Drought Tolerance in Quercus douglasii in the California Mediterranean Savanna: A study of photosynthetic functional responses, limitations, and changes during annual seasonal drought
Quercus douglasii, blue oak, is endemic to the California mediterranean-type climate region characterized by annual summer drought. Because blue oak is winter deciduous, an understanding of the strategy for surviving under current climate conditions is vital to improving the predictions for survival and adaptation to predicted climate change. This is especially important because the region may experience strengthened climate-change type drought, a decrease in precipitation accompanied by higher temperatures. As such, this dissertation was focused on studying the ecophysiology of Quercus douglasii by intense measurements of the seasonal trends in leaf structure and function, photosynthetic functional response to three environmental variables (PAR, CO2, and temperature), leaf water potentials, and the limitation imposed on photosynthetic carbon fixation by stomatal and internal (mesophyll) conductance. The study was realized in situ in a grazed oak-grass savanna near Ione, CA, USA, in multiple years and on multiple trees of different water use strategies (i.e. both isohydric and anisohydric).
Instantaneous gas-exchange and leaf-fluorescence was measured with the Li-Cor 6400-40 in order to estimate photosynthetic capacity via photosynthetic response curves to CO2, mitochondrial respiration in the dark during the day (Rd) via photosynthetic light response curves, as well as instantaneous maximum rates of photosynthesis, stomatal conductance, and transpiration. Additionally, by combining gas-exchange and fluorescence measurements, I calculated internal conductance (the conductance of CO2 from the intercellular airspaces to the site of carboxylation, also referred to as mesophyll conductance) employing the `variable J' method. In order to estimate the relative contribution of electron transport to each photosynthesis and alternative electron sinks, I conducted photosynthetic response curves under 2% O2. I also developed and utilized a novel method of performing photosynthetic response curves to leaf temperature in in situ. Finally, I routinely measured leaf water potential in the mid-day and pre-dawn, leaf mass, area, nitrogen content, chlorophyll content, and leaf absorptance.
The results from this study indicate that the majority of photosynthetic carbon fixation occurs during the spring when photosynthetic capacity of blue oaks is exceptionally high (Vcmax = 118 μmol m-2 s-1) and prior to the extreme drop of mid-day leaf water potential brought on by the onset of the summer drought. During this peak period, stomatal and internal conductances also peaked and the negative response of photosynthesis to increased leaf temperatures was minimal especially in the isohydric individual. During the summer drought when trees increasingly rely on the water table, mid-day leaf water potentials reached -5.1 MPa. While values of photosynthesis and conductance were low in all individuals, the isohydric individual displayed less suppression. In the anisohydric species, values approached zero toward day of year 200 and photosynthesis decreased nearly linearly with an increase in temperature and leaf-to-air vapor pressure difference, approaching 0 at 40°C. In all individuals in all years, Rd normalized to 25°C decreased exponentially from an initial value around 3 μmol m-2 s-1 throughout the season.
While photosynthetic activity, transpiration, and stomatal and internal conductances decreased throughout the summer drought, leaf absorptance and chlorophyll content tended to reach a plateau rather than a peak. Additionally, except in 2007 when the drought was exceptionally strong, leaf nitrogen on an area basis did not continually decrease. Similarly, temperature-normalized rates of electron transport tended to remain constant throughout the summer drought despite decreased photosynthetic demand for electron transport. Additionally, the ratio of electron transport to photosynthesis increased exponentially with temperature. This information indicated that despite reduction of photosynthesis during the summer drought, the leaves did not translocate resources, but rather acted to conserve water through minimized stomatal conductance and maintained the ability to process the high levels of incoming radiation at the site. Additionally, as temperature increased and PAR exceeded demand, alternative electron sinks were increasingly utilized to avoid photo-oxidation of the leaf. The presence of alternative electron sinks was emphasized by the offset of the linear regression between electron transport required for photosynthetic activity (Ja) and the actual electron transport rate measured by chlorophyll fluorescence (Jf), with Jf having a value of 30 μmol m-2 s-1 in the absence of photosynthesis and photorespiration.
This research has provided an increased understanding of the strategy for survival of blue oak during summer drought by demonstrating the strong peak of photosynthetic activity during the spring, the maintained capacity to process radiation beyond that required for photosynthesis, and the presence of alternative electron sinks to avoid photo-oxidation of the leaf especially during the drought. Additionally, it describes a protocol developed to perform photosynthetic temperature response curves in situ in drought conditions which can be easily implemented at other field sites. Finally, the seasonal trend of internal conductance and limitations in measuring internal conductance under extreme drought, are described. The information provided is valuable to ecosystem modelers because it improves estimates of photosynthetic capacity often used in models through the incorporation of internal conductance. Finally, the understanding of the seasonal plasticity in photosynthetic response to environmental conditions and capacity for tolerating high temperatures and incoming PAR can be utilized to improve predictions of the capacity of blue oaks to adapt and survive under predicted future climate scenarios.