Complex environmental factors affecting the decline of Pinus densiflora in the Seto Inland Sea area of western Japan

This study reconsidered the cause of forest decline of Japanese red pine (Pinus densiflora Sieb. et Zucc.) in the Seto Inland Sea area, western Japan. Although the decline in pine forests was attributed to pine-wood nematode infection, the stress of air pollution was also shown to have negative physiological effects on pine needles. Secondary pollutants related to NOx accumulate in the morning dew then liquid-phase free radicals (•OH) form by photochemical reactions on the needle surface. The stress of polluted dew induces faster needle senescence and defective needle gas exchange. The influence of atmospheric O3 and SO2 was not significant. Understory vegetation also had negative physiological effects on the overstory pine needles through competition of roots for water and nutrients. Abandonment of management accelerates growth of understory vegetation causing decline of P. densiflora plantations. Each of these stresses was shown to independently and significantly reduce needle photosynthesis, which consequently seemed to reduce pine resistance to pathogens. In many cases, air pollution and progressing succession have not been considered as important causes of P. densiflora decline. Careful on-site atmospheric chemical and eco-physiological measurements are therefore important for accurate evaluations of the conditions of pine growth and the causes of decline