Xylem Sap Tension in Three Woodland Oaks of Central California

In mid—August bottomland Quercus agrifolia and Q. lobata trees had low xylem sap tensions, as estimated by a pressure chamber. After a very dry winter, pre—drawn minimum tensions were less than 6 atm, suggesting that these rates were rooted to a water table. Widely spaced Q. lobata trees in upland savannas also had low pre—dawn tensions, presumably due to the lack of woody plant competition. In contrast, all strands of Q. douglasii had high pre—dawn tensions even after a very wet winter. They seem well adapted to dry slopes where the water table is usually unavailable. Quercus douglasii trees appear to drop their leaves when the water supply becomes critical. Trees in some north aspect stands had higher tensions in mid—August than trees in south aspect stands due to soil moisture supply and tree density interactions. During the dry season a size—class effect appeared with tension inversely correlated with crown size and probably root depth.