COS 37-2
Response to nitrogen deposition in a savanna by oak understory and open grassland plant communities: Crossing thresholds or resilience?
The California oak savanna is increasingly exposed to atmospheric nitrogen (N) deposition from anthropogenic activities. In response to N deposition, the plant community may exhibit threshold behavior in which the community crosses a tipping point and reaches an alternative stable state at a certain level of N deposition. Alternatively, the plant community may be resilient to N deposition and not experience change. In savanna landscapes, tree canopies introduce spatial heterogeneity to the ground layer plant community and ecosystem processes. The community beneath oaks may exhibit a different response to N deposition than the adjacent open community. In this experiment, we asked 1) is there a threshold level of N deposition prompting a state shift in the plant community and 2) does that level differ between understory and open areas? We simulated a range of N deposition rates by fertilizing plots in the understory and open. Productivity, species richness, and cover of invasive species and soil chemical and physical properties were measured for two years. We performed a hierarchical cluster analysis to characterize community composition in the system, and linear mixed effects modeling to analyze the effect of N fertilization and landscape position (understory versus open) on our plant community measures.
Results/Conclusions
Community composition differed between understory and open. Species richness was lower beneath oaks and many species were restricted to the understory. Contrary to expectations, N fertilization did not significantly affect plant community measures, except at the highest level of N treatment. After the first year of fertilization, high N decreased productivity in the understory relative to control plots, but after the second year of fertilization, high N decreased productivity in the open relative to control plots. Species richness decreased under high N in the understory relative to control plots after the second year. There was no effect of N on species richness in open plots. Invasive species did not respond to N fertilization. Cluster analysis did not show any clustering based on N fertilization. The high N treatment that resulted in a shift to a species poor, low productivity community was well above realistic potential N deposition rates. Therefore, the results of the field experiment suggest that the oak savanna plant community is resilient against current N deposition levels and is not subject to a threshold response. Resilience of the plant community could be due to high leaching from soils, causing fertilizer to be lost before it can affect plants.