Non-linear responses of native shrubs to invasion and altered rainfall regimes

Background/Question/Methods

Southern California is predicted to be a “hot-spot” for climate change, due to declining precipitation totals as well as more variable precipitation.  Coastal sage scrub is a shrub-dominated plant community in this region, which has been impacted by habitat loss, disturbances such as fire, and subsequent invasion by non-native species, especially exotic annual grasses.  We hypothesize that the response of this vegetation type to future shifts in precipitation regimes will be influenced by interactions between native shrubs and exotic grasses, particularly as shrubs re-establish following disturbance.  To test this we first performed a greenhouse experiment growing seedlings of two native shrubs (Encelia californica & Eriogonum fasciculatum) at eight levels of soil moisture, both with and without competition from an exotic annual grass (Avena fatua).  We subsequently initiated a rainfall manipulation experiment in the field, using rainfall shelters to achieve five levels of rainfall from drought (complete exclusion) to deluge (a doubling of ambient rainfall).  Vegetation at the site is still regenerating from the 2003 Cedar Fire, and we evaluated the growth and physiological response of established adult shrubs, existing exotic grasses, and eight species of native shrubs planted as seedlings into the experimental plots.

Results/Conclusions

The greenhouse experiment revealed that invasion and soil moisture interacted to influence native shrub growth.  Shrub seedling growth was positively correlated with water addition when grown alone, but negatively correlated when shrubs were grown with the highly productive exotic grass.  Exotic grass production increased with water addition, and this response saturated at high levels of water addition, where we documented increased nitrate leaching and leaf transpiration.  In the field experiment, pre-dawn water potentials showed that by the second year the rainfall treatments impacted soil water access similarly for adult and juvenile shrubs, in contrast to our expectation that deeply rooted adult shrubs would be less impacted by our plot-level treatments.  Despite this, physiological performance (photosynthesis, water-use efficiency) of adult shrubs was not influenced by the rainfall treatments. Juvenile shrubs, however, responded non-linearly to changing rainfall, with lower survival, physiological performance, and growth under both drought and deluge conditions. The decline under high rainfall may be partly due to increased growth by exotic grasses, or unmeasured factors such as root pathogens. These results show the potential for regenerating shrub communities to respond non-linearly to climate change, particularly when simultaneously impacted by invasion.