Tracing Seasonal Changes in Water Use of an Invasive Acacia and a Native Pine in Southern Portugal by Measurement of Sap Flow

Invasion of the exotic Acacia longifolia in Portugal has serious ecosystem level impacts. In this paper we test the hypothesis that the invasion of A. longifolia into seasonally water limited southern Portugal significantly affects the ecosystem water balance by decreasing the water available for native species. From October 2007 through July 2008 we monitored sap flow using Granier-type sensors in three groups of trees in a secondary dunes pine forest in southern Portugal: (1) overstory Pinus pinaster trees with an A. longifolia invaded understory, (2) P. pinaster trees without an A. longifolia understory and (3) the understory A. longifolia trees. Averaged over the entire study, stand water use on a ground area basis, E-T, was reduced by 26% in the invaded as compared to non-invaded P. pinaster trees. Furthermore, A. longifolia water use contributed 31% on average and up to 43% of total E-T in the invaded area. Total E-T was 6.5% greater in the invaded than non-invaded area. A detailed examination of daily sapwood-related sap flux density, Q(S), during a rain pulse, dry-down event in the fall of 2007 provided further evidence that differences in sap flux density between the non-invaded and invaded P. pinaster populations were directly affected by A. longifolia water use. Hence, the invasion of A. longifolia significantly impacts the water cycle in a pine forest in southern Portugal.