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 P. 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 P. 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 1.5 μmol 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 P. 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.