Afforestation with Pinus halepensis reduces native shrub performance in a Mediterranean semiarid area

It has been often suggested that the introduction of Pinus halepensis with afforestation could facilitate long-term colonisation and development of late-successional species in Mediterranean dry and semiarid areas, but this affirmation has rarely been experimentally tested in the field. In this study, we analyse the effects of planted R halepensis density (0, 1500 and 3470 trees-ha(-1)) on soil moisture at two depths (0-10 and 10-30 cm), and on the physiological status of three shrub species (Quercus coccifera, Erica multiflora and Rhamnus lycioides) in a semiarid area of SE Spain. Afforestation with R halepensis had a negative effect on soil moisture, an effect that increased with tree density. This effect was more pronounced at 0-10 cm depth than at 10-30 cm depth, and was especially evident after the spring and late summer rainfalls. During spring and summer, mean predawn water potentials in the area without P. halepensis were -1.5, -2.4 and -2.2 MPa for Q. coccifera, E. multiflora, and R. lycioides, respectively. Mean net photosynthesis values in the same area and period were 1.2, 1.0 and L5 mumol CO2 m(-2) s(-1) for Q. coccifera, E. multiflora, and R. lycioides, respectively. Similarly, mean stomatal conductance was 15.1, 16.4 and 21.7 mmol H2O m(-2) s(-1) for Q. coccifera, E. multiflora, and R. lycioides, respectively. Values were substantially reduced (up to 56%) with increasing P. halepensis density, suggesting the presence of a strong competition for water. Our results provide evidence that the introduction of R halepensis with afforestation has a negative effect on existing late-successional shrubs, and suggest that this introduction does not stimulate successional processes in Mediterranean semiarid areas. (C) 2003 Elsevier SAS. All rights reserved.