The Janzen-Connell hypothesis predicts that high densities of specialist enemies surrounding established trees leads to higher mortality of conspecific seeds and seedlings. Where conspecific seedlings fail, heterospecific seedlings have a higher probability of survival, leading to greater spatial mixing of tree species. Although this conspecific negative density dependence (CNDD) can lead to maintenance of tree diversity in forests, not all tree species exhibit evidence of CNDD. In attempts to better evaluate CNDD, some studies have used plant traits (in particular, shade tolerance and mycorrhizal type) as a predictor of differential susceptibility to CNDD. Here, we simultaneously test the importance of shade tolerance and mycorrhizal type as modifiers of CNDD susceptibility in common tree species of mixed hardwood forests in the Piedmont of North Carolina. More specifically, we compare the ability of these focal traits to explain CNDD susceptibility against species-specific models, testing to what extent this trait-based approach can explain differential conspecific sensitivity. We test models using a Bayesian hierarchical approach and evaluate seedling and sapling growth and survival based on conspecific and heterospecific densities at a series of spatial scales (5, 10, 15, and 20m).
Results/Conclusions
We present evidence that shade tolerance and mycorrhizal type can, to a small degree, explain differential CNDD susceptibility among temperate forest tree species. However, species-specific models outperformed the trait-based models for most spatial scales, indicating unaccounted-for interspecific variation that may modify a species' susceptibility to conspecifics. In other words, there may be other traits correlated with tissue defense or growth strategy that better explain interspecific variation in the magnitude of CNDD effects on temperate trees. Alternatively – or coinciding with this – environmental conditions may influence CNDD susceptibility. For example, literature focused on plant-pathogen dynamics has established the role of soil moisture in predicting pathogen infection of plants. It is possible that micro-environmental heterogeneity across plots – or large-scale climatic differences across regions – is in part responsible for differential CNDD susceptibility. We argue that there may be more important drivers of differential CNDD susceptibility in temperate trees than those that are commonly addressed in CNDD literature