INFLUENCE OF NUTRIENT AND WATER-STRESS ON NORWAY SPRUCE PRODUCTION IN SOUTH SWEDEN - THE ROLE OF AIR-POLLUTANTS

A field experiment primarily designed for simulating the indirect effects of air pollutants for a 25-year-old Norway spruce stand in SW Sweden is presented (The Skogaby project). Treatment include irrigation; artificial drought; ammonium sulphate addition; nitrogen-free-fertilization and irrigation with liquid fertilizers including a complete set of nutrients. The experiment has a randomized block design with four replicates per treatment. Growth response on an areal basis of basal area, height and dry mass of stems, branches and needles after up to four years of treatment are presented. Dry mass is estimated using allometric equations based on destructive samplings of trees. The stand suffered from temporary water stress during all four years investigated despite 970-1160 mm of annual precipitation. Irrigation resulted in improved above-ground dry mass production (stem, bark, branches, needles, litter fall) by 20% during the first 3 years of treatment, whereas 2 years of drought treatment followed by 1 year of recovery led to 10% reduced dry mass growth. During year 2 of recovery, however, basal area growth was only about half of that of the control. Nitrogen, markedly, was a growth limiting nutrient, although the stand got approx. 20 kg N ha-1 y-1 from deposition. Ammonium sulphate addition (100 kg N ha-1 y-1) resulted in 31% improved dry mass production whereas irrigation with liquid fertilization (100 kg N ha-1 y-1) including all important nutrient elements led to 57% increased dry mass growth after 3 years of treatment. Basal area growth of the latter treatment gradually increased and during year 4 of treatment was 123% larger than the control. Nitrogen-free-fertilization resulted in a small improvement of dry mass production (+10%). After 3 years of treatment, the amount of needles had increased markedly for both treatments including irrigation, whereas drought treated trees instead had decreased their needle amount vs control. The increase in needle amount occurred as a result of both larger formation of needles and higher preservence of old needles, the opposite relations being found for the drought treated trees. At the same point larger needle formation in combination with a higher shedding of older needles was found for trees treated with ammonium sulphate and nitrogen-free-fertilizer. It is concluded that there is no stage of N saturation in the Skogaby site as there is no leaching of N from the control plots and N fertilization results in both increased tree growth and N uptake.