Distance- and density-dependent seedling mortality caused by several diseases in eight tree species co-occurring in a temperate forest

To examine whether the Janzen–Connell mechanism applies to temperate forests, seedling survival and causes of mortality were investigated at two distances (beneath, far) from conspecific adults and at two densities (high, low) at each distance for seedlings (n = 7935) of eight tree species co-occurring in a hardwood forest. Six of the eight species showed distance- and/or density-dependent seedling mortality mainly caused by diseases and rodents. In four of the five species primarily killed by disease (i.e. damping-off, blight, rot, powdery mildew), the infectivity (probability of infection by the disease) and/or the virulence (proportion of seedlings killed to those infected by the disease) were higher beneath than far from conspecific adults. These findings suggest that host specificity and/or spatially heterogeneous activity of natural enemies play an important role in the reciprocal replacement of tree species, maintaining species diversity in temperate forests.