Did larger urban oaks have more tree microhabitats?

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Urban trees provide important habitat structures, but factors associated with tree-related microhabitats (TreMs) may differ from those driving short-term change. Researchers surveyed 165 native pedunculate oaks (Quercus robur) and non-native Northern red oaks (Quercus rubra) in parks and along streets in Karlsruhe, Germany, in 2020 and 2024.

Generalized additive models (GAM) showed that baseline 2020 TreM richness and abundance increased strongly with tree diameter and had nonlinear associations with crown dieback. Visible cut branches, used as a proxy for tree care intensity, were negatively associated with baseline 2020 TreM abundance, though this association was less robust to individual observations than the effects of size and crown dieback. Models using baseline 2020 data explained 46.8% of deviance in TreM richness and 47.4% in abundance. By contrast, models of four-year change explained only 9.6% of deviance in richness and 7.9% in abundance, indicating weak predictability from baseline size, crown condition, and pruning change.

Ecologically important TreMs, including rot holes, cavities, dendrotelms, and saproxylic fungi, changed significantly in ways not captured by general richness and abundance measures. For example, rot holes and hollow cavities increased from 3.6% to 15.2% of trees, while woodpecker breeding cavity levels stayed largely the same. The researchers say studies of TreM dynamics should focus on changes in key microhabitats and urban areas as well as richness and abundance. They strongly recommend retaining large, habitat-bearing trees where safety permits, combining routine tree inspections with repeated TreM assessments, and avoiding unnecessary removal of habitat structures.