Competing vegetation effects on initial growth of planted Picea abies

Abstract

Growth during the first two growing seasons in response to different levels of aboveand below-ground competition was evaluated for Norway spruce (Picea abies L. Karst) seedlings on a clay moraine in southern Sweden. Spruce seedlings were planted in plots and left undisturbed, or mowed, or mowed and shaded, or treated with herbicides, or treated with herbicides and shaded. The aim of the shading treatment was to simulate the light levels below the canopy of the ground vegetation in the undisturbed plots. Spruce seedling growth was reduced by the presence of competing vegetation regardless of whether the vegetation was mown or not. Shading only reduced growth to a minor extent. Soil water potentials were reduced in all treatments during periods of drought. However, undisturbed plots showed the lowest levels of soil moisture and the periods of drought extended over longer periods of time than in plots where the vegetation was either mown or removed with herbicides. Carbon isotope analysis and measurement of needle lengths indicated no differences in water stress for seedlings in undisturbed plots compared to seedlings in herbicide-treated plots. Seedlings in undisturbed and mown plots had lower needle nitrogen concentrations than seedlings in herbicide-treated plots. Therefore, it was concluded that competition between weeds and seedlings planted on clay moraine was mainly below ground and that lower availability of nitrogen in untreated and mown plots may have been an important limitation for growth.