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Variation in insect herbivory across an urbanization gradient: the role of abiotic factors and leaf secondary metabolites
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Edité par CCSD -
International audience. Urbanization impacts plant-herbivore interactions, which are crucial for ecosystem functions such as carbon sequestration and soil fertility. While some studies have reported reductions in insect herbivory in urban areas, the trend is inconsistent and its causes unclear. We conducted a continental-scale study on insect herbivory along urbanization gradients for three European tree species: Quercus robur, Tilia cordata, and Fraxinus excelsior, and further investigated their biotic and abiotic correlates. We quantified insect leaf herbivory and foliar secondary metabolites (phenolics, terpenoids, alkaloids) in 176 solitary trees across eight European cities. Additionally, we collected data on microclimate (air temperature) and soil characteristics (pH, carbon, nutrients) to test for abiotic correlates of urbanization effects directly or indirectly (through plant secondary chemistry) linked to herbivory. Our results showed that urbanization was negatively associated with herbivory for Q. robur and F. excelsior, but not for T. cordata. In addition, urbanization was positively associated with secondary metabolite concentrations, but only for Q. robur. Urbanization was positively associated with air temperature for Q. robur and F. excelsior, and negatively with soil nutrients (magnesium) in the case of F. excelsior, but these abiotic variables were not associated with herbivory. In particular, we did not find indirect effects of abiotic factors on herbivory for Q. robur and F. excelsior. Thus, despite substantial variation in temperature and soil conditions coinciding with variation in herbivory and plant secondary chemistry along the studied urban-to-rural gradients, herbivory patterns appear to respond also to other sources of biotic or abiotic forcing yet to be determined.
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