Woodpecker cavity establishment in managed forests: relative rather than absolute tree size matters

What kind of tree should be preserved from logging for biodiversity conservation is a matter of debate. Large and old trees are potential candidates due to the structures they can bear, like cavities and other tree-related microhabitats (TreMs). One of the most studied TreM is woodpecker-made cavities, which, in addition to be breeding sites for primary cavity-nesters, are often the main breeding sites for secondary cavity-nesters, especially in managed forests. Therefore, understanding which trees woodpeckers select for cavities is relevant to forest management, especially in management regimes where individual trees are logged or spared, as in retention forestry. We used data from a forest inventory, TreM inventory and woodpecker counts in one-hectare plots in the Black Forest (southwest Germany) to investigate which features make a retention tree suitable for woodpeckers. By employing a resource selection probability function, we tested several variables for their influence on the probability of tree choice by woodpeckers including altitude, tree species, TreM richness and abundance, diameter at breast height (DBH) and deviation from the mean DBH per plot. The results show that the probability of selection by woodpeckers does not correlate with individual tree diameter. Instead, the probability is driven mainly by the deviation from the mean DBH per plot. We were able to identify a relative size for the selection of trees indicating that woodpeckers prefer trees that are about 15-20cm larger than the mean DBH per plot. Thereby, we argue, that using absolute diameter thresholds to select retention might not be the best management solution in the short-term, as in managed forests woodpeckers might select sub-optimal trees. Apparently, more knowledge concerning relative thresholds, as detected in our study, is required to improve our understanding of the potential ecological value of retention trees.