Mechanisms underlying the interaction between Pinus halepensis and the native late-successional shrub Pistacia lentiscus in a semi-arid plantation

Pinus halepensis has been extensively planted in semi-arid areas throughout the world. This has often led to slow-growth stands that: a) suffer from insect plagues, b) promote nutrient depletion and c) fail to promote the recovery of native vegetation. The introduction of native late-successional shrubs in these stands could stimulate successional processes, improve soil conditions and enhance their resilience against disturbances. The main objective of this study was to evaluate the mechanisms underlying the interaction between Pinus and the native late-successional shrub Pistacia lentiscus in a semi-arid plantation. By using manipulative field and laboratory experiments, we evaluated direct (competition for soil resources and allelopathic effects) and indirect (competition with herbaceous understorey) interactions between Pinus and Pistacia. We found no effect of Pinus litter and root exudates on Pistacia growth. In the field, Pistacia seedlings planted under the canopy of Pinus showed higher survival than those planted in open areas with sparse vegetation. Girdling of Pinus trees did not affect the performance of planted Pistacia seedlings, but suppression of the herbaceous understorey significantly enhanced both survival and physiological status of Pistacia seedlings planted under the canopy of Pinus. The magnitude of the interference by herbaceous understorey was considerably higher than that by Pinus. Our results provide evidence that a negative indirect interaction between Pinus and Pistacia, mediated by the herbaceous understorey, is taking place in the afforestation studied, and can help to explain the low rates of colonisation of late-successional woody shrubs typically observed in semi-arid Pinus halepensis plantations.