COMPETITION FOR WATER AND NUTRIENTS BETWEEN GROUND VEGETATION AND PLANTED

A field experiment was established between 1989 and 1993 on four sites in southern Sweden to study the effects of the clearcut age on damage by pine weevils and competing vegetation. On each site, Norway spruce (Picea abies (L.) Karst.) seedlings were planted on 0- to 4-year-old clearcuts. In the study reported here, the effects of clearcut age, mounding, herbicide, mowing, and removal of slash on growth of ground vegetation were investigated. The study was restricted to analysis of second-year growth of planted Norway spruce seedlings, i.e., established seedlings. Ground vegetation was sparse on fresh and 1-year-old clearcuts. On older clearcuts the dry weight of ground vegetation ranged between 1 and 4 Mg/ha, with a considerable variation between years and sites. Seedling growth was negatively influenced by ground vegetation. Mounding and herbicide treatments reduced ground vegetation and increased seedling growth, especially on older clearcuts, while mowing had no effect. Slash removal had no significant effect on the amount of ground vegetation and did not affect seedling growth. Carbon isotope analysis (13C abundance), predawn water potential, and needle conductance did not reveal a consistent difference iri water stress between seedlings in undisturbed and vegetation-controlled plots, even during dry periods. There was no evidence that competitor effects on radiation were related to seedling growth response. Therefore, it was concluded that competition between ground vegetation and planted seedlings for water and light alone could not explain the observed differences in growth. The results indicate that growth was restricted by nitrogen availability