The present study was carried out on natural Korean fir forests (Abies koreana) growing in Mount Halla in Jeju Island, Korea (33 degrees 13-36' N and 126 degrees 12-57' E). Mount Halla is the highest mountain (1,950 m a.s.l.) in South Korea. On the Korean fir forests near the top of Mount Halla in Korea, we established permanent plots between dieback and healthy population. Each permanent plot includes both dieback and relatively healthy Korean fir individuals. Three sites in this study showed similar altitude, topographic position, aspects, slope, diameter at breast height, average height and ages. Net photosynthetic rates (P(N)) on different temperature regimes were evaluated to explain the forest dieback phenomenon on Korean fir populations. Light response curves were determined on three different temperature regimes: 15 degrees C, 20 degrees C and 25 degrees C. The irradiance response curve showed higher values in lower air temperatures. Generally, irradiance response curves of healthy Korean fir populations were higher than the dieback population at all sites.