Due to its heterogeneity and complexity, oak-grass savanna ecosystems are poorly understood by biometeorologists, in term of energy and water fluxes and their response to environmental perturbations. Here we present the results from a study on seasonal variation of energy and water vapor fluxes of an oak-grass savanna and a nearby grazed grassland under the conditions of severe water stress and extreme high temperature. The experiment site is located on the foothills of the Sierra Nevada Mountains in California. The site has a Mediterranean climate with wet, cold winter and dry, hot summer. The predominant tree species is blue oak (Quercus douglasii) and grasses of understory are C₃ species. Latent heat (LE) and sensible heat (H) over- and understory of the oak/grass savanna and over the grazed grassland were measured using eddy covariance technique. Other meteorological, soil and plant physiological parameters, including solar radiation, net radiation (R_n), PAR, air temperature, soil moisture, predawn water potential and stomatal conductance, were also measured at different time intervals.

It was found that seasonal variations in LE over the grazed grassland and understory of oak-grass savanna followed changes in grass leaf area index (LAI), which in turn was controlled by soil moisture content. This response is due to the shallow rooting depth of grass. During the dry summer, when the grass was dead, virtually all the available energy was partitioned into H.

Seasonal changes in oak tree transpiration were obtained by subtracting overstory LE from understory LE. Transpiration decreased exponentially with time from the beginning of the measurement period (d127) and it did not follow the seasonal variation of tree LAI. Instead, stomata manage water loss by responding to severe water stress and high temperature during the summer; the leaf stomatal conductance decreased from 0.5 mol m^-2s^-1 in the early summer to less than 0.05 mol m^-2s^-1 in the fall when the predawn leaf water potential reached –7.0 MPa.

Net radiation balance (R_n) over the grazed grassland peaked (around 600 W m^-2) as early, at the end of April, just before the grass senescence. While for the oak-grass savanna, Rn peaked (around 700 W m^-2) more or less around summer solstice. More than one-third of the total energy exchange was attributed to the understory throughout the season. Since a substantial amount of energy exchange occurs under the oak tree, multi- or two-layer models are needed as a tool to estimate energy and water vapor fluxes from this open savanna ecosystem.