Functional groups affect seedling survival both through a negative soil feedback and changes in abiotic conditions.

Seedling establishment, growth and survival are influenced by the competition from neighbouring plants for resources and their effect on the environment, including plant–soil feedback. Do species-level mechanisms operate at the functional group level? We used a long-term removal experiment in a temperate grassland to study seedling survival and growth of two forb species and two species of grass in plots with either, only forb, only grass or mixture of grass and forb plants. The seedlings were followed for 16 months and environmental factors measured. All species survived best and grew larger in plots with plants of the same functional group. The largest differences in seedling survival between grass and forb plots were recorded in winter. We therefore carried out an additional experiment that focused on the effects of above-ground biomass and plant–soil feedback on overwintering of seedlings. Removal of above-ground plant biomass at the beginning of winter increased seedling survival of all four species in grass plots, but only of the forb species in forb plots. Negative plant–soil feedback was detected only for forbs during the winter experiment. At a finer scale in the main experiment, an increasing abundance of conspecific neighbours (of the same functional group) had a negative effect on seedling survival, but in some cases had a positive effect on seedling size. The environmental conditions measured were generally more favourable for seedling establishment in the forb plots (higher PhAR transmission and R/FR ratio in early spring, lower summer soil–surface temperatures, higher soil moisture throughout the season, higher long-term nitrate flow), but in the grass plots seedling establishment was probably enhanced by a higher spatial heterogeneity. Our results indicate that seedlings of both functional groups thrive better among plants of the other functional group, although reasons differ for the two functional groups compared: negative plant–soil feedback effect on the forb functional group combined with a negative effect of biomass and the high spatial heterogeneity in the grass plots.