Forest structure, productivity and soil properties in a subtropical evergreen broad-leaved forest in Okinawa, Japan

Abstract

Structure, species composition, and soil properties of a subtropical evergreen broad-leaved forest in Okinawa, Japan, were examined by establishment of plots at thirty sites. The forest was characterized by a relatively low canopy and a large number of small-diameter trees. Mean canopy height for this forest was 10 m and stands contained an average of 5400 stems·ha−1 (≧3.0 cm DBH); 64% of those stems were smaller than 10 cm DBH. The total basal area was 54.4 m 2·ha−1, of which Castanopsis sieboldii contributed 48%. The forest showed high species diversity of trees. 80 tree species (≧ 3.0 cm DBH) from 31 families was identified in the thirty sampling plots. C. sieboldii and Schima wallichii were the dominant and subdominant species in terms of importance value. The mean tree species diversity indices for the plots were, 3.36 for Diversity index (H′), 0.71 for Equitability index (J′) and 4.72 for Species richness index (S′), all of which strongly declined with the increase of importance value of the dominant, C. sieboldii. Measures of soil nutrients indicated low fertility, extreme heterogeneity and possible Al toxicity. Regression analysis showed that stem density and the dominant tree height were significantly correlated with soil pH. There was a significant positive relationship between species diversity index and soil exchangeable K+, Ca2+, and Ca2+/Al3+ ratio (all p values <0.001) and a negative relationship with N, C and P. The results suggest that soil property is a major factor influencing forest composition and structure within the subtropical forest in Okinawa.