Past studies of how plants respond to increases in atmospheric CO2 indicate decreases in stomatal density and increases in stomatal size; together modifying stomatal conductance such that WUE increased. This hopeful physiological result is expected to significantly buffer ecosystem water use efficiency in response to a predicted warmer, and for the California Floristic Province (CFP), potentially drier climate. Here we test whether changes in CO2 since 1900 have affected species that dominate California chaparral, an extensive vegetation that extends throughout much of the CFP. We sampled 5 species each from two genera, Arctostaphylos and Ceanothus, with half the species from coastal maritime zones and half from inland regions. Density and sizes of stomata along with 13C of bulk leaf tissue were measured from herbarium specimens with collections from every decade since 1900. Using these data, we calculated stomatal conductance, WUE, and potential rates of photosynthesis. We correlated these measurements against changes in CO2, as well as temperature, precipitation and PDSI during the growing season for each leaf measured.
Results/Conclusions
We found significant morphological differences between coastal and inland taxa, but contrary to our expectations, little change in stomatal traits over the last century suggesting that responses to CO2 increases were lacking. Almost no regressions indicated significant responses to changes in CO2, although there were some trends. GLM models that included climatic variables as well indicated shifts in stomatal density or size were more frequently correlated with variation in either seasonal precipitation or PDSI, sometimes with temperature. Already having highly reduced stomatal conductance, these species do not appear to be responding to continued increases in CO2 over the last century. These plants live in environments that generally lack summer rain for months at a time; predicted changes in climate suggest higher summer temperatures and longer rainless periods, although total rainfall may not change much. Given their lack of response to CO2 and greater sensitivity to climatic variables, climate change may have a much greater impact on this vegetation that earlier work would suggest.