Influence of ammonia and ozone on growth and drought sensitivity of Pinus sylvestris

Abstract

Four-year-old Pinus sylvestris trees were exposed to ammonia (16, 55, 110 ppb for 24 h d -1) and ozone (0, 45 and 68 ppb, 9 h d -1) in a factorial design in open-top chambers for 15 months. Treatment effects on tree growth and architecture were assessed during two growing seasons; effects on sensitivity to drought stress were determined during the second growing season. Tree height and stem diameter increments were stimulated by NH 3. The tree architecture was influenced only by NH 3 giving the trees a stunted appearance. Exposure to NH 3 resulted in lower needle water potentials both in fully watered trees and in droughted trees, indicating an increased susceptibility to drought. When the trees were drought stressed, the water potential in needles exposed to NH 3 alone decreased linearly with the NH 3 concentration. Trees exposed to NH 3+O 3 maintained a less negative needle-water potential after a drought treatment, indicating an ameliorative effect of O 3 on drought sensitivity although the relative decrease in water potential during the drought treatment was larger than in trees exposed to only NH 3. Drought-stress phenomena were also enhanced by NH 3 by increasing the amount of needles relative to the root biomass. Although the biomass of fine roots was increased by NH 3 as well as by O 3, NH 3 increased the needle biomass more, increasing the needle:root ratio and thus drought stress.