Plant-mycorrhiza association in urban forests: Effects of the degree of urbanisation and forest size on the performance of sycamore (Acer pseudoplatanus) saplings

Environmental change as result of urbanisation can influence soil microbial communities in green areas. In cities, forests are among the most frequent types of green areas and provide a wide variety of ecosystem services for the residents, such as air cleaning, nutrient cycling and area for recreational activities. Mycorrhizal fungi play an important role for the establishment of trees and therefore for the natural regeneration of forests. We examined the effect of urbanisation on the arbuscular mycorrhizal (AM) fungal symbiosis and performance of Acer pseudoplatanus saplings in a controlled field experiment in forests situated in the city of Basel and its suburbs in Switzerland. A total of 660 one-year-old A. pseudoplatanus saplings were planted in plots in 11 forests, which differed in degree of urbanisation in the surroundings and size. Saplings including their full root systems were harvested after 4, 6 and 13 months. AM fungal colonisation was 15–45 % lower on roots of saplings growing in forests located in areas with a high degree of urbanisation than that on sapling roots in forests in less urbanised areas. Furthermore, AM fungal colonisation of saplings decreased with decreasing forest size. Sapling survival was not influenced by urbanisation, but by forest size. In particular, sapling survival was reduced in small and medium-sized forests. Sapling survival also increased with increasing percentage of AM fungal colonisation after 13 months. These urbanisation- and forest-size induced changes became more pronounced with the time elapsed since the saplings were planted. Our findings demonstrate the negative impact of urbanisation on AM fungal symbiosis and the performance of A. pseudoplatanus saplings and indicate that urbanisation is a threat for the long-term maintenance of intact small urban forests.