Native pedunculate oaks support more biodiversity than non-native oaks, but non-native oaks are healthier than native oaks: A study on street and park trees of a city

Abstract

Trees in cities provide multiple ecosystem services. However, simultaneously ensuring healthy trees with high habitat diversity can be challenging in a harsh urban environment. We compared health, microhabitats, and bat activities between native (Quercus robur L.) and non-native (Quercus rubra L.) oaks growing in different urban habitats (street vs. park) in Karlsruhe, southwestern Germany. We randomly selected 167 oak trees with a diameter at breast height (DBH) >20 cm across the city from Urban Tree Registrar. We performed tree health assessment, dendrometric, and microhabitat inventory. We recorded the four-day bat activities on 45 native and non-native oaks with acoustic loggers installed on the trees. We found that non-native oaks were healthier than native oaks but provided less abundance and richness of microhabitats. Tree size (positive effect) and pruning (negative effect) strongly influence microhabitat richness and abundance. In addition, park trees hosted significantly more microhabitats than street trees. We recorded the activities of 9 bat species from 4 genera. Pipistrellus bats were more active in park trees than street trees. Long-eared bats (Plecotus) were more active near the native than non-native oaks. Bats are likely favored by microhabitats such as fork split, lightning scar, and woodpecker “flute” that are more common in less healthy trees. We conclude that non-native red oak can be planted alongside streets, where the conditions are harsher than in parks to better adapt to climatic changes and stay healthy with less maintenance. The preservation of native pedunculate oak trees, especially within parks, is paramount for urban biodiversity conservation because of their potential to provide microhabitats and supporting bats.