Evaluating the effect of climate change on population demographics of Ceanothus verrucosus

Background/Question/Methods

C. verrucosus, a shrub species restricted to the coastal chaparral and sage scrub of southern California, is considered vulnerable to both very long and short fire intervals.  Fire kills adult plants and the population re-establishes from soil seed banks which are not restocked to replacement levels for several decades after fire.  In addition to shifts to the species’ distribution, climate change poses risks to this species due to both potential effects on vital rates and increases in fire frequency.  We consider two scenarios evaluated under the U.S. Climate Sciences Program.  One predicts a warmer and wetter climate while the other a warmer and drier climate for southern California.  Under both scenarios the probability of extreme wet and dry events increase in the future and the probability of fire could increase as well.  We use a spatially explicit stochastic matrix model to evaluate the direct effects of changes in temperature and precipitation on population dynamics by considering likely changes in the means and variances of vital rates and fire frequencies.  
Results/Conclusions

Our model is age structured with a two year time step and includes seeds for a total of 48 stages.  Sensitivity analysis was conducted by perturbing mean vital rates by plus or minus 10 percent and showed that the model is sensitive to changes in the survival rates of individuals up to 25 years old.  As was expected increasing the variance increases the risk of extinction.  Based on data from other obligate post-fire seeding Ceanothus species, C. verrucosus is expected to experience relatively high mortality from predation and drought stress in its first few years and then density dependent thinning through 15 to 20 years.  After this time, based on ring counts from dead standing C. verrucosus, mortality drops to very low levels.  Changes in vital rates resulting from predicted changes in mean temperature and precipitation and their variances are expected to disproportionately affect survival of the younger plants competing for moisture where the model is most sensitive.  Our model shows that for a warmer drier climate with increased variance the risk of extinction increases.  For a warmer wetter climate the situation is less clear because it is expected that vital rates will be indirectly affected by increased competition from plants already in the system and plants whose ranges may shift as a consequence of climate change.