Effects of nitrogen deposition on interactions between native and exotic grasses of the California oak savanna are life stage specific

Background/Question/Methods

Nitrogen (N) deposition in the California oak savanna is spatially and temporally heterogeneous.   Hotspots exist locally beneath solitary oak canopies within a site, and regionally, near N emission sources.  Periodic flushes of N deposition occur throughout the growing season, and, consequently, plant community response may be driven by changes that occur at a particular growth phase for the plants.  This spatial and temporal heterogeneity may influence native and exotic competitive interactions.  To investigate the influence of N deposition on species interactions relative to other critical factors in the system, a greenhouse experiment was conducted.   The effects of N fertilization, soil type, and light environment on native-exotic community combinations during early growth, peak biomass, and senescence were tested.  A native perennial, a naturalized exotic annual, and an invasive exotic annual grass species were used.  Soil was collected from 3 sites that represented the range of background fertility and N deposition levels in the oak savanna.  Pots were fertilized at N concentrations to simulate deposition levels found across the region and a shading treatment simulated oak understory light levels.  Plant performance was measured at each growth phase as: total biomass – above and belowground –, height, and seed production and analyzed using ANOVA.

Results/Conclusions

The naturalized exotic and native perennial responded significantly to high N fertilization, but the invasive exotic showed no response.  Nitrogen had the strongest effect on the performance of the naturalized exotic, and the response did not depend on the presence of competitors.  The effect of N on plant performance varied among life stages depending on the plant response variable.  For both native and naturalized exotic, N increased aboveground biomass and height during early growth, but increased belowground biomass during peak growth.  During the senescent phase, N had no effect on seed production.  Soil type, light, and competitive mixture were better predictors of plant performance than N fertilization.  There was no interaction between N and soil type indicating plant response was not to soil N levels, but perhaps to soil structure or other variables.  The importance of N for the naturalized exotic species, but not the invasive species contradicts ecological theory.  Overall, under realistic scenarios in the oak savanna, N deposition is not likely to produce a drastic change in plant communities.  However, small changes in the performance of particular species, especially during early growth, could eventually lead to increased exotic species abundance.