Background: There is growing interest in mixed-species forests but a lack of studies that analyse them for regeneration phases or any stage other than mature stands. Information is scarce about relatively unproductive species such as Pinus pinaster and Pinus halepensis in Mediterranean ecosystems. The objective of this study was to investigate inter- and intra-specific interactions of both species in different tree densities during the first years of establishment. Five Nelder wheel plots were planted to test densities ranging from 1000 to 80000 seedlings/ha and simulate establishment sub-processes at high densities. Pinus pinaster and Pinus halepensis were mixed along the spokes, to obtain three mixture levels in which 100%, 80% or 60% of the seedlings were of the same species. Cox proportional-hazards models and binomial logistic regressions were fitted to analyse seedling survival. Early growth (basal diameter and height at one and four years after plantation) was analysed by fitting linear mixed-effects models.
Results: Pinus halepensis showed higher survival rates and basal diameter increments but more time is needed to know how Pinus pinaster responds to density and mixture.
Conclusions: Both competitive and facilitating seedling interactions were observed at higher densities, which facilitate seedling survival but decrease early growth. Pinus halepensis showed higher survival rates and basal diameter increments but more time is necessary to determine Pinus pinaster response to density and mixture.