Invasion of the exotic Acacia longifolia into coastal dunes causes ecological problems in Portugal. A. longifolia exhibits properties characteristic of woody invaders (high growth rates, profligate seed production, symbiotic N-fixation) and follows a water spender strategy. Most studies regarding A. longifolia invasion in Portugal have been conducted in primary dunes; however A. longifolia also invades secondary dune forests. Our objective was therefore to evaluate effects of A. longifolia invasion on community structure and the water cycle in a Pinus pinaster forest. We established a 1000m² study area encompassing both invaded and uninvaded areas. The impact of invasion on forest structure was determined by dividing the study area into 10m² plots and measuring structural parameters of all plants within one-third of the plots. Hemispherical photos were also taken to quantify canopy cover, light transmission and leaf area index. Invasion effects on regeneration were studied in 0.04m² subplots in which all seedlings were counted, identified by species and classified into height classes. The impact of A. longifolia on the water cycle was evaluated by continuously measuring xylem sap flow in invaded P. pinaster trees, uninvaded P. pinaster trees and the understory A. longifolia.

Results/Conclusions

A. longifolia altered forest structure by creating a new understory layer, small trees (between 1.5m and 5m tall), which was an empty stratum in the uninvaded forest. Furthermore, seedlings of the invader were more prevalent than those of native species. A. longifolia seedling density was greater and the range of height classes was broader the closer that a plot was to an adult A. longifolia while the opposite was true for native seedlings – indicating that A. longifolia inhibited seedlings of native species while facilitating its own regeneration. A. longifolia presence increased leaf area index and nitrogen content of leaf litter while decreasing canopy openness and transmitted light at 1.3m. On average, A. longifolia contributed 30% to the total water flux of the invaded area. Although total water use on a ground area basis was similar between invaded and uninvaded subareas, the sap flow of the invaded P. pinaster was 26% reduced relative to the uninvaded P. pinaster. Furthermore, when soil water content was very limiting (<5%) the difference in sap flow between the invaded and uninvaded P. pinaster correlated linearly with A. longifolia sap flow rates. In conclusion, A. longifolia invasion into a P. pinaster forest altered forest structure and decreased the water available to the native trees.