Effects of virus infection and light environment on population dynamics ofEupatorium makinoi (Asteraceae)

We studied the effects of virus infection on dynamics of three Eupatorium m akinoi populations in contrasting light environments, Gora-dani (a shaded p opulation) and Minou 1 and Minou 2 (open-site populations). Censuses of the plants were taken for 8 yr in Gora-dani and 3 yr in Minou 1 and Minou 2. A fter the epidemics of virus infection, most plants were virus infected at b oth sites. The number of plants and the proportion of flowering individuals decreased rapidly and simultaneously in the shaded population in Gora-dani . By contrast, in the open-site populations of Minou, the proportion of flo wering plants decreased first, and then the number of plants decreased grad ually. Growth analysis of the plants in the Gora-dani population revealed t hat stem growth was significantly suppressed by infection and that flowerin g and survivorship of the infected plants decreased with reducing plant hei ght. Since light availability affected plant growth and thereby flowering a nd survivorship, the differences in population dynamics between the two fie ld sites could be caused by the differences in light environments. Although populations in open sites may persist for considerable periods after virus epidemics, the individual local populations of E. makinoi would eventually become extinct irrespective of light environments.