Negative impact of ozone on the stem basal area increment of mature Norway spruce in south Sweden
Journal article, 2006
The relative annual basal area increment of mature Norway spruce trees in south-central Sweden during 9 years was used as the response variable and analysed in relation to ozone exposure, meteorological conditions, soil moisture and stand characteristics. The method used was a modified multiple regression analysis, allowing for dependencies between observations from the same plots. The selected statistical model explained 91% of the variation in the annual relative basal area increment. The strongest explanatory variable was the stand basal area, followed by the temperature sum and the soil moisture index. After these three variables, the ozone index was the most important variable. Its effect was negative and highly significant. The average daylight ozone concentration gave a slightly better model fit as compared to the accumulated exposure during daylight hours above a threshold of 40 nmol mol−1 (AOT40). The predicted effect of ozone within the range of annual ozone exposures found in this study (18008700 nmol mol−1 h AOT40), was in absolute values a 0.8% decrease in the relative annual basal area increment. This could be compared with the mean relative annual increment measured during the study period of 4.6%. Our results provide statistical evidence that ground level ozone can have a negative impact on the stem growth of mature Norway spruce trees under field conditions.