Physiological Performance in Thinned vs. Non-thinned Ceanothus spinosus, Ceanothus megacarpus, and Malosma laurina of the Santa Monica Mountains

In fire prone environments, such as Southern California, native vegetation is often artificially thinned around structures in order to reduce fuel load and protect buildings from fire damage. We tested the hypothesis that thinning of chaparral shrubs increased their physiological performance in comparison to neighboring un-thinned plants. As experimental subjects, we chose three dominate chaparral species in coastal exposures of the Santa Monica Mountains: green bark ceanothus (Ceanothus spinosus), big pod ceanothus (Ceanothus megacarpus) and laurel sumac (Malosma laurina). We compared thinned to un-thinned water status (pre-dawn and mid-day water potential using a Scholander-Hammel pressure chamber), water use rates (stomatal conductance to water vapor diffusion using a field portable gas- exchange system), and photosynthetic rates (also using a gas-exchange system). We also compared the mechanical strength of stems and leaves using a mechanical testing device (Instron 5544A). There was no difference in mechanical strength between treatments of either leaves or stems; however, we found that thinned plants had significantly higher photosynthetic rates (ranging between 5 to 8 μmol CO2 /m2 s), higher stomatal conductance rates (ranging between 43 to 149 mmol H2O / m2 s) , and elevated water potentials (ranging between 1.2 to 2.1 MPa). These significant differences illustrated that the thinned chaparral were in fact performing better than the non-thinned chaparral species; therefore, the effect of thinning chaparral directly correlates with improving the thinned chaparral’s performance, presumably because there is less competition for water