THE RESPONSES OF SHOOT GROWTH OF EUCALYPTUS SPECIES TO CONCENTRATION AND FREQUENCY OF EXPOSURE TO NITROGEN-OXIDES

TO manage forests in areas with high concentrations of nitrogen oxides in the air, it is important to know the concentrations and frequencie s of exposure which produce responses in trees. Four Eucalyptus specie s (E. microcorys F. Muell., E. globulus Labill., E. pilularis Smith an d E. marginata Don ex Smith) were exposed to a range of nitrogen oxide s concentrations (< 5, 25, 50, 91 and 187 nl l-1) at fixed frequency ( 2 h day-1, three times per week), and to fixed concentration (about 10 0 nl l-1), but variable frequency (never, once only, once per month, o nce per week and three times per week) for 169 days. The responses of growth to these treatments were determined using open-top chambers wit h plants grown directly in the soil. Generally, increasing frequency o r concentration of nitrogen oxide fumigations had effects of similar m agnitude. The effects of nitrogen oxides on growth were consistent wit h bivariant response models having constants which varied between spec ies. Eucalyptus microcorys grew taller and heavier with increasing exp osure, with a significant response in the ascending and plateau region s of the curve. Eucalyptus globulus and E. pilularis grew taller and h eavier at low exposures but this effect reversed at higher exposures. The response encompassed the ascending, plateau and descending regions of the curve. The height and weight of E. marginata were not statisti cally significantly affected by nitrogen oxides fumigation, although t he response curve suggested a similar response, but with smaller ascen ding and narrower plateau regions of the curve than the other species.