Nitrogen deposition and early growth of native and exotic grasses of the California oak savanna

Background/Question/Methods

Nitrogen deposition in the California oak savanna is spatially heterogeneous at local and regional scales.  Hotspots exist beneath solitary oaks within a site and near N emission sources.  This heterogeneity may influence spatial patterns of competitive interactions between native and exotic species.  Nitrogen deposition is also temporally heterogeneous.   A seasonal flush of deposition coincides with early growth in most grasses, and, consequently, plant community response may be driven by changes in performance at the germination and seedling phases.   To tease apart the influence of N deposition relative to other critical factors in the system, a greenhouse experiment was conducted to test the effects of N fertilization, soil type, and light on native-exotic communities during early growth.  A common native perennial, a naturalized exotic annual, and an invasive exotic annual grass species were used.  Soil was collected from 3 oak savanna sites that represented the range of background fertility and N deposition across a regional gradient.  Pots were fertilized with N to simulate the gradient of deposition and a shading treatment simulated oak understory light levels.  Percent germination success was measured after one month and percent seedling establishment was measured after two months for each pot and analyzed with ANOVA.

Results/Conclusions

Of the main treatments, N fertilization had no effect, partial shade and high fertility soils had significant positive effects, and species composition had significant effects on germination and seedling establishment.  For the species composition treatment, native species growing alone performed significantly better than when grown in combination, whereas the two exotic species performed significantly better when grown with at least one other species.  Nitrogen fertilization did have a significant positive effect when interacting with species composition.  Specifically, high N fertilization increased invasive species’ seedling establishment when growing alone, but had no effect when growing with neighbors.  The lack of an experiment-wide significant effect of N fertilization indicates that deposition levels in oak savannas are not high enough to affect most plants.  Differences in soil type, light, and community composition appear to have a much stronger effect on early growth compared to N deposition.  However, the sole N treatment effect on our invasive species suggests plant communities may be altered by these chronic, relatively low levels of N input.  A longer term experiment that includes the entire life cycle of these grasses will help to further elucidate effects of N deposition on oak savanna plant communities across local and regional scales.