Light and competition fail to explain seedling coexistence

The coexistence of forest tree species has often been linked to differences in their response to light availability during the regeneration stage. Van Couwenberghe et al. study natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings and find that no rank reversal occurs between the two species along a light gradient, or along density, mixture or seedling-size gradients. The results thus do not support the classical assumption that spatial heterogeneity in a canopy opening would explain the coexistence of the two species studied. Instead, it is suggested that the main driver of the dynamics of these mixed stands is spatial variation in local size hierarchies among seedlings, which may be caused by differences in seedling emergence time or initial seedling performance.