Irradiance in young stands of Picea abies (L.) Karst. and Pinus sylvestris L. and the possibilities to prevent suckers of broad-leaved trees

Incoming shortwave global radiation (Qg) and photosynthetically active radiation (PAR;Qpa) as a fraction of full daylight, relative irradiance (%Q), were measured at the same time in young stands of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.). Measurements were made on three levels above ground: 20 cm above ground and 50 and 75% of stand height. Stands of three heights (75, 150 and 300 cm) were studied during two months. The stands were created by arranging young trees cut from natural stands, in nine quadratic spacings: 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0, 1.4, and 2.0 m. The leaf area index (L) was estimated. Differences in %Q-values for Qpa and Qg in the same species at the same stand height and level of light measurements above ground were significant only in 150 and 300 cm stands. In 75 cm high stands of Scots pine, the %Q was less than 60% at spacings less than 0.2 m and in 75 cm high stands of Norway spruce, the %Q was less than 60% at spacings less than 0.3 m. Only at 0.1 x 0.1-0.3 x 0.3 m, %Q was less than 20% in Norway spruce stands. In 150 cm stands %Q less than 20% was measured at spacings less than 0.7 m in Norway spruce and less than 0.5 in Scots pine. In 300 cm stands of Norway spruce it was measured up to 1.2 and in Scots pine less than 1.1 m. Light extinction coefficients, K and Kg for spruce and pine stands were 0.17-0.40 and 0.16-0.31 respectively. Some practical implications of the study are presented. Lack of light (%Q less than 10%) as a single factor of seriously suppressed growth and development of broad-leaved plants and suckers by competition in young stands of spruce and pine only occur in dense stands 0.3 x 0.3-1.1 x 1.1 m (8000-100000 stems per hectare). Competition by light on a regenerated area generally occurs in the level of 50% of tree height (150-300 cm) and higher due to the rapid growth of broad-leaved trees (sprouts) compared with planted conifers but the light intensity at these levels %Q greater than 10%.