Ozone visible symptoms and reduced root biomass in the subalpine species Pinus uncinata after two years of free-air ozone fumigation

Concentrations of ozone often exceed the thresholds of forest protection in the Pyrenees, but the effect of ozone on Pinus uncinata, the dominant species in subalpine forests in this mountainous range, has not yet been studied. We conducted an experiment of free-air ozone fumigation with saplings of P. uncinata fumigated with ambient O3 (AOT40 May–Oct: 9.2 ppm h), 1.5 × O3amb (AOT40 May–Oct: 19.2 ppm h), and 1.8 × O3amb (AOT40 May–Oct: 32.5 ppm h) during two growing seasons. We measured chlorophyll content and fluorescence, visible injury, gas exchange, and above- and below-ground biomass. Increased exposures to ozone led to a higher occurrence and intensity of visible injury from O3 and a 24–29% reduction of root biomass, which may render trees more susceptible to other stresses such as drought. P. uncinata is thus a species sensitive to O3, concentrations of which in the Pyrenees are already likely affecting this species.

► We assessed sensitivity to O3 in Pinus uncinata using a free-air O3 fumigation system.
► Occurrence and intensity of visible injury from O3 correlated with exposure to O3.
► Increased O3 reduced root biomass 24–29%.
► O3 weakens P. uncinata, making it more susceptible to other stresses.
► Ambient [O3] in the Pyrenees is thus likely to already be affecting P. uncinata stands.