Plant invasion as an emerging challenge for the conservation of heritage sites: the spread of ornamental trees on ancient monuments in Rome, Italy

Laura Celesti-Grapow,Carlo Ricotta

doi:10.1007/s10530-020-02429-9

Abstract

Cultural heritage sites such as historical or sacred areas provide suitable habitats for plants and play an important role in nature conservation, particularly in human-modified contexts such as urban environments. However, such sites also provide opportunities for the spread of invasive species, whose impact on monuments has been raising growing concerns. The aim of this study was to investigate the patterns of distribution and spread of invasive plants in heritage areas, taking the city of Rome as an example. We focused on woody species as they pose the greatest threat to the conservation of monuments, owing to the detrimental effects of their root system. We analysed changes in the diversity and traits of native and non-native flora growing on the walls of 26 ancient sites that have been surveyed repeatedly since the 1940s. We found that the diversity of the native flora has steadily decreased, while there has been an increase in non-native, larger and more damaging species. The introduced species that have expanded most are ornamental wind- or bird-dispersed trees, which represent a major management problem as their propagules can reach the upper sections of the monuments, where they become more difficult to control. The most widespread and damaging of such species is Ailanthus altissima, which has recently been included among the invasive species of European Union concern (EU Regulation 2019/1262). Our findings show that plant invasion is an emerging challenge for the conservation of heritage sites and needs to be prioritized for management to prevent future expansion.

Highlights

Protected sites, such as monumental or sacred sites, play an important role in nature conservation throughout the world (Bhagwat and Rutte 2006; Verschuuren et al 2010; Gao et al 2013; Frascaroli et al 2016; Woods et al 2017), when they are located in environments that have been markedly modified by man, such as agricultural landscapes (Frosch and Deil 2011)
Our findings show that plant invasion is an emerging challenge for the conservation of heritage sites and needs to be prioritized for management to prevent future expansion
Keywords Ailanthus altissima Á Biodeterioration Á EU regulation on invasive alien species Á Impacts Á Ornamental horticulture Á Urban flora

Summary

Introduction

Protected sites, such as monumental or sacred sites, play an important role in nature conservation throughout the world (Bhagwat and Rutte 2006; Verschuuren et al 2010; Gao et al 2013; Frascaroli et al 2016; Woods et al 2017), when they are located in environments that have been markedly modified by man, such as agricultural landscapes (Frosch and Deil 2011). Larger sites can, owing to their size, heterogeneity and moderate human-mediated disturbance, provide key habitat for a wide range of plants and wildlife, and harbour a rich diversity of species, which often even exceed that found in more intact natural elements of the urban green, such as remnant forests and urban parks (Kowarik et al 2016) Since parts of these structures, such as the tops of high monuments and walls, are difficult and expensive to access to man, some sites provide a refuge for relict populations of species of high conservation value, which have become rare in the surrounding rural environments (Shwartz et al 2014; Ives et al 2016). Smaller sites form a key part of the urban green infrastructure and enhance biodiversity by creating a series of green patches in the urban mosaic or by providing habitat continuity and improving connectivity within the urban ecological network when they stretch over considerable distances in built-up areas, as in the case of ancient city walls (Shwartz et al 2013)

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