Checklist of Ornamental Trees, Shrubs, and Succulents of Apulia (Southern Italy)

Abstract

In this study we focused on the need to fill a knowledge gap among Italian botanical studies namely that of ornamental species census. In particular, we addressed one of the regions in southern Italy with less knowledge in the field of such studies and with an obvious presence of non-native species. A widespread census of the Apulian territory was carried out between 2021 and 2024 in both urban and suburban areas including street trees, parks, and private and historic gardens. The inventory of ornamental trees, shrubs, and succulents of Apulia (southern Italy) was carried out in six provinces, i.e., Bari, Barletta-Andria-Trani, Brindisi, Foggia, Lecce, and Taranto. The checklist comprises 287 taxa (including 265 species, 6 varieties, 5 subspecies, and 11 forms) included in 179 genera belonging to 78 families. We evaluated the number of taxa per families and genera, the presence of each taxa in the provinces of Apulia, the number of taxa per occurrence status, growth forms, geographical origin, and the number of native and alien taxa and also the artificial hybrids. Remarks of the most significant taxa and evaluation of geographical distribution in Italy were also taken into consideration. A large number of surveyed taxa (51.74%) are comprised in the list reported in the recent study on allochthonous vascular flora in Italy with a marked prevalence of Neophyte Casual Alien and Neophyte Naturalized Alien species.

1. Introduction

Ornamental plants are important elements of urban areas useful in improving air quality, water quality, climate regulation, nutrient recycling, and pollination. Within urban parks and historic gardens, they also play a cultural role by providing recreational opportunities for citizens resulting in improved well-being [1].

Ornamental plants are also used in the form of city street trees for aesthetic beauty but also to help reduce air pollution and temperature by shading asphalt and buildings with a direct impact on Urban Heat Islands [2]. Some of these ornamental species are of particular aesthetic value and, at times, of considerable age and therefore considered by citizens as part of the urban landscape.

On the other hand, the prevalence of alien ornamental species over native species leads to a number of critical issues that have been studied by several authors [3,4,5]. Many of the invasive plants recorded in Italy were intentionally introduced for ornamental reasons than for any other purpose.

The number of allochthonous species is gradually and steadily increasing, and the phenomenon of biological invasions is known to cause damage to biodiversity, ecosystem services, human health, and the economy [6,7]. This is in line with the growing trend of biological invasions resulting from the intentional or accidental introduction of species as ornamentals by humans into areas outside their native range. Anthropogenic factors play a key role in the establishment and subsequent spread of alien species. Emblematic is the example of Ailanthus altissima (Mill.) Swingle, a tree that belongs to the large group of ornamental trees dispersed by wind or birds, which is an important management problem for the conservation of cultural heritage sites [4]. Also not to be underestimated is the spread of novel plant diseases through the introduction of alien ornamental species both in nurseries and within urban centers [8,9].

However, the analysis of urban street trees is often overlooked, although they are key green elements within cities, because of their specific ecological and socio-economic functions [10]. In Italy, previous studies have dealt marginally with ornamental species, and some references can be found in papers dealing more generally with allochthonous vascular flora or alien species [11]. On the contrary, reports from other countries demonstrated a wide interest in ornamental plants as a source of multiple ecosystem services, including their utility in the phytoremediation of contaminated soil, air, and water [12], their capacity to remove volatile organic compounds from indoor air [13], and their role as sinks and bioindicators [14], as well as for the analysis of the toxicity of many ornamental shrubs used for their aesthetics due to accidental ingestion [15], etc.

In 2021, we launched a census of ornamental plants in Apulia, a region in southern Italy, with the aim of providing an inventory of trees, shrubs, and succulents grown in street trees, parks, private gardens, and historic villas.

The unpublished data collected during the census are the subject of this survey which provides an important contribution to the knowledge of an important aspect of the alien and native flora of Apulia.

A further purpose of this research is to highlight the presence of ornamental species in the Apulian territory, an under-researched area of botanical studies in the region. The objective of this study is to provide a list as close to reality as possible of the ornamental species in Apulia that will allow subsequent assessments by local governments of the aesthetic value of the plants but also of the possible risks related to invasiveness and public health mainly in terms of increased allergies.

2. Results

A total of 287 taxa (including 265 species, 6 varieties, 5 subspecies, and 11 forms) included in 179 genera belonging to 78 families were surveyed in the six provinces of the Apulia region, i.e., Bari, Barletta-Andria-Trani, Brindisi, Foggia, Lecce, and Taranto.

