Effects of elevated ozone on growth and foliar traits of European and hybrid aspen

We studied growth and foliar responses of two clones of both European aspen (Populus tremula) and hybrid aspen (P. tremula x P. tremuloides) to elevated O 3 (45 ppb, 14-h mean) over one growing season using a free-air fumigation system in central Finland. All clones exhibited 03-specific foliar injury and accelerated leaf senescence under elevated O 3 . Yet, exposure to 1.5 x ambient O 3 had only minor effects on the growth and biomass production of clones grown under optimal nutrient and water supply, and no O 3 effects on leaf morphology were observed. Slower-growing European aspen was more sensitive to elevated O 3 than hybrid aspen. Exposure to O 3 decreased the root/stem ratio (-11%) and leaf N concentration (-9%) of European aspen. Inter- or intraspecific differences in the O 3 sensitivity of the trees could not be explained by stomatal conductance, but some xeromorphic leaf traits were related to increased susceptibility to O 3 . Intraspecific differences in the O 3 sensitivity have implications e.g. for nurseries producing commercial tree material.