SPECIES RICHNESS IN A SUCCESSIONAL GRASSLAND: EFFECTS OF NITROGEN ENRICHMENT AND PLANT LITTER

We conducted a field experiment in a successional grassland to investigate the short-term effects of nitrogen enrichment and plant litter on plant species richness and on the establishment of a native grass (Andropogon gerardi) that was experimentally introduced to the study site as seed. Additions of nitrogen fertilizer to experimental plots over two growing seasons increased plant production as indicated by increases in both living plant biomass and litter biomass. Increased productivity reduced species richness by effectively preventing the seedling establishment of the subordinate forb species and reduced the recruitment of Andropogon gerardi by inhibiting both germination and survival. Litter removals carried out at each of two levels of nitrogen enrichment (no nitrogen, nitrogen added) showed that litter significantly reduced species richness by the same amount in fertilized and unfertilized plots, suggesting that living biomass and litter were purely additive, rather than interactive in their effects. In contrast, the results of litter additions suggested that the declines in richness associated with fertilization could largely be due to the suppressive effects of increased litter biomass alone. As a whole, the results indicate that litter and living biomass are largely substitutable in their inhibitory effects on species richness in highly productive successional grasslands due to their independent and equivalent capacities to attenuate light to very low levels. This study highlights the combined roles of competition and plant litter in influencing the diversity of grasslands through effects on seedling establishment.

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