Ozone affects Scots pine phenology and growth

Abstract

An open-top chamber (OTC) fumigation experiment with 10- to 15-year-old Scots pines ( Pinus sylvestris L.) was started in autumn 1997 at the University of Oulu (65°N, 25°E). There were six non-filtered air (NF) and six open-field (AA) control pines. The six NF+O 3 pines were exposed from summer 1998 onwards to target ozone (O 3 ) concentrations of ambient air +40 ppb in May, ambient air +30 ppb in June, ambient air +20 ppb in July, ambient air +10 ppb in August and ambient air in September. The accumulated O 3 exposure over a threshold of 40 ppb (AOT40) in the NF+O 3 OTCs was 12.9 ppmh in the summer of 1998, but only 1.1 and 1.8 ppmh in the summers of 1999 and 2000, respectively, because fumigation started late. The respiration of previous-year needles was increased by exposure to O 3 ; and they also showed a decreasing trend in net photosynthesis and an increasing trend in the internal CO 2 concentration with increasing O 3 exposure in the summer of 1998. The results on needle carbon (C) contents suggested O 3 -related changes in C allocation, and the chlorophyll a+b/carotenoid ratio in the current-year needles of the NF+O 3 pines was also lower than that in the current-year needles of the NF controls in November 1999. The slightly elevated O 3 concentrations caused clear physiological responses in Scots pine needles, which may, over a longer period, result growth reductions, as was suggested by the non-significant changes in the current-year shoot (18% increase in main shoots vs. 19% decrease in branches) and needle (15% and 10% decrease in main shoots and branches, respectively) growth of the NF+O 3 trees recorded in late July 2000. It seems that peak O 3 episodes during early summer are harmful to subarctic Scots pines.