Shortage of declining and damaged sun-exposed trees in European mountain forests limits saproxylic beetles: a case study on the endangered longhorn beetle Ropalopus ungaricus (Coleoptera: Cerambycidae)

European mountain forests are unique ecosystems, and they harbour specific saproxylic beetle fauna, including relict species such as the longhorn beetle Ropalopus ungaricus (Herbst, 1784). This endangered species is endemic to European mountain forests and is considered to be monophagous on living sycamore trees (Acer pseudoplatanus L.). Insufficient knowledge of the species ecology limits its effective conservation. Hence, to investigate its habitat requirements, 175 sycamore trees (87 occupied and 88 unoccupied by the species) were surveyed in the Hrubý Jesenik Mts. and Moravskoslezske Beskydy Mts. in northern Moravia (Czech Republic) in 2016. The species strongly preferred trees with reduced vitality, including variously damaged trees. The species was also documented to be much more abundant on sun-exposed trees, and the parts of the trunk mantle (south- and east-facing quarters) with the warmest microclimates were preferred. Surprisingly, trunk diameter seems to be an unimportant characteristic for this species (the diameters of occupied trees varied between 5 and 72 cm). In conclusion, R. ungaricus is a specialised species whose survival depends upon a continuous supply of sun-exposed declining and damaged sycamores in mountain forests. Based on the results of the present study, we suggest concrete conservation measures to support the species, and these will benefit other saproxylic species associated with the same habitat.