“Lifespan” of non-excavated holes in a primeval temperate forest: A 30 year study

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Numerous forest organisms rely on non-excavated tree-holes, which are often limited in managed European woods. Holes’ supply depends on formation rates and persistence times. I use 30years of data from a primeval forest (strictly protected reserve, Białowieża National Park, Poland) to determine how long non-excavated holes persist, whether their persistence varies across forest types, tree species and conditions, or bird species using them. From 1979 to 2010, I followed the fate of 1794 holes, used by 14 non-excavator bird species. Almost half of them were destroyed during this time, most often (40%) due to tree fall or break off of the hole-bearing section, growing over injury (28%) or decay of walls (24%). Holes were retained for a median of 12years, longer in deciduous habitats (11–13years) than in coniferous forests (4.4years). These differences were due to different sets of tree species used in different habitats. Lifespan of holes varied by tree species, ranging from 5 (Populus tremula) to 16.5years (Quercus robur). Persistence was much lower for holes in dead (5years) than in living (14years) substrates. It increased with increasing tree size. Differences in persistence times of holes used by various bird species was mostly a by-product of them selecting trees of different qualities. Holes in large living trees, with relatively hard wood, persisted longest. Their retention should become a conservation priority. Current forestry policies should be modified, to assure that hole-bearing trees are retained, and trees in which replacement holes could be formed are maintained.