Interactive effect of springtime frost and elevated ozone on early growth, foliar injuries and leaf structure of birch (Betula pendula).

•   Impacts of ozone and late frost on six birch (Betula pendula) genotypes from south-eastern Finland were studied in an 8-wk chamber experiment. •   The plants were measured for bud burst, growth, visible foliar injuries caused by ozone and frost, structural leaf properties and changes in chloroplasts. •   Ozone delayed bud burst but stimulated subsequent growth. Acute frost injuries were compensated by increased leaf production. Early bud burst predisposed to frost damage, whereas late bud burst increased the vulnerability to ozone. In combined ozone + frost treatment, freezing reduced visible ozone injuries, counteracted ozone-induced growth enhancement and stomatal changes, and exacerbated ozone-caused reduction in palisade cell, chloroplast and starch grain size. Rapid changes in epidermal cell differentiation towards stomata and/or glandular trichomes occurred to enhance ozone/frost tolerance. •   The results showed large genetic variation within birch population in response to frost and ozone. Generally, birch seem to recover from acute frost occurrence efficiently through compensating leaf production, but co-occurring ozone enhancement may disturb the recovery processes mechanistically through structural damage in photosynthetic tissue, especially in chloroplasts.