Visible ozone-like injury, defoliation, and mortality in two Pinus uncinata stands in the Catalan Pyrenees (NE Spain)

Ozone concentrations in the Pyrenees have exceeded the thresholds for forest protection since 1994. We surveyed the severity of visible O-3 injuries, crown defoliation, and tree mortality of Pinus uncinata, the dominant species in subalpine forests in this mountain range, along two altitudinal and O-3 gradients in the central Catalan Pyrenees and analysed their relationships with the local environmental conditions. The severity of visible O-3 injuries increased with increasing mean annual [O-3] when summer water availability was high (summer precipitation/potential evapotranspiration above 0.96), whereas higher [O-3] did not produce more visible injuries during drier conditions. Mean crown defoliation and tree mortality ranged between 20.4-66.4 and 0.6-29.6 %, respectively, depending on the site. Both were positively correlated with the accumulated O-3 exposure during the last 5 years and with variables associated with soil-water availability, which favours greater O-3 uptake by increasing stomatal conductance. The results indicate that O-3 contributed to the crown defoliation and tree mortality, although further research is clearly warranted to determine the contributions of the multiple stress factors to crown defoliation and mortality in P. uncinata stands in the Catalan Pyrenees.