3.1. Short-term greenhouse experiment

The plant material in the short-term Belgian greenhouse experiment consisted of four provenances of 3- to 4-year-old nursery-grown Scots pine (Pinus sylvestris L.) from latitudes 45N to 65N and three provenances of nursery-grown seedlings and one of cloned Norway spruce (Picea abies Karst. L.) from latitudes 45N to 52N (Fig. 1). The seedlings were planted in a standardized soil mixture consisting of sand, unfertilized peat and composted soil. The seedlings were randomly distributed into the treatment categories, with three or four trees per provenance per dose treatment. The experiment was conducted during 13 weeks from April 13 till July 13, 1994, in the Faculty of Agricultural Sciences of Gembloux, Belgium. The temperature in the greenhouses was regulated between +16C and +20C in ambient light conditions. To simulate UV enhancement conditions, the plants were irradiated with UV-lamps (Philips UV-B TL 12 40 W), using four lamps in each experimental area. The UV-C radiation was filtered out by cellulose diacetate filters (Courtaulds Ltd, 95 mm). The biologically effective doses (UV-BBE) were 0, 11.3 and 22.6 kJ m⁻² d⁻¹. The UV measurements were done with a Jobin & Yvon UV spectroradiometer from the Institute for Spatial Aeronomy (Brussels, Belgium) (II).