Influence of patch scarification and insect herbivory on growth and survival in Fagus sylvatica L., Picea abies L. Karst. and Quercus robur L. seedlings following a Norway spruce forest

Abstract

Southern Sweden is dominated by relatively uniform coniferous forests where Norway spruce is the most common tree species. Ecological and economic considerations have created an interest in re-establishing broadleaves in these forests. To study the influence of patch scarification and compare the influence of insect herbivory on growth and survival in beech ( Fagus sylvatica L.), Norway spruce ( Picea abies L. Karst.) and oak ( Quercus robur L.) seedlings, a field experiment was established. The experiment was carried out on a new clear-cut following Norway spruce, and in a Norway spruce shelterwood. Seedlings were spring-planted in two consecutive years in untreated ground (control) and in patch site preparation treatment in combination with or without repeated insecticide treatment. The influence of patches on seedling growth and survival of all species studied was low. No pronounced differences in photosynthetic photon flux density at seedling level and recolonization of ground vegetation were observed between patch scarification and control. Patches had higher soil water potentials than the control in the shelterwood, but this did not result in any positive growth response in the seedlings. The number of trapped pine weevils ( Hylobius abietis L.) was about equal in both patch scarification and control treatment. Insecticide treatment had a significant influence on growth and survival of seedlings. On the clear-cut, herbivory by pine weevils was lower on beech and oak seedlings than on Norway spruce seedlings. In the shelterwood, herbivory by pine weevils was low on all species. The conclusion is that less efforts are needed to protect beech and oak from herbivory by pine weevil compared to Norway spruce. Herbivory by short-snouted weevils ( Strophosoma melanogrammum Forst. and Otiorhynchus scaber L.) constitutes another threat, but was about the same on beech, oak and Norway spruce. Although the highest growth in the seedlings occurred in the open, this study shows that high survival can be achieved without using site preparation and insecticides when beech and oak are planted under shelterwood trees of Norway spruce.