Radial stem growth of Picea abies in relation to spatial variation in soil moisture conditions

Soil water potential was measured for 5 yrs (1988-1992) in different plots established in a 26-yr-old stand of Norway spruce in southern Sweden. The plots were subjected to different nutrient regimes but all had the same precipitation input. Content of stone and gravel varied between plots. Stem growth, estimated as basal area increment, varied between plots. This variation was related to soil water potential. Lower rates of stem growth were found on soils where low soil water potentials appeared during dry periods. These results showed that soil moisture has been a major factor limiting stem growth for many years and that stem growth is generally hampered on shallow soils despite an annual precipitation of around 1100 mm in this part of Sweden. Analysis of two control plots (C) and two plots treated with ammonium sulphate (N) showing within-treatment differences in soil moisture conditions revealed two different trends: the annual basal area growth on the plot on the wet soil became 50% higher than that on dry soil for the N-fertilized plots whereas control plots showed no change with time. This indicated that trees on dry plots were unable to utilize the increased supply of nitrogen.