Anthropogenic nitrogen deposition can have harmful effects on native plant species, and beneficial effects on non-native invasive plant species. Stipa pulchra, a dominant native species in Californian grassland communities, appears to have been extirpated from many lowland habitats but is more often found on moderate slopes. Stipa’s extirpation may be explained by increased levels of nitrogen in lowland areas. During rainfall events, water carries nitrogen in runoff following topographical patterns from uphill areas to lowland areas where nitrogen and water can accumulate. Nitrogen can limit Stipa either directly through a negative effect on growth, survival, and reproduction, or indirectly by increasing the competitive effect of invasive species. Here, we examine the direct effects of topography and nitrogen on Stipa growth, survival, and recruitment. The objectives of this study were to 1) analyze the differences in soil moisture and soil nitrogen created by a topographical gradient and 2) to determine the areas within the topographical gradient that are the most suitable for Stipa persistence. We established 36 1.5m x 1.5m plots in three different slope classes (steep 22-32o, medium 10-16o, and flat 0-10o), and replicated three nitrogen treatments (addition 2.3g N/m2/yr, removal 68g C/m2, and ambient). Five seedlings of Stipa were planted in each plot. Soil moisture, soil nitrogen content, plant biomass, growth, and leaf water status were measured.
Results/Conclusions
Results show that topography can constrain the growth of Stipa through its effects on water and nitrogen availability. Flat slope plots contained significantly more NH4+ and had consistently higher soil moisture than steep slope plots. However, Stipa growth rates were the lowest in flat slope plots compared to both medium and steep slopes. According to predawn leaf water potentials, Stipa was also the most stressed in flat slope plots compared to medium and steep slopes. Plant biomass was significantly the largest at medium slopes under ambient nitrogen conditions, compared to all other treatment combinations. These results show that flat slopes should be the most favorable habitat for plant growth because they contain the highest amount of NH4+ and soil moisture. However, these areas were found to be the least conducive to Stipa growth, whereas the medium slope appeared to be the most beneficial to Stipa. Therefore, nitrogen and water availability alone cannot explain Stipa’s current distribution pattern. Our ongoing field and greenhouse experiments will explore these patterns further.