Persistence of a keystone microhabitat in boreal forests: Cavities of Eurasian Three-toed Woodpeckers (Picoides tridactylus)

Abstract

Tree cavities are microhabitats used by multiple taxa and are considered indicators of forest biodiversity. The factors that affect cavity occurrence and its dynamics are poorly known. We studied tree- and stand-level factors that affect cavity persistence in boreal forests. Cavities of Eurasian Three-toed Woodpeckers (Picoides tridactylus) (n = 654) were surveyed in a 170 km2 area in southern Finland for 31 years during 1987–2017. In total, 447 cavities were lost during the study period: 329 to tree fall or breakage, 72 to cavity damage, and 46 to logging. With the Kaplan-Meier method and Cox proportional hazards models, we analysed which tree- and stand-level factors affected the risk of a cavity to be damaged or lost due to tree fall. The median lifespan of cavities was 10 years, and both tree- and stand-level factors affected the persistence of cavities. Cavities in managed forest areas with low territory occupancy rates were more at risk of tree fall and cavity damage than in other types of forest areas. In all nest tree species, with the exception of European aspen (Populus tremula), the risk of cavity loss to tree fall declined with increasing tree diameter. Moreover, the risk of cavity loss to tree fall was higher in dead trees than in healthy and weakened trees. Cavities in deciduous tree species other than aspen were more at risk of damage than in other tree species. Furthermore, the risk of cavity damage was greater in cavities located lower in a tree. This study showed that factors like the tree species, and size and condition of a tree modify the persistence of woodpecker-made cavities. Consequently, it is likely that these factors influence the value of cavities as microhabitats and how these cavities can be used as indicators of forest biodiversity. Understanding the dynamics of cavities in forest habitats appears critical for the use of cavities as general biodiversity indicators.