Ozone and/or water stresses could have influenced the Betula ermanii Cham.: Forest decline observed at Oku-Nikko, Japan

A serious forest decline of Betula ermanii Cham. has been observed at Mt. Mae-Shirane, Oku-Nikko, Japan, where high ozone (O-3) concentration and severe water deficiency have been measured. In order to consider the possibility whether O-3 and/or water stresses could have been the causes of the forest decline of B. ermanii, plant growth experiments were conducted in environment-controlled growth cabinets. Two-year-old seedlings of B. ermanii were exposed to either charcoal-filtered air (O-3 concentration < 5 ppb) or 50 ppb O-3 (daily average, ranging between 20-100 ppb) for 123 days at 20.0/12.5 +/- 1.0 degrees C (day/night) and 70/80 +/- 7% relative humidity (day/night). Simultaneously, seedlings were treated with three watering regime: 1.0 < pF 1.8 (no water stress), 1.8 < pF < 2.5 (mild water stress) or 2.5 < pF < 3.0 (severe water stress). O-3 exposure significantly reduced the dry weights of leaf, root and the whole plant, while water stress significantly reduced the dry weights of each organ and the whole plant. Significant reductions of net photosynthesis, transpiration and stomatal conductance were also observed under O-3 and/or water deficiency treatments, while contents of RuBP carboxylase/oxygenase (Rubisco), chlorophyll (a+b) and some essential nutrient elements (N, P, K, Mg and Ca) were not markedly changed. It was suggested that the decrease in net photosynthetic rate induced mainly by stomatal closure was the major cause of the growth reduction under O-3 and/or water stresses. No significant interactions between O-3 and water stresses were observed in terms of the depression of dry matter production, which suggested that simultaneous stress treatments of O-3 exposure and water deficiency could affect the tree growth of B. ermanii additively.