Light and competition gradients fail to explain the coexistence of shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings

Abstract

Background and Aims The coexistence of forest tree species has often been linked to differences among species in terms of their response to light availability during the regeneration stage. From this perspective, species coexistence results from growth-growth or mortality-growth trade-offs along spatial light gradients. Experimental evidence of growth-growth trade-offs in natural conditions is sparse due to various confounding factors that potentially hinder the relationship. This study examined growth hierarchies along light gradients between two tree species with contrasting shade tolerance by controlling potential confounding factors such as seedling size, seedling status, seedling density and species composition. Methods Natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings were used, and growth rankings over a 4-year period were compared in 8- to 10-year-old tree seedlings. Key resultsNorank reversal occurs between the two species along the light gradient, or along the density, mixture or seedling size gradients.The shade-tolerant specieswas always themore competitive of the two. Pronounced effects of initial size on seedling growth were observed, whereas the effects of light and competition by neighbours were of secondary importance. The paramount effect of size, which results fromthe asymmetric nature of interseedling competition, gives a strong advantage to tall seedlings over the long term. ConclusionsThis studyextendsprevious effortsto identifypotentialdriversof rankreversals in young treemixtures. It does not support the classical assumption that spatial heterogeneity in canopy opening explains the coexistence of the two species studied. It suggests that spatial variation in local size hierarchies among seedlings that may be caused by seedling emergence time or seedling initial performance is the main driver of the dynamics of these mixed stands. © The Author 2013.