Fertilization triggers 11 yr of changes in community assembly in Mediterranean grassland

QuestionFertilization, i.e. nutrient addition, and changes in grazing intensity are important factors associated with global change that threaten grasslands worldwide. How does the plant community assembly in Mediterranean grasslands respond after 11yr to a single fertilization event?LocationSpecies-rich Mediterranean grassland, Navarra, northern Spain.MethodsIn 2003, 11 permanent plots were established in a Mediterranean grassland: six plots were fertilized and five were used as controls. Fertilization consisted in the addition of sewage sludge on the soil surface. Grazing was excluded during the course of the experiment. Changes in vegetation abundance were monitored from 2004 to 2014. In 2012, key plant functional traits (height, leaf area, SLA, LDMC) of the most frequent species were collected. We measured taxonomic and functional species composition and diversity. Niche overlap compared to null models was used to assess the mechanisms influencing community assembly.ResultsSpecies composition changed markedly in the fertilized plots. These changes were accompanied by a reduction in taxonomic diversity. One yr after starting the experiment there were no differences in functional composition (i.e. CWM trait values) between fertilized and control plots (excluding SLA), but 11yr later fertilized plots were composed of taller plants with larger SLA and lower LDMC. We found a major reduction in niche overlap in plant height andSLA after fertilizing associated with a replacement in several species that could be indicative of competitive exclusion (principle of limiting similarity). Conversely, we found an increase in niche overlap in leaf area indicating that competition increases similarity among species (weaker competitor exclusion or abiotic filtering). At the end of the experiment, we found large differences in SLA and LDMC between fertilized and control plots.ConclusionsA single fertilization event that occurred 11yr ago triggered a wave of changes in grassland community assembly that are still on-going. Fertilization enhanced the establishment of taller species with larger leaf areas, which displaced, by competitive exclusion, the species that had previously been present. Community assembly patterns in Mediterranean grassland are highly sensitive to abrupt changes in resource availability.