Conspecific distance-dependent seedling performance, and replacement of conspecific seedlings by heterospecifics in five hardwood, temperate forest species.

The mechanisms driving species diversity in the context of Janzen-Connell model are best understood by evaluating not only conspecific distance-dependent (CDD) seedling performance, but also replacement of conspecific seedlings by heterospecific seedlings beneath adult trees. We evaluated CDD and replacement as a log response ratio of seedling performance (height, age) directly beneath and at a distance from adult plants in a temperate forest, and examined the log response ratio of that between conspecifics and heterospecifics beneath adults for five hardwood species with different ecological traits (e.g., seed size, mycorrhizal type, relative abundance). CDD was greater in three small-seeded species with arbuscular mycorrhizae (AM) associations than it was in two large-seeded species with ectomycorrhizae (EM) associations. Replacement was also higher for small-seeded AM species compared to large-seeded EM species, resulting in a strong, positive relationship between CDD and replacement. The traits suggest that small-seeded AM seedlings are more likely to be replaced by heterologous seedlings beneath the adults than large-seeded EM seedlings, probably due to that the small-seeded AM species are more susceptible to attack by plant natural enemies (e.g., soil pathogens, leaf diseases). As a result, small-seeded AM species had lower relative abundances compared to large-seeded EM species. This study suggests that either seed size or associations with microorganisms play an important role in driving forest diversity by regulating replacement and CDD, although relative importance of the two traits (i.e., seed size, mycorrhizal type) remains unclear, because of the autocorrelation between the two traits for the five species studied.