Effects of nitrogen load on Asian trees

Abstract

The effects of increasing nitrogen (N) load on Asian forest trees have been studied mainly in Japan and China during the past decade. In this chapter, we summarize the expected mechanisms of the harmful effects of excessive N load on the eco-physiological function of trees, and introduce experimental studies on the growth responses and foliar nutrient status of young Asian tree seedlings to increasing N loads. By comparing the growth responses to various N loads in 12 tree species in Japan and China, we confirmed that: (1) the threshold of the N load that induced growth reduction was quite different among the species; (2) the threshold value ranged between 50 and 100 kg N ha-1 year-1 in relatively sensitive (low-tolerance) species. Furthermore, a significant relationship between foliar nutrient balance and growth indicated that (3) the threshold N/P ratio for growth reduction was slightly higher than that in European tree species, and (4) an Mn/Mg ratio of over 0.8 was observed, together with growth reduction, in the sensitive tree species.