy 3 attempting to explain distribution and abundance patterns of plant communities. Determining 674 whether such large fractions of unaccounted for variation are caused by lack of sufficient data, or 675 are an indication of stochastic features of forest communities globally, will remain an important 676 challenge for ecologists in the future. 677 678

Herbaceous plants are a key component of tropical forests. Previous work indicates that herbs contribute substantially to the species richness of tropical plant communities. However, the processes structuring tropical herb diversity, and how they contrast with woody communities, have been underexplored. Within the understory of a 50-ha forest dynamics plot in central Panama, we compared the diversity, distribution, and abundance of vascular herbaceous plants with woody seedlings (i.e., tree and lianas <1 cm DBH and ≥20 cm tall). Beta-diversity was calculated for each community using a null model approach. We then assessed the similarity in alpha and beta-diversity among herbs, tree seedlings, and liana seedlings. Strengths of habitat associations were measured using permutational ANOVA among topographic habitat-types. Variance partitioning was then used to quantify the amount of variation in species richness and composition explained by spatial and environmental variables (i.e., topography, soils, and shade) for each growth form. Species richness and diversity were highest for tree seedlings, followed by liana seedlings and then herbs. In contrast, beta-diversity was 16-127% higher for herbs compared to woody seedlings, indicating higher spatial variation in this stratum. We observed no correlation between local richness or compositional uniqueness of herbs and woody seedlings across sites, indicating that different processes control the spatial patterns of woody and herbaceous diversity and composition. Habitat associations were strongest for herbs, as indicated by greater compositional dissimilarity among habitat types. Likewise, environmental variables explained a larger proportion of the variation in species richness and composition for herbs than for woody seedlings (richness = 25%, 14%, 12%; composition = 25%, 9%, 6%, for herbs, trees, and lianas, respectively). These differences between strata did not appear to be due to differences in lifespan alone, based on data from adult trees. Our results point to contrasting assembly mechanisms for herbaceous and woody communities, with herbs showing stronger niche-derived structure. Future research on tropical herbaceous communities is likely to yield new insights into the many processes structuring diverse plant communities.

Since the 1984 passage of the Waxman-Hatch Act, generic prescription drugs have become central to disease treatment and generic drug entry and price competition has been vigorous in the U.S. Nonetheless, recent policy concern has focused on potential supply inadequacy and price increases among selected generic drugs. Details regarding the supply of generic drugs throughout the product life cycle are surprisingly unstudied. Here, we examine manufacturer entry, exit, the extent of competition and the relationship between supply structure and inflation adjusted prices among generic drugs. Our empirical approach is descriptive and reduced form, following recent innovations on the older structure-conduct-performance tradition. We employ quarterly national data on quantities, wholesale dollar sales and manufacturers from QuintilesIMS National Sales Perspective data, 2004Q4-2016Q3. Defining a market as the molecule-dosage-form, we observe that median sizes of drug markets are predominantly small, with annual inflation adjusted sales revenues of less than $10 million but increasing over time. The median number of manufacturers in each market is about two, the mean about four. We find evidence to suggest decreasing numbers of suppliers over our study period, particularly following implementation of the Affordable Care Act in 2010 and the Generic Drug User Fee Amendments of 2012, attributable both to more exit and less entry. Approximately 40 percent of markets are supplied by one manufacturer; the share of markets supplied by one or two manufacturers is observed to increase over time and is more likely among non-oral drugs and those belonging to selected therapeutic classes. We find evidence to suggest prices of generic drugs are statistically significantly increasing over time, particularly following the implementation of the 2010 Affordable Care Act and the 2012 Generic Drug User Fee Amendments. Price increases are positively correlated with reduced manufacturer counts and alternative measures of increased supplier concentration, holding all else constant. Our results suggest the market for generic drugs is largely comprised of small revenue products the supply of which has tended towards duopoly or monopoly in recent years. Therefore, it is surprising generic drug prices have not been observed to be higher and potentially risen more over our study period. This issue merits further study; we suggest several testable hypotheses based in economic theory.