Where does the community start, and where does it end? Including the seed bank to reassess forest herb layer responses to the environment

Question Belowground processes are key determinants of aboveground plant population and community dynamics. Still, our understanding of how environmental drivers shape plant communities is mostly based on aboveground diversity patterns, bypassing belowground plant diversity stored in seed banks. As seed banks may shape aboveground plant communities, we question whether concurrently analyzing the above- and belowground species assemblages may potentially enhance our understanding of community responses to environmental variation. Location Temperate deciduous forests along a 2000 km latitudinal gradient in NW-Europe. Methods Herb layer, seed bank and local environmental data including soil pH, canopy cover, forest cover continuity and time since last canopy disturbance were collected in 129 temperate deciduous forest plots. We quantified herb layer and seed bank diversity per plot and evaluated how environmental variation structured community diversity in the herb layer, seed bank and the combined herb layer-seed bank community. Results Seed banks consistently held more plant species than the herb layer. How local plot diversity was partitioned across the herb layer and seed bank was mediated by environmental variation in drivers serving as proxies of light availability. The herb layer and seed bank contained an ever smaller and ever larger share of local diversity, respectively, as both canopy cover and time since last canopy disturbance decreased. Species richness and β-diversity of the combined herb layer-seed bank community responded distinctly different compared to the separate assemblages in response to environmental variation in, e.g., forest cover continuity and canopy cover. Conclusions The seed bank is a belowground diversity reservoir of the herbaceous forest community, which interacts with the herb layer, though constrained by environmental variation in e.g. light availability. The herb layer and seed bank coexist as a single community by means of the so-called storage effect, resulting in distinct responses to environmental variation not necessarily recorded in the individual herb layer or seed bank assemblages. Thus, concurrently analysing above- and belowground diversity will improve our ecological understanding of how understorey plant communities respond to environmental variation. This article is protected by copyright. All rights reserved.