Occurrence probabilities of tree cavities classified by entrance width and internal dimensions in hardwood forests in Hokkaido, Japan

To estimate cavity availability for species with diverse cavity requirements, we recorded cavities in eight deciduous hardwood forests in Hokkaido, Japan, and classified them into two categories based on their entrance width (small, 2.5–5 cm; large, ≥5 cm). In two of the forests, we inspected cavity interiors and classified cavities into three further categories based on their internal dimensions (shallow, moderate, and deep). We then estimated the occurrence probabilities of cavities of different sizes at the individual-tree level. Although the cumulative occurrence probabilities of small and large cavities increased with increasing diameter at breast height (DBH) for all tree species, the occurrence probabilities of large cavities were <0.08 at 40 cm DBH except for 0.20 in Acer mono and 0.22 in snags. Snags were more likely than live trees to have cavities especially at smaller DBH. The cumulative probabilities that cavities were classified as deep or moderate were positively correlated with DBH and cavity entrance width, and negatively correlated with height of cavity entrance above ground. The variation in occurrence probabilities among cavities of different sizes shows that the classification of cavities by both entrance width and internal dimensions contributes to reliable estimates of cavity availability for different cavity users.