SAP FLOW IN CORK OAK TREES AT TWO CONTRASTING SITES IN PORTUGAL

Abstract

The water use of cork oaks (Quercus suber L.) was studied in two Portuguese oak woodland (montado) sites that differed in soil quality and depth and in stand structure. The thermal dissipation technique was employed on a total number of 26 trees continuously over a period of twelve months. The results indicate that the sap flux density, Js, in the outer sapwood (0–2 cm depth) showed no statistically significant trends with stem diameter, and supplementary data for Js in 2–4 cm sapwood depth seem to back this. Trees from a less dense stand on poorer shallow soil did not differ in daily values of Js compared to trees with similar stem diameter from a nearby, denser stand on less poor, less shallow soil. Sap flow in the outer sapwood (Q) reached highest values > 100 kg d -1 for a dominant tree with a stem diameter of 50 cm. Generally, daily and half-hourly maxima of Js and Q were higher in spring and lower at the peak of the summer drought. Reductions in Js or Q in summer compared to spring were similar at the two sites. OBJECTIVE Cork oaks (Quercus suber L.) dominate ca. 60% of the Portuguese broadleaved evergreen oak woodlands (“montado” in Portuguese). These typical and economically important anthropogenic agroforestry-ecosystems of the Iberian Peninsula cover 33% of the forest area of Portugal. Montados are characterised by a savannah-like physiognomy, with a relatively sparse tree layer mainly composed of evergreen cork oak and Portuguese holm oak (Q. rotundifolia Lam.) and an herbaceous layer of cultivated cereals or fodder for livestock (Pereira et al., 2007). One objective of the study was to quantify tree water use along gradients in site quality and stand structure, as a prerequisite to interpolate canopy transpiration to stand and landscape levels. Therefore trees from two stands were selected for xylem sap flow measurements from a large number of stands in the area, representing extremes regarding stand structure and edaphic site conditions. Here we compare sap flux density (and sap flow) in individual cork oak trees and its relation to easily accessible scalars of tree size and stand structure, and follow this through the summer to determine whether drought affects the trees differently at the two sites.