Coastal sage scrub (CSS) is a diverse plant community found in Southern California, and provides critical habitat for several endangered species.  The current extent of CSS is a fraction of its original area, due to land use change over the past century.  The remaining CSS, along with other ecosystems in Southern California, are also impacted by nitrogen (N) deposition from fossil fuel combustion, and shifting precipitation regimes associated with climate change.  In recent years there has been growing concern about whether CSS ecosystems will continue to regenerate due to accelerating fire frequency and invasion by exotic annual grasses following fire.  A greenhouse experiment was designed to evaluate how environmental changes such as N deposition, shifting precipitation and invasion will influence CSS regeneration.  Seedlings of two native shrub species (Artemesia californica & Eriogonum fasciculatum) were planted in 1.5 gallon pots, both with and without exotic annual grasses, along orthogonal gradients of soil N and water availability.  There were eight levels each of N and soil moisture, for a total of 64 treatment combinations, with 3 replicates of each. Germination, growth, and mid-day water potential were collected over three months and analyzed using linear or polynomial regressions, chosen to maximize model fit.

Results/Conclusions

In general, shrub seedlings experienced non-linear responses along the soil moisture gradient.  Germination, in particular, displayed non-linear, threshold responses.  Seeds of both species did not germinate at the lowest two levels of soil moisture, few seeds germinated at the 3rd level of soil moisture, but there was no difference in germination rate between the 4th and 8th levels of soil moisture.  The exotic annual grasses germinated in all treatments but had low survivorship in the lowest two levels of soil moisture. Germination was not influenced by soil N treatments.  Mid-day leaf water potential was highly correlated with soil moisture.  Two months after germination, water potential for both species of shrubs was positively and non-linearly related to soil moisture treatment.  Leaf water potential was consistently lower for shrub seedlings planted in a matrix of exotic annual grasses, and was depressed at the highest level of N availability, when compared with lower levels.  On average, E. fasciculatum had lower fluorescence values than A. californica, possibly caused by the presence of dense leaf hairs.  In sum, it will be important to assess the potential for non-linear responses of native vegetation to present and future environmental changes.