A review of ozone responses in Scots pine (Pinus sylvestris)

The data on Scots pine responses to elevated ozone (O3) mainly come from experimental studies with young seedlings and trees. Based on the 38 experiments reviewed here, Scots pine may be considered as an O3-sensitive conifer species, with mature pines more sensitive than younger trees. This is due to their relatively small proportion of current (c) year needles with the highest photosynthetic capacity. Moreover, young seedlings and trees seem to acclimate to slightly elevated realistic O3 exposures, and hence do not often exhibit growth and biomass reductions in spite of the visible and microscopic needle injuries and changes in needle chemistry. The O3 sensitivity in Scots pine is thought to relate to impaired water status due to the malfunction of stomata and subsequent increase in transpiration. This may lead to reduced wood biomass in the long term, if Scots pines try to maximise the biomass of c needles and root biomass to maintain efficient water and nitrogen (N) supply to support the photosynthesis of c needles. Tree water status also contributes to the spring-time recovery of photosynthesis. We call especially for studies on atmosphere–needle surface interaction that would yield novel information on the impact of O3 on epicuticular waxes and stomatal functioning, which both regulate O3 flux and tree water status and hence also modify photosynthesis. The need for flux-based field studies is especially important in the light of future climatic change, since the risk presented by O3 to Scots pine forests in Northern and Central Europe seems to be equal.

► Under chronic ozone exposure, Scots pine seedlings seem to invest resources on current-year needles to support photosynthesis.
► Young Scots pines also seem to invest resources to roots to support water and nitrogen uptake, when exposed to slightly elevated ozone levels.
► The needle surface may account for ∼50% of the total foliar ozone deposition.
► Ozone effects on the chemistry of Scots pine epicuticular waxes are unknown though.
► Long-term field monitoring studies on the interactive effects of ozone and climatic factors on Scots pine are needed.