Tar spot disease on Norway maple in North America: Quantifying the impacts of a reunion between an invasive tree species and its adventive natural enemy in an urban forest

The invasion of Norway maple (Acer platanoides) into North American forests is considered to have deleterious effects on native flora richness. One explanation for Norway maple's success is that it may have escaped its natural enemies and thus is less predated upon than its indigenous congener, sugar maple (Acer saccharum). However, Norway maple has been recently reunited with one of its natural enemies from Europe, Rhytisma acerinum, an adventive fungus that causes tar spot disease. The study reported here quantified the impacts of the disease on the growth and survival of Norway maple. In the urban forest of Mount Royal, we compared the growth of maple saplings and trees before the disease was first observed and after tar spot outbreaks of 2006 and 2007. We found a strong link between the outbreak of the disease and a very sharp decline in sapling and tree growth, together with high mortality of Norway maple saplings that could not be attributed to normal senescence or detrimental climatic conditions. While Norway maple usually exhibits higher growth rates than sugar maple, the reverse situation was observed after the epidemics. Our results suggest that the invasion potential of Norway maple could be reduced by the exotic disease.