Litter accumulation, not light limitation, drives early plant recruitment

Abstract

Abstract Theory predicts a decline in grassland diversity under nutrient enrichment and loss of herbivory, and one possible cause is hampered seedling recruitment. Two potential drivers for reduced diversity at the seedling level are diminished light availability caused by surrounding vegetation and accumulation of dead biomass. To test the importance of these two mechanisms on early recruitment, we added seeds of 15 herbaceous grassland plant species and monitored sown and natural seedling emergence during one growing season in a full factorial field experiment with light addition and litter removal under fertilization and exclusion of mammalian herbivores in an experimental grassland in Central Germany. We used modern LED lamps, mimicking the spectrum of natural sun light, to provide light to small‐statured understorey plants. This novel experimental set‐up allowed us to specifically disentangle the roles of light limitation and litter accumulation independently and in combination. In general, herbivore exclusion, but not fertilization increased the amount of litter biomass. Litter removal increased seedling number and richness by 83% and 33%, respectively, while light addition had no significant main effect on seedling recruitment, nor did it interact with any other factors, and did not affect recruitment even when litter was removed. In addition, fertilization had a negative and herbivore exclusion a negligible impact on recruitment, and these effects were independent of litter removal. Furthermore, seedling number and richness were unrelated to light intensity and quality, litter depth, soil moisture, temperature and C:N ratio. Synthesis: These results provide novel insights into the role of light limitation versus litter accumulation driving early recruitment and help understanding the mechanisms that affect diversity in grassland communities via recruitment. Our results highlight the detrimental role of litter accumulation as opposed to surrounding vegetation induced light deficiency driving early recruitment from seeds and call for management actions that reduce the amount of litter when maintaining or restoring diversity.