Early development of pure and mixed tree species plantations in Snogeholm, southern Sweden

There is a lack of experimental studies comparing the forest production of mixed tree species stands and monocultures. As a case study, in 1994 an experiment was established in an afforestation landscape in southern Sweden with 66 plots: 18 planted with single tree species (including most native Swedish trees, plus Populus and Larix hybrids) and 48 with various non-replicated mixtures. Fifteen years after planting, stand growth did not differ significantly between the mixtures and the monocultures. However, the monocultures did exhibit a higher variation in growth levels relative to the moderate but comparatively consistent growth levels exhibited by the mixtures; particularly those comprised of more than two species. A specific analysis of mixtures containing either the tree species Picea abies or Quercus robur and their corresponding monocultures demonstrated the importance of the tree species admixed as a determinant of production outcomes early in the rotation. In the case of P. abies, tree species mixtures tended to reduce production, whereas in the case of Q. robur, mixtures tended to increase production. In addition no consistent differences in the mean height and mean diameter growth of P. abies and Q. robur between mixtures and monocultures were detected. A major conclusion is that adding more than two species did not increase stand volume growth.