The role of alien and native weeds in the deterioration of archaeological remains in Italy

Plants growing on ancient buildings and archaeological remains pose a severe threat to their conservation. Controlling such weeds is costly, and the use of herbicides may lead to serious ecological problems. We surveyed 20 archaeological sites in Mediterranean Italy and estimated the relative effect of alien and native weeds on archaeological remains on the basis of their abundance, frequency, and danger index (DI), which is a measure of the potential damage that each species can cause to buildings and remains. DI is assessed according to plant morphology, vigor, and life-form. The results show that, although alien plant species are potentially harmful, as revealed by their significantly higher DI, they rarely grow on remains, whereas native plants thrive. Despite human disturbance and abundant propagule supply, factors that generally enhance the establishment of aliens, the flora growing on archaeological remains was relatively resistant to alien invasion. Among the aliens observed, only tree-of-heaven turned out to be relatively harmful. Although this invasive tree is not frequent on remains, it is expected to become more widespread on them, as areas surrounding archaeological sites become increasingly urbanized.