The highest number of taxa (Figure 1) is found in the families Rosaceae (18 taxa), Fabaceae (17 taxa), Asparagaceae (16 taxa), Malvaceae (14 taxa), Cupressaceae (13 taxa), Pinaceae (13 taxa), Arecaceae (12 taxa), Oleaceae (12 taxa), Lamiaceae (10 taxa), Fagaceae (8 taxa), Solanaceae (6 taxa), Bignoniaceae (5 taxa), Rutaceae (5 taxa), Ulmaceae (5 taxa), Anacardaceae (4 taxa), Apocynaceae (4 taxa), Berberidaceae (4 taxa), Euphorbiaceae (4 taxa), Lauraceae (4 taxa), Poaceae (4 taxa), and Sapindaceae (4 taxa).

Figure 1. Number of taxa per family based on taxa surveyed in the Apulian territory.

The largest number of taxa is found in the genera Quercus L. (eight taxa), Prunus L. (seven taxa), Tamarix L. (seven taxa), Pinus L. (six taxa), Asparagus Tourn. ex L. (five taxa), Ficus Tourn. ex L (five taxa), Ligustrum L. (five taxa), Ulmus L. (five taxa), Brachychiton Schott and Endl. (four taxa), Citrus L. (four taxa), Euphorbia L. (four taxa), Salvia L. (four taxa), and Yucca L. (four taxa).

A total of 31.2% (90 taxa) of the taxa listed in Appendix A Table A1 are found in all six provinces where the census was conducted, while 103 taxa (35.76%) have only been surveyed in one province.

Bari (264 taxa) is the province in Apulia with the largest number of taxa surveyed (Figure 2) followed by Lecce (151 taxa), Foggia (143 taxa), Brindisi (142 taxa), Barletta-Andria-Trani (141 taxa), and Taranto (133 taxa).

Figure 2. Number of taxa surveyed in each Apulian province.

A large number of the surveyed taxa (51.74%) are included in the list reported in the recent study on allochthonous vascular flora in Italy [16]. According to Galasso et al. [17], most of them fall into the category Neophyte Casual Alien (N CAS) (52 taxa) followed by Neophyte Naturalized Alien (N NAT) (43 taxa), Neophyte Invasive Alien (N INV) (24 Taxa), Archeophyte Naturalized Alien (A NAT) (9 taxa), Neophyte Invasive Alien Feral (N INV FER) (5 taxa), Archeophyte Invasive Alien Feral (A NAT FER) (5 taxa), Taxonomically Doubtful Neophyte Naturalized Alien (T N NAT) (3 taxa), Neophyte Naturalized Alien Feral (N NAT FER) (2 taxa), Archeophyte Casual Alien Culton (A CAS CLT) (2 taxa), Neophyte Unclear Alien Status (N NC) (2 taxa), Naturalized Data Deficient N DD (2 taxa), Neophyte Casual Alien Culton (N CAS CLT) (1 taxa), and Neophyte Casual Alien Naturalized (N CAS NLT) (1 taxa) (Figure 3).

Figure 3. Number of taxa per occurrence status (sensu Galasso et al. [17]) based on taxa surveyed in the Apulian territory.

Based on what can be gleaned from “Plants of the World Online” edited by the Royal Botanic Gardens, Kew, among the plants surveyed, scapose phanerophytes (P scap) clearly predominate followed by caespitose phanerophytes (P caesp) (Figure 4) while there is a clear prevalence of Asian, African, and Central and South American geographic origins over European ones (Appendix A Table A2). From Appendix A Table A2, it is possible also to see the clear predominance of alien species (217 taxa) over native species (60 taxa) and 10 artificial hybrids (AH).

Figure 4. Number of taxa per biological form categorized according to Plants of the World Online (POWO 2024).

Most of the species surveyed are also commonly used as ornamentals in the other regions of Italy.

Taxa at the upper limit of their distribution in Italy are Bauhinia variegata L. var. candida Voigt, Brachychiton sp. pl., Brahea armata S. Watson, Ceiba speciosa (A. St.-Hil., A. Juss., and Cambess.) Ravenna, Erythrina crista-galli L., Erythrina herbacea L., Euphorbia ingens E. May. ex Boiss, Euphorbia murielii N.E.Br, Euphorbia tirucalli L., Ficus sp. pl., Hibiscus ×rosa-sinensis L., Jacaranda mimosifolia D. Don, Musa ×paradisiaca L., Persea indica (L.) Spreng, Sabal palmetto (Walter) Lodd. ex Schult. and Schult.f., Sideroxylon spinosum L., and Yucca rostrata Engelm. ex Trel.

