SOIL-NITROGEN AND CHRONIC OZONE STRESS INFLUENCE PHYSIOLOGY, GROWTH AND NUTRIENT STATUS OF PINUS-TAEDA-L AND LIRIODENDRON-TULIPIFERA-L SEEDLINGS

The effects of soil nitrogen availability and chronic ozone stress on carbon and nutrient economy were investigated in loblolly pine (Pinus taeda L.) and yellow-poplar (Liriodendron tulipifera L.). One-year-old seedlings were planted individually in pots in forest soil of low (58-mu-g g-1), medium (96-mu-g g-1) or high (172-mu-g g-1) initial concentrations of soluble nitrogen. The seedlings were exposed to ozone in open-top field chambers at sub-ambient (charcoal-filtered air), ambient, and elevated (ambient + 60 nl l-1 O3) (32, 56, 108 nl l-1 O3, 7 h seasonal mean, respectively) levels for 18 weeks. At final harvest loblolly pine dry matter increased by 50 % at the highest soil N level relative to the low with the largest gains in new needle biomass. Elevated ozone reduced the biomass of current-year needles by 20 % in plants grown at the highest N level. Higher soil N supply increased the concentration of nitrogen in needles, stimulated current-year needle photosynthesis and increased needle and whole-plant water-use efficiencies. Ozone treatment had no significant effect on photosynthesis or water-use efficiency in either species, although ozone exposure tended to reduce stomatal conductance in loblolly pine. The low N treatment increased the proportion of dry matter allocated to fine roots in yellow-poplar, but whole-plant dry weight had not responded to N fertilization at the final harvest, suggesting other limitations on growth. Ozone exposure increased leaf abscission and doubled leaf turnover in yellow-poplar. Although yellow-poplar was highly sensitive to ozone-induced leaf abscission, final whole-plant dry weights were not affected. The indeterminate growth habit of yellow-poplar permitted compensatory leaf growth which may have ameliorated effects of chronic ozone stress on biomass gain. Ozone exposure also decreased shoot weight more than root weight, resulting in higher root:shoot ratios in loblolly pine and a similar trend in higher fine root:leaf ratios in yellow-poplar. Greater proportional allocation of carbon to roots in response to nutrient deficiency may preclude an increased allocation to shoots often observed in response to air pollution stress. Interspecific differences in growth response to chronic ozone and nutrient stress may be influenced by differences in leaf growth habit.