1998 Volume 35 Issue 4 Pages 153-163
This study examined the relationship between the decline of red pine (Pinus densiflora) forests peculiar to the middle slope of inland basins and the appearance of a cold air lake in Saijo Basin, Hiroshima. From comparisons between the inside and the outside of the basin, and between the mountainridge and middle slope, it was found that the mortality of red pine trees was the highest in the middle slope (260 to 280 m a.s.l.), and the thickening growth was severely suppressed during the few years before death. Vertical profiles of temperature and relative humidity measured by a captive balloon indicated that the surface inversion layer, which was created by radiative cooling during the night, covered almost the whole basin up to 500 m a.s.l. However, the cold air lake appearing below 290 m contained especially low temperature and high humidity air. A nitrogen dioxide (NO2) level higher than 20 ppb/24 hours was recorded in the upper half (230 to 290 m a.s.l.) of the cold air lake. Acidic matter originating from NO2 can cause a stress for red pines under these meteorological conditions. The results suggest that the zonal forest decline within the basin is caused not only by nematode but also by the high NO2 layer created by the cold air lake.
