Light can modify density-dependent seedling mortality in a temperate forest

Questions There is growing evidence on how density-dependent drivers of plant performance are influenced by habitat heterogeneity such as variation in understorey light availability. How these processes operate in temperate forests still remains an open question, however. We examined the interactive effects of density dependence and light availability on woody seedling mortality across multiple species, two size stages and growth forms. Location Alluvial old-growth temperate forest, southeastern Czech Republic. Methods We mapped seedlings of woody species (>= 10 cm tall) and recorded their mortality over a two-year period on 21 square seedling plots (25 m(2)) within one large square plot with all trees mapped (>= 1 cm diameter at breast height). Seedling mortality was studied using a combined approach of generalised linear mixed models and spatial point pattern analyses. Results Density-dependent effects of tree and seedling neighbours influenced seedling mortality at different spatial scales, depending on their size class, growth form and by taxon. Conspecific inhibition raised the mortality of seedlings other than Acer campestre at low light levels, indicating that host-specific enemies and/or intraspecific competition may be important in unfavourable habitats. While small seedlings experienced conspecific inhibition at low light levels, tall seedlings were additively affected by low-light conditions and competition with trees. Both conspecific and heterospecific neighbours tended to affect tree seedlings at low light levels, whereas shrub seedlings experienced light- and density-independent mortality. Some negative interspecific interactions indicated ongoing expansion of species like Acer campestre in currently flood-free habitats. Conclusions How density-dependent effects translate into demographic outcomes for woody seedling populations may be largely determined by variation in understorey light availability. Given that seedlings differ in size, growth form and taxa, we also demonstrate that both seedling and tree neighbours may contribute to the strength and spatial variation in density-dependent effects at seedling neighbourhood scales.