Tree cavity abundance and formation in subtropical evergreen broad-leaved forests on Amami Oshima Island, Japan: Implications for conservation of cavity-using species

Abstract

Tree cavities are a vital microhabitat in forested ecosystems, providing nesting and roosting sites for cavity-using species. The factors regulating tree-cavity formation and abundance vary substantially depending on the climate zone and/ or region, necessitating regional-level investigations. In this study, we investigated tree size, tree species composition, and the numbers of tree cavities in mature and secondary subtropical moist evergreen broad-leaved forests on Amami Oshima Island, Japan, with a focus on differences in cavity characteristics among tree species. Tree cavity abundance was high relative to findings from tropical or subtropical forests in other regions. Nearly all cavities were formed by decay, rather than being excavated by woodpeckers. In large trees with a diameter at breast height (DBH) . 30 cm, the numbers of tree stems, stems bearing a cavity, and cavities were higher in mature forests than in secondary forests. Castanopsis sieboldii was the dominant species in both forest types (48 and 66% of trees . 15 cm DBH in the mature and secondary forests, respectively). Other dominant tree species included Schima wallichii (10.8%) and Distylium racemosum (10.3%) in mature forests and S. wallichii (9.9%) and Pinus luchuensis (7.6%) in secondary forests. We used a generalised linear mixed model to examine the factors affecting the number of decay-formed cavities per stem. The results showed that stem DBH was significantly associated with the number of cavities, and D. racemosum had the greatest number, followed by C. sieboldii and S. wallichii. Almost all cavities (90%) found were in D. racemosum and C. sieboldii, and cavity entrances were significantly higher in D. racemosum. Observations using a charge-coupled device camera showed that cavities were occupied by multiple taxa, including Otus elegans, Garrulus lidthi, and Diplothrix legata. These results demonstrate the value of conserving the limited remaining mature subtropical forest on Amami Oshima Island, as these forests contain large D. racemosum and are vital habitats for cavity-using birds and mammals.