Species turnover and diversity patterns along an evergreen broad-leaved forest coenocline



Abstract. Direct gradient analysis was applied to the evergreen broad-leaved forest coenocline in the Tatera Forest Reserve, Japan. 10 quadrats of 0.1 -0.05 ha were laid out from 140 m to 560 m above sea level at intervals of 25–70 m. Gradient analysis revealed that distributions of many species terminated or started at ca. 400 m. Community similarity, calculated in Percentage Similarity (PS) and Community Coefficient (CC), changed abruptly below and above the 400 m contour, suggesting a change of vegetation structure at this altitude, which was also clear from population distributions.

The spatial turnover rate of species along the altitudinal gradient was calculated in two ways: as the Average turnover rate along the whole range of the gradient, and as the Zone turnover rate at individual altitudes. The overall rates calculated for five categories of populations: DBH > 10 cm, DBH >3 cm, all woody species, herb-layer, and total vegetation, were- 0.0011 to- 0.0021 for PS, and - 0.0009 to- 0.0019 for CC. The calculated rates (PS basis) indicate that a 95% change in species composition is reached at 1120 to 620 m altitude. Similarly, the rates -0.0009 to - 0.0019 (CC) correspond to 1410 - 680 m. The altitudinal range expected here for a 95% change agrees with the actual elevation of forest zonation in northwestern Kyushu. The average rate of both PS and CC in the herb-layer population was 1.56 times higher than the rate in the woody species population, showing a more rapid change in herb-layer population than in the woody ones along the gradient.

The Zone turnover rates were higher at the 370–440 m belt than those below and above the belt. This coincided with the interchanging pattern in population distributions and the abrupt change in similarity at about 400 m above sea level. This may be due to the change in environmental conditions such as physiography and air humidity. In the diversity measurements, the species density per 100 m2 showed a gradual increase in the DBH >3 cm population but a constant level in the DBH >10 cm population along the whole range of the forest coenocline studied, while index values of S(100) and Shannon's H showed decreasing trends in the same gradient with a few exceptionally high and low values.