Sapling recruitment in mixed stands in the Northern Plateau of Spain: a patch model approach

Abstract

Our modelling approach shows the predominance of the facilitative over the competition effects in controlling the regeneration of the studied species, thus pointing out to intra- and inter-specific coexistence mechanisms. Natural regeneration is a main issue in the management of continental Mediterranean forests. Ensuring natural forest regeneration is therefore vital for preserving ecosystem function under climate warming and increased frequency and intensity of extreme droughts. In this study, we evaluated intraspecific and inter-specific relationships between saplings and adult trees in mixed Pinus pinea-Quercus ilex-Juniperus stands in the Spanish Northern Plateau. Data were gathered from five 50 m × 60 m plots that covered different stand densities. Adult trees, saplings, and stumps were positioned, and height and diameter were measured. We developed a patch model to predict the probability of occurrence and abundance of saplings of the three species in 2.5 m × 2.5 m patches using a zero-inflated model. We considered as predictive variables abundance of adult trees, stumps, and saplings of each species, both in a patch and in its adjacent patches. Saplings of the three species showed a cluster pattern. The abundance of saplings of the three species was positively related with the current or past presence of their conspecific adult trees. The presence of adult trees of Pinus pinea plays a main role in both Quercus ilex and Juniperus dissemination. Our modelling approach shows the predominance of the facilitative effects over the competition effects in controlling the regeneration of the three species, pointing out to intra- and inter-specific coexistence mechanisms. The simulations suggested that to perpetuate the mixture, it is necessary to apply intense release cuttings in Pinus pinea trees once seedlings have been successfully established.