The effect of light availability and spatio-temporal heterogeneity on the soil seed bank diversity in temperate forests

Light is a key driver of understory plant diversity, also important for seed germination, yet its effect on forests soil seed bank diversity remains underexplored. This study assessed the impact of both light availability and spatio-temporal heterogeneity on soil seed bank diversity. We hypothesized that increased light availability and heterogeneity at plot and landscape scale enhance soil seed bank species richness, particularly when accounting for historical canopy cover changes. Additionally, we expected that higher light availability increased similarity between the soil seed bank and understory vegetation. To test this, we established six subplots (25 m² each) within each of 30 plots (1 ha), in the Southern Black Forest, Germany. Within each subplot, soil cores (10 cm diameter, 0–15 cm depth) were collected and subsequently pooled to produce one composite sample per plot (n = 30). Soil seed bank species were identified through a germination experiment. Light availability and understory composition were recorded in the field. To test the effect of both present versus past landscape structure on soil seed bank diversity, percentage canopy cover and patch density (for a 500 m buffer) were calculated from contemporary and historical remote sensing data highlighting areas with continuous forest canopy cover since 1963. At the plot scale, light availability and heterogeneity did not have an effect on species richness but the dissimilarity between the soil seed bank and the understory significantly increased with light heterogeneity. Stands with more heterogeneous light conditions promoted the occurrence of different subsets of species in the soil seed bank or the understory. At the landscape scale, soil seed bank species richness increased significantly with increased patch density of continuous canopy cover, highlighting the role of past canopy openings. Overall this study highlights how light heterogeneity and historical disturbances shape soil seed bank diversity in managed forests.