WATER-BALANCE OF EUCALYPTUS-GLOBULUS AND QUERCUS-SUBER FOREST STANDS IN SOUTH PORTUGAL

The pulp industry is one of the major export industries of Portugal at present. Eucalyptus globulus is widely planted for this purpose. Nevertheless, the use of Eucalyptus is confronted with strong objections because of possible environmental effects. The effect on soil moisture appears to be particularly relevant in the southern and interior semi-arid zones of the country. The study utilized two watersheds near the town of Odemira in Alentejo, a southern province of Portugal. The region has a relatively humid climate because of its proximity to the sea. One 6-ha watershed contained scattered native cork oak (Quercus suber) with a newly developing understory of shrubs. An adjacent 19-ha watershed contained a mature Eucalyptus globulus stand, which was harvested for the study. The study included measurements of precipitation, runoff and soil water storage. Measurements of the tree stands and canopies were included for the characterization of throughfall and stemflow. The results demonstrated that the summer-dry climate depleted soil moisture to such a degree that wintertime recharge of the cork-oak watershed generated only one percent of annual rainfall into runoff. Annual rainfall increased during the study period, confounding wetter soil conditions with the effect of Eucalyptus harvesting. Canopy interception by the mature Eucalyptus stand was 16.8%, and stemflow was 2.7% of annual rainfall. Stemflow from the scattered oak trees was 1.3% of annual rainfall, but canopy interception could not be evaluated and was estimated at 4.2%. Interception by the shrub vegetation in the cork oak watershed was not measured. measurements showed no difference in monthly evapotranspiration (E(t)) between the two vegetation types during the pre-treatment period. However, the 11 mm month-1 reduction of E(t) lasted only for three months. After this period the regenerating Eucalyptus coppice matched water use of the cork-oak stand and appeared to exceed this by 4 mm month-1 at the end of the drought cycle. Harvesting increased first-year runoff only by 4 mm and second-year runoff by 36 mm after soil water recharge by