Relationship between distribution of large-diameter stems and topography in approximately 65-year-old secondary forest in the Yambaru area of Okinawa Island

Abstract

Because large trees are considered to be important for conservation of subtropical evergreen forest, we surveyed the relationship between distribution of largediameter stems and topography, in a small watershed of secondary forest, approximately 65 years old, in the Yambaru area of Okinawa Island. The density of stems with a diameter at breast height ≥ 30 cm was 91.0 ha^-1 for all species, of which 65.2% were Castanopsis sieboldii, 20.9% were Schima wallichii, and 6.4% were Quercus miyagii. The density of large-diameter stems was less than half that found in a previous study that surveyed a mature non-clear-cut forest in the Yambaru area. We divided the target watershed into three zones (ridge, midslope, and valley)according to topography, and compared the zonal densities and sizes of large-diameter stems. The density of large-diameter stems on the ridge was less than half that in the valley and on the mid-slope. The stem sizes of C. sieboldii and S. wallichii were larger in the valley than on the ridge. Our results indicate that, in approximately 65-year-old secondary forest in the Yambaru area of Okinawa Island, large stems distributed around the valley zone. In addition, the small number of old large-diameter stems that escaped past logging were left in the valley and on the mid-slope. Therefore, the valley and the mid-slope are important topographic zones that might play a key role in ecosystem management and biota conservation, even in secondary forests.