Comparison of Leaf Water Potential, Stomatal Conductance, and Chlorophyll Fluorescence between an Invasive Weed, Schinus molle, and a native Chaparral Shrub, Ceanothus spinosus, in the Santa Monica Mountains

The Peruvian peppertree, Schinus molle, is an invasive species in California that has escaped cultivation to competitively exclude native species in the landscape. One example of such competition is between a dominant member of the chaparral shrub community, greenbark ceanothus (Ceanothus spinosus), and S. molle. The mechanism of competitive displacement of chaparral by S. molle is not understood nor has it been previously studied. We hypothesized that during severe drought, such as occurred during the sumer/fall of 2016, S. molle would have a more favorable water balance then its native competitor C. spinosus, eventually leading to higher survival and species displacement. We tested this hypothesis by comparing leaf water potential, stomatal conductance to water vapor diffusion, and chlorophyll fluorescence of adjacent individuals of each species (n = 6) during severe drought. We found that leaves of S. molle had more favorable water status at predawn (-1.7 MPa versus -8.5 MPa) and at midday (-2.0 MPa versus -7.6 MPa) than C. spinosus. S. molle also had leaves with higher stomatal conductance to water vapor diffusion (27 µmol m-2 s-1 versus 12 µmol m-2 s-1) and higher chlorophyll fluorescence (Fv/Fm of 0.82 versus 0.72) indicating higher photosynthetic performance. Taken together, these data support our initial hypothesis and suggest that competition for water resources during severe drought may favor the invasive weed S. molle at the expense of the native shrub C. spinosus. Furthermore, this infers that increasing dry and hot conditions with climate change in California may increase the threat of weed invasion and vegetation-type conversion