Plasticity in preferential nitrogen uptake by perennial woody shrubs

Coastal sage scrub is a threatened and highly fragmented plant community that is habitat for over 90 rare, threatened, or endangered taxa. Communities of long-lived shrubs persist for long time periods in a nutrient and water-limited environment. Previous work has shown that adult shrubs exist in spatially clustered patterns, raising questions of coexistence vs. competition for limiting nutrients. We examined two sites at Santa Margarita Ecological Reserve, Riverside County, CA. We estimated δ¹⁵N ratios in leaf materials from several species of native and exotic plants. We also estimated soil organic matter content and the concentrations of nitrogen (N) in NO₃^-, NH₄⁺, and organic pools. δ¹⁵N ratios ranged from -2.09 to 1.80 ‰ and were site-specific. Eriogonum fasciculatum had the highest δ¹⁵N ratio (average 0.41 ‰), and Gutierrezia californica had the lowest (average -1.31 ‰). Salvia apiana and Artemisia californica were not significantly different in their δ¹⁵N ratios. All δ¹⁵N ratios were higher at site 2 than site 1, but A. californica showed a significantly larger difference between sites than other species. Site 2 showed lower levels of NO₃^- (9.40 μg/g), higher levels of NH₄⁺ (3.93 μg/g), and a lower NO₃^-/NH₄⁺ ratio (2.91) than site 1 (12.98 μg/g NO₃^-, 1.39 μg/g NH₄⁺, and 12.21 NO₃^-/NH₄⁺). No differences were detected in organic or total inorganic N pool sizes. Organic N does not appear to be an important resource for direct uptake of N in this system, which may increase competition for inorganic forms. The change in A. californica leaf δ¹⁵N ratios between sites implies that A. californica is more plastic in its ability to acquire N than other species in this community, and is able to access a wider range of N resources. Further work is underway to examine interactions with the soil microbial community, and explore ecosystem impacts of invasive plants.