Functional leaf traits, plant communities and acclimation processes in relation to oxidative stress in trees: a critical overview

Tropospheric ozone exercises pressure on vegetation in combination with other oxidative factors such as strong sunlight, UV-B radiation, high temperatures and water shortage. The relationship established between these factors and the plants can be either antagonistic (cross-resistance) or synergistic. Response mechanisms are activated involving physiological, chemical and morphological features. Overall sensitivity (or resistance) is determined by the amount of total oxidative pressure in relation to the plant's physiological plasticity [i.e. its ability (within its own resilience potential) to alter its structure and functions in order to react to the oxidative pressure]. Within different populations of the same species, functional leaf traits (leaf area and thickness, leaf mass per area or specific leaf area, tissue thickness) may vary in relation to the extent that the plant succeeds in acclimating to ambient oxidative pressure. The behaviour of a tree in an oxidative environment can be interpreted by means of leaf structure analysis. Leaves presenting high tissue density (and thus low intercellular space content) display a high degree of acclimation to stressors, react little to environmental changes and are characterized by remarkable longevity. Leaves of this type also have a high photosynthesis capacity per surface unit (due to the N content per surface unit) and a high P-N/G(W) (or water use efficiency, WUE). Thus, they are able to support detoxification processes. These morphological traits are to be found in adult plants in late successional stands and, above all, in ambients that have already been subjected to oxidative pressure. However, in early successional species and during the dynamic stages of growth (e.g. in the renewal occurring along forest edges), it is the opposite leaf traits that prevail: low leaf density, high photosynthesis capacity per dry weight unit, low WUE, low leaf longevity. These traits make plants far more reactive to environmental changes (e.g. they exploit the light from sun flecks much more effectively), but enable them to achieve only a low degree of acclimation and a poor ability to support detoxification processes. Whereas adult forests display a high level of ecological resilience and have a relatively good tolerance of ozone, the renewal stages are (at least potentially) more vulnerable. In these environments, ozone can alter the competition among genotypes and favour more resistant ones. Among the sectors most at risk, we must include communities growing at the edge of their ecological range, for whom even a slight increase in oxidative pressure can trigger substantial degenerative processes.