Erythrina crista-galli and Jacaranda mimosifolia are species of high ornamental value that are fairly common but have only been expanding in southern Italy for a few years and, moreover, only locally.

Widespread in the Apulian provinces is the use of plants providing both productivity and ornamental value such as Annona cherimola Mill., Citrus ×limon (L.) Osbeck, Citrus reticulata Blanco, Citrus ×sinensis (L.) Osbeck, Corylus avellana L., Feijoa sellowiana (O. Berg) O. Berg, Ficus carica L., Juglans regia L., Musa ×paradisiaca L., Punica granatum L., Prunus domestica L., and Prunus dulcis (Mill.) D.A. Webb.

Moreover, Sideroxylon spinosum is of particular botanical interest providing ecological services and socioeconomic value. Its fruits provide an edible and marketable oil for cooking, cosmetic, and medicinal purposes. The vigorous tree cultivated in the University Campus of Bari is one of the few plants to date grown in Italy [18].

Another plant of application interest is Euphorbia tirucalli whose oil obtained from the modified stems and leaves is a valuable alternative energy source for biofuel production in some Arab and African countries [19].

Finally, extensive use of species of the genus Quercus L. is noted. Quercus ilex L. is widely used in the streets of the city of Lecce where the crowns of the trees make contact with each other forming tunnels that provide coolness on hot days for citizens. In the city of Lecce, it is also possible to observe a monumental tree of Quercus ithaburensis subsp. macrolepis (Kotschy) Hedge and Yalt, (Figure 5), a species found in Italy only in Apulia and more precisely in Salento, mainly in the province of Lecce, and to a lesser extent in the province of Bari and Brindisi. This is a relict species from an old botanical garden that was dismantled in the early 20th century. Since 1993, Q. ithaburensis subsp. macrolepis is included in the Red List of the endangered botanical species in Italy. At the international level, the need to establish reserves to safeguard its genetic heritage has been recognized.

Figure 5. Monumental tree of Quercus ithaburensis subsp. macrolepis in the city of Lecce.

Trees of Quercus pubescens Willd. are used as ornamentals in the cities of Bari and Foggia.

Instead, Quercus cerris L., Q. petraea (Matt.) Liebl, Q. trojana Webb, Q. robur L., Q. suber L., and Q. trojana Webb are used in the newly planted gardens in the city of Bari.

In the cities of Bari and Taranto, among the newly planted species (Figure 6), we found Liquidambar styraciflua L., a medium-to-large tree, which is an excellent choice as it can provide protection from the sun’s rays in summer. Also, from a decorative point of view, it provides distinctive color effects in autumn thanks to the golden-yellow, orange, or red color of its leaves. In addition, it is a fast-growing species that does not require special care. On the contrary, the choice to plant Pyrus calleryana Decne is open to criticism since in Canada it is considered an invasive species, demonstrating the not-always prudent choices made by municipal administrations.

Figure 6. Pyrus calleryana (left) and Liquidambar styraciflua (right), two species widely used as ornamentals along the streets of Bari and Taranto.

Finally, among the various palms of ornamental use, Syagrus romanzoffiana (Cham.) Glassman is widely used in towns along the Apulian coast, as it is partially resistant to marine aerosols, and particularly in Barletta where it characterizes one of the main streets of the city (Figure 7).

Figure 7. An unusual row of Syagrus romanzoffiana along the streets of Barletta.

From the comparison made on the Portal to the flora of Italy 2024.2 https://dryades.units.it/floritaly/ (accessed on 15 June 2024) with other Italian regions, the following interesting results emerged: 111 taxa (38.5%) out of 288 taxa surveyed are new to Apulia, meaning by new taxa that those taxa were erroneously reported for Apulia previously but are now confirmed by us or other taxa that have so far escaped reporting because of poor exploration of the Apulian territory or because they have not yet begun the process of spontaneization. Cedrus libani A. Rich, Phoenix dactylifera L., Photinia serratifolia (Desf.) Kalkman cv. Red Robin, Phyllostachys nigra (Lodd. ex Lindl.) Munro, Picea abies (L.) H. Karst., Pinus halepensis Mill. subsp. brutia (Ten.) Holmboe, Platanus ×hispanica Mill. ex Münchh. Portulacaria afra Jacq. Prunus cerasifera Ehrh, and Schinus terebinthifolia Raddi are new reports for Apulia.

Abies alba Mill., Berberis vulgaris L., Crataegus rhipidophylla Gand., Myoporum laetum G. Forst., Platanus orientalis L., Quercus petraea (Matt.) Liebl., and Solanum laciniatum Aiