Expansion dynamics of introduced Pinus halepensis Miller plantations in an oceanic island (La Gomera, Canary Islands)

Abstract

Aleppo pine (Pinus halepensis Miller) has been one of the most used Pinus species in reforestation in the Mediterranean Basin and in many other regions outside its native range. Its massive use in plantations combined with its low environmental requirements, relatively short juvenile period and adaptability to human disturbance make P. halepensis a great competitor for local species, becoming invasive in many natural and anthropized areas. As little is known about the effects of these plantations on oceanic islands ecosystems, we evaluated in this study the expansion dynamics of a series of P. halepensis plantations carried out on the island of La Gomera (Canary Islands) in the 1960s and 1970s of the past century. At short distance, we counted and measured P. halepensis individuals along transects of 140 m from the inside of the plantation to the outside, assessing also the proportion occupied by other native, endemic and ruderal species in two habitat types: thermo-sclerophyllous woodlands and laurel forests. To examine seed dispersal over longer distances, we selected one of the areas that showed a greater expansion and we analyzed factors influencing regeneration. Our results revealed that expansion is occurring mostly in open or disturbed areas instead of closed-canopy humid forests and we found a clear negative relationship between the presence of the species and the abundance of other trees or the endemic and native flora. Furthermore, we detected potential new foci of regeneration away from the plantations too. Overall, we suggest that the status of P. halepensis as a non-native species and its classification for the purposes of management in the Canary Islands needs to be re-examined.