Photosynthetic capacity and its implications for the competitive ability of Ceanothus megacarpus, Ceanothus crassifolius and Rhamnus californica

Background/Question/Methods

The pocketed microclimates of the Santa Monica Mountains contain distinct species types depending on their location and site temperature.  Ceanothus megacarpus is not able to withstand cold canyon temperatures and is extirpated at sites where temperatures reach below -9°C; however, C. megacarpus is commonly the dominant species at warm coastal sites.  Ceanothus crassifolius and Rhamnus californica both can withstand cold canyon temperatures and both appear frequently at sites where C. megacarpus cannot.  At warm sites where C. megacarpus is dominant, we do not find these species.  This informed the hypothesis that a tradeoff may exist between cold tolerance and competitive ability.  To test this, photosynthetic capacity was examined as a surrogate for competitive ability.  Photosynthetic capacity was measured for each species using a Li-Cor 6400XT Portable Gas Exchange Measurement System.  CO₂ assimilation was recorded while exposing plants to a range of light levels (Photosynthetic Photon Flux Densities) and CO₂ values, respectively.  

Results/Conclusions

Photosynthetic performance was found to be significantly different among the three species, but data did not support the hypothesis that a tradeoff exists.  C. crassifolius, a plant with high cold tolerance, had the highest performance based on A_max, gs_max, ETR_max, PhiPsi2 and Vc_max.  However, when leaf specific area (g/m²) was factored into the A_max, C. crassifolius no longer had the highest photosynthetic performance;  R. californica had a higher value than the other species.  R. californica was the most efficient species in leaf carbon investment.