Syuzo Itow

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

. 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 Shannons H showed decreasing trends in the same gradient with a few exceptionally high and low values.

Population structure,stand-level dieback and recovery of Scalesia pedunculata forest in the Galapagos Islands

Scalesia pedunculata is a fast growing, short-lived tree of the Asteraceae, which forms dense, monospecific stands in the humid forest zone at middle elevations on Isla Santa Cruz of the Galfipagos Islands (Bowman, 1961, Itow, 1965; Wiggins and Porter, 1971). The species apparently evolved from a herbaceous ancestor. Between 1935 and 1940 Scalesia pedunculata underwent mass mortality according to observations recorded by a long-term local resident (Kastdalen, 1982). The forest thinned down to only a few scattered remnants until the flowering, coincident with a dry period during the early 1940s, and the forest area became densely restocked with seedlings of the same species. The first named author of the present paper has been interested in the ecology and behavior of the species and carried out investigations of the size distribution of Scalesia pedunculata populations on his visit to the Galfipagos in 1970, 1978 and 1981. The data he gathered showed that each stand of the forest consists of one or a few even-aged cohorts without any young generations, but the data were not published (a part of which are appearing in the present paper). Based on his field studies he noted some ecological characteristics of S. pedunculata such as (1) its pioneering behavior, (2) the species-poor constitution of the forest, (3) the absence of young tree generations in the stands, (4) the synchronized collapse of canopy tree populations at a post-mature life-stage, and (5) a self-cyclic or build-up-and-collapse succession (Itow, 1978, 1983: Original in Japanese). Hamann (1979) also reported on the pioneering behavior of the species and the cyclic replacement of the forest. The 1982-1983 E1 Nifio brought extremely high rainfall to the Galfipagos Islands (Robalino, 1985), and mature populations of S. pedunculata died then and afterwards (Hamann, 1985; Lawesson, 1987). The present paper focuses on the population structure of S. pedunculata before the 1982-83 E1 Nifio and its change afterwards, and further on a comparison of stand-level dieback phenomena with other Pacific island forests. The study area is Los Gemeros, a nearly level terrain at an altitude of ca. 560 m, on Isla

Species diversity of mainland- and island forests in the Pacific area

Alpha diversity, or species richness, of East Asian mainland evergreen broadleaved forests, expressed by indices of Fisher’s alpha (α) and S(100), a new index showing species number in a 100-individual sample, is significantly correlated with the climatic favorableness, expressed by Kira’s warmth index. On the contrary, diversity values of insular forests studied on Kyushu satellites of Japan, the Bonins, the Eastern Carolines of Micronesia, and the Galapagos in the eastern Pacific, are below those expected from the climate of respective oceanic islands. Species-individual curves, comparing mainland-and insular communities, also support clearly the above conclusion of species poverty in the insular communities studied.

Species diversity of equatorial insular forests on Ponape and Kosrae, Micronesia

The species richness, or alpha diversity, of plant communities depends on various factors, of which climatic favorableness is one of the important factors (Row, 1985). Under the most favorable conditions, the tropical rain forest shows an extremely species-rich constitution, which is supported by high temperatures combined with plentiful rainfall throughout the year. On oceanic islands far from a continent, however, the flora is poor due to isolation. In this context, a comparison of species diversity made between continental (Ryukyu) and oceanic (Bonin) islands in the subtropical climate (Itow et al., 1984; Itow, 1985) clearly showed that remoteness, or isolation, is another important factor involved in the species diversity of insular communities. It was considered that the next step along this line should be a study of the natural forest on tropical oceanic islands and its comparison with continental rain forest communities. Ponape and Kosrae of Micronesia provide a good field for this purpose, since they are located in the equatorial Pacific and in a climate with high temperatures and plentiful rainfall.

Aboveground biomass and soil nutrient pools of a Scalesia pedunculata montane forest on Santa Cruz, Galápagos

We estimated the live aboveground biomass (AGB) and soil nutrient pools of the Scalesia pedunculata monodominant tropical montane forest at 600 m above sea level on Santa Cruz, Galápagos, an isolated oceanic island. The estimated AGB was 60.4 Mg ha−1, which was considerably lower than that of other montane forests of similar climates elsewhere. Nutrient pools were ample for inorganic N, soluble P, and exchangeable cations. We suggest that the low AGB, in spite of the ample nutrients, is related to the absence of tall-statured climax species, that have high demands for nutrients (particularly N) to fix C, due to the isolation.