Defense characteristics of seral deciduous broad‐leaved tree seedlings grown under differing levels of CO 2 and nitrogen

Mixed broad‐leaved forests are abundantly populated by several kinds of herbivore species in northern Japan. The life of herbivores depends strongly on the quality of food leaves. Leaf quality is changing with increasing atmospheric CO 2 and nitrogen deposition. Four seral species of deciduous broad‐leaved tree seedlings (alder, birch, oak and maple) were raised in all four combinations of two levels of CO 2 and two levels of nutrient, to examine foliar defense traits. To evaluate the relative defense capacity of the four tree species, we used wild silkworms (Erisan; a generalist herbivore) as a bioassay material. Except with alder, the survival rate and longevity (ML 50 ) of the silkworms were least when they were fed with leaves of seedlings raised under high CO 2 and infertile soil conditions, and longest on a diet of leaves grown in ambient CO 2 and fertile soil, especially in birch and maple. The longevity of Erisan decreased in the order birch, oak, and maple. The longevity of Erisan fed with alder leaves was independent of CO 2 levels and was longer on alder seedlings grown in infertile soil conditions. Alder is an actinorhizal plant that can fix atmospheric nitrogen in root nodules formed by the actinomycetes Frankia sp. The activity of symbiotic microbes would have been enhanced by the greater amount of photosynthates received from the host plants at high CO 2 , improving the food quality for the silkworms. It was concluded that in all but alder, leaf chemical traits, especially C/N ratio, affect the ML 50 of Erisan larvae.