Effects of nutrient supply on understory Picea abies seedling growth in partially cut stands in south-east Norway

Abstract

Norway spruce (Picea abies (L.) Karst.) understory seedlings, growing in partially harvested plots with different canopy cover in a boreal spruce stand, were spot fertilized (Hydro 15-4-12) 9 years after planting. The principal aim was to test the hypothesis that nitrogen (N) availability influences growth of understory seedlings at intermediate but not at low levels of irradiance. In addition, we tested the combined influences of N and light availability on selected morphological and phenological traits, covering a 2-year period after treatment. Diffuse radiation (DIFR) at the seedling level was estimated from hemispherical photographs and ranged from 19 to 46 per cent of DIFR in open conditions. Fertilizer application was associated with a marked increase in foliar N concentration. The fertilized seedlings grew better in height and root collar diameter compared with unfertilized controls. While the absolute growth in both diameter and height increased with increasing DIFR, seedlings also responded to improved nutrient availability across the range of light conditions studied. Fertilizer treatment did not affect the number of nodal buds, but we observed a higher apical dominance ratio and advanced bud burst in fertilized seedlings. In conclusion, nutrient availability influenced growth and bud phenology of understory Norway spruce seedlings at least down to 20 per cent DIFR.