Oak-grass savanna ecosystems are among the most complex ecosystems to be studied by biometeorologists. They are horizontally and vertically heterogeneous, they experience summer water deficits and extreme air temperatures, and they rely on a multiple plant functional approaches to acquire carbon and manage water loss. Consequently, this ecosystem is an analogs for studying how the carbon metabolism and energy partitioning of ecosystems will respond to environmental perturbations.

We report on a field study being conducted near Ione, CA, in the foothills of the Sierra Nevada Mountains. The predominant tree species is blue oak (Quercus douglasii). The stand is 7 m tall and its leaf area index is about 0.60. The eddy covariance measurements of the flux densities of CO2 and water vapor were made under and over the forest stand using a sonic anemometer and open-path infrared gas analyzer. Physiological capacity, sap flow and soil-root respiration were measured to evaluate fluxes associated with constituent compartments.

Canopy scale carbon exchange of the oak grass savanna experienced much seasonality, and is out-of-phase with the majority of temperate forest stands. The understorey grassland was green and photosynthesizing during the winter, when the forest was dormant and deciduous. Frost events and grazing limited photosynthesis of the grass understorey, but a supply of soil moisture and nutrients, by winter rains and the cows, promoted respiration. Leaf out of the oak woodland occurred in late March. Peak rates of photosynthesis lasted only a month. Then stomata progressively closed and photosynthetic capacity diminished, in response to soil moisture deficits, as low as –70 bars, and rising air temperatures, exceeding 40 oC. By June the grass understorey was dead, but soil respiration persisted under the trees due to a supply of carbohydrates from the trees, and a conserved amount of soil moisture.