Stem growth of Norway spruce in south Sweden in relation to soil moisture, nitrogen deposition, ozone exposure and meteorological variables

Abstract

Associations between the annual stem basal area increment growth and soil moisture, nitrogen deposition, ground level ozone exposure, air temperatures and the timing of the start of the growing season have been investigated for a twenty four-year period, 1990–2013, based on tree-ring width measurements from seventeen monitoring sites with Norway spruce (Picea abies) forests in southern Sweden. The stem growth-environment associations were analyzed using a fixed effect regression model, with annual stem basal area increment (BAI) as the dependent variable and annual values for a soil moisture index, ozone exposure estimated as AOT30, bulk deposition of nitrogen, summed air temperatures above a threshold and the timing of the start of the growing season as explanatory variables. The statistical analysis was made with and without taking clustering of the sampled trees into account, i.e. that several different tree observations were made at the same monitoring site. The annual number of days with soil moisture below a threshold was the only explanatory variable that could be demonstrated to be negatively associated with changes in BAI, regardless of statistical approach. Positive associations between temperature sums as well as nitrogen deposition with changes in BAI were indicated by low p values using standard p-values, but not when clustering was taken into consideration. Associations between ozone exposure as well as the start date of the growing season with changes in BAI could not be demonstrated since the estimated p values were high regardless of statistical approach. The results show that soil water deficit may considerably limit forest growth in northern European forests.