An in-depth study of the seasonal trends in photosynthesis in Q. douglasii in a Mediterranean savanna

Background/Question/Methods The Californian Mediterranean savannas are characterized by severe drought during the summer. However, this is the period of foliation for Quercus douglasii (Blue oak). This project aims to understand how Blue oaks are able to remain photosynthetically active during the dry, hot summer conditions. We specifically study if and how the response of photosynthesis to temperature changes throughout the onset and duration of the drought. Photosynthetic response curves to light, CO2, and temperature were measured regularly throughout the growing season. Additionally, the electron transport rate (ETR), leaf nitrogen content (NL), leaf chlorophyll content , leaf absorptance (α), and leaf water potential were measured. All measurements except NL and α were done in situ. Photosynthetic measurements were done with a Li-Cor 6400-40 (Lincoln Nebraska). An integrating sphere was used to measure α, and a Minolta SPAD-502 was used to measure chlorophyll content.

Results/Conclusions We found a strong seasonal trend in Vcmax, with a strong peak in the spring. We also found a linear response in photosynthesis to temperature. Although the magnitude of photosynthesis values changed, the slope is nearly constant throughout the summer drought. The response, however, was not uniform on all trees measured. In addition, stem water potential was measured. Our site experienced water potentials down to -5.5 MPa. However, the difference between pre-dawn and mid-day water potential decreases throughout the drought showing that Q. douglasii is acting to conserve water. The strong response to temperature shows that some trees are capable of increasing productivity during periods of optimal temperature despite water stress (such as in the morning period). This also helps to explain how the blue oak can maintain a positive carbon budget throughout the summer drought.