We tested a hypothesis that elevated ozone was an eliciting or contributing factor in outbreaks of the ‘top dying’
(or ‘subtop dying’) syndrome in Norway spruce (Picea abies). Progeny were used from open-pollinated trees
within a stand with the ‘top dying’ syndrome. The mother trees were classified in relation to the expression of the
‘top dying’ syndrome, and progeny from the healthiest and least healthy thirds of the population were exposed to
high and low concentrations of ozone for three seasons. Elevated ozone did not affect height growth of the trees.
It did not measurably affect net photosynthesis, stomatal conductance or instantaneous water use efficiency.
Chlorophyll and carotenoid contents were also not significantly affected by ozone concentration. In the first year,
instantaneous water use efficiency was lower in the progeny of the unhealthy mother trees than in the healthy
mother trees. Furthermore, the unhealthy mother trees tended to produce longer annual shoots and showed more
winter damage at the end of the experiment. None of these parameters were related to ozone concentration in the
atmosphere. These results do not support a hypothesis that elevated ozone is a significant contributory factor or
an eliciting factor in the development of the syndrome.