Seasonal Trends in Photosynthesis and Mesophyll Conductance in a Mediterranean Oak Savanna

Understanding how and why leaves respond to extreme temperature and drought stress is an important aspect of modeling carbon uptake in a Mediterranean oak savanna. We made leaf-level measurements of CO2 (ACi) and light response (APar) curves as well as measurements of mesophyll conductance (gm), leaf nitrogen (NL), and specific leaf area (SLA) on blue oak (Quercus douglasii) leaves throughout the growing season during two consecutive years. From the ACi curves, photosynthetic capacity was calculated as well as gm using a model developed by Ethier and Livingston (2004). The modeled values were then compared to those measured in the field. In both years we found the same Vcmax trend with a peak in the early growing season, before the drought begins and after light availability is sufficient. It has been hypothesized that seasonal variance in Vcmax is actually an artifact of seasonal variance in gm. We, however, found a seasonal trend in both Vcmax and gm. This is supported by our data showing a trend in NL parallel to that of Vcmax. Mesophyll conductance was comparable to stomatal conductance (gs), ranging from 0.03 to 0.26 and 0.03 to 0.14 mol m-2 s-1 respectively. Additionally, both conductances followed the same seasonal trend. This research shows the importance of including mesophyll conductance in models of carbon uptake, as mesophyll conductance is often just as limiting as stomatal conductance to photosynthetic carbon uptake in a Mediterranean oak savanna.