Predicting community response to N-enrichment with SLA: A multisystem test

Background/Question/Methods Human activities have increased the amount of available nitrogen (N) globally. Increased N-availability can change plant community structure and function, and lead to diversity loss. Species traits associated with differential resource limitation may predict how plant communities will respond to N-enrichment across ecosystems. We focused on specific leaf area (SLA), leaf area per unit leaf mass, as a candidate trait because it is correlated with high relative growth rates, photosynthetic rates, and leaf N-concentrations. Species with high SLA are often associated with high-N, light-limited environments. Thus, we expected that N-enrichment will cause 1) a shift in favor of species with higher SLA, and 2) stronger diversity declines in communities with low SLA and/or with species with a narrower range of SLA prior to enrichment. In unfertilized conditions, we measured SLA of 10 individuals of 260 species comprising the top 80% cover of 35 long-term N-enrichment experiments within the United States. These experiments were located in 10 herbaceous communities, ranging from tundra to salt marshes to grasslands. Using relative abundance data from control and N-fertilization treatments we examined if SLA could predict changes in abundance or diversity with N-enrichment within and across sites.

Results/Conclusions Across all experiments, species with lower SLA were more likely to be lost with N-enrichment. For species that were not lost, SLA weakly predicted changes in abundance in response to N-enrichment. Additionally, this relationship was statistically significant in only 3 of 35 experiments, and only 2 of 10 ecosystems. Community-aggregated SLA was not related to the effect of N-enrichment on species richness; however, communities containing species with a narrow range of SLA experienced greater diversity declines than communities containing species with a wider range of SLA. Our findings suggest that SLA in control conditions can predict species loss due to N-enrichment, but can only weakly predict changes in abundance of the species that persist in both environments. We also found support for our hypothesis that trait diversity provides resilience against diversity loss due to N-enrichment.