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The Janzen–Connell Hypothesis (JCH) predicts that density‐responsive and host‐specific natural enemies limit the population sizes of abundant species. Importantly, these interactions help to maintain local community diversity through time. While ample evidence exists for the demographic predictions of the JCH, it remains unclear which natural enemies drive these dynamics across different plant communities.While large mammalian herbivores are often assumed to lack the specialized diet needed to drive Janzen–Connell effects, they do show a degree of host‐preference that could drive density‐dependent plant demography. However, the potential role of large mammalian herbivores in Janzen–Connell interactions has only rarely been investigated.Using 204 seedling transects (1 m × 10 m) at 51 sites across a 900‐ha forested reserve in southwestern Pennsylvania (USA), we examined the role that large mammals play in driving conspecific negative density dependence (CNDD) in temperate tree seedlings. Individual fences were erected around half of the transects (n = 102) to exclude large mammals, and were paired with adjacent unfenced transects. Within transects, a total of ~15,000 individual seedlings were monitored over three growing seasons.Demographic neighbourhood models were constructed to examine the influence of neighbourhood composition and density on seedling survival and growth. An interaction term between conspecific neighbour density and fencing treatment was included to test the hypothesis that large herbivores cause CNDD.We found that seedling survival was influenced by both conspecific neighbour density and fencing. CNDD was strongest when large mammals were allowed access to seedlings, and these results were driven by two abundant taxa (Prunus serotina and Fraxinus spp). Despite evidence that large mammals mediate CNDD, we found no effect of fencing on rarified species richness or evenness in seedling transects during the study.Synthesis. Understanding the specific natural enemies driving conspecific negative density dependence remains vital for understanding the maintenance of forest diversity across the globe. Our results indicate that large mammalian herbivores are capable of driving CNDD in temperate tree species. These results suggest that large mammals may be an important and generally overlooked agent contributing to Janzen–Connell interactions in forest communities. We expect that further research examining large mammals in other systems will be important in the future. [ABSTRACT FROM AUTHOR]

Environmental gradients act as potent filters on species distributions driving compositional shifts across communities. Compositional shifts may reflect differences in physiological tolerances to a limiting resource that result in broad distributions for tolerant species and restricted distributions for intolerant species (i.e. a nested pattern). Alternatively, trade‐offs in resource use or conflicting species' responses to multiple resources can result in complete turnover of species along gradients.We combined trait (leaf area, leaf mass per area, wood density and maximum height) and distribution data for 550 tree species to examine taxonomic and functional composition at 72 sites across strong gradients of soil phosphorus (P) and rainfall in central Panama.We determined whether functional and taxonomic composition was nested or turned over completely and whether community mean traits and species composition were more strongly driven by P or moisture.Turnover characterized the functional composition of tree communities. Leaf traits responded to both gradients, with species having larger and thinner leaves in drier and more fertile sites than in wetter and less fertile sites. These leaf trait–moisture relationships contradict predictions based on drought responses and suggest a greater role for differences in light availability than in moisture. Shifts in wood density and maximum height were weaker than for leaf traits with taller species dominating wet sites and low wood density species dominating P‐rich sites.Turnover characterized the taxonomic composition of tree communities. Geographic distances explained a larger fraction of variation for taxonomic composition than for functional composition, and community mean traits were more strongly driven by P than moisture.Synthesis. Our results offer weak support for the tolerance hypothesis for tree communities in central Panama. Instead, we observe functional and taxonomic turnover reflecting trade‐offs and conflicting species' responses to multiple abiotic factors including moisture, soil phosphorus and potentially other correlated variables (e.g. light). [ABSTRACT FROM AUTHOR]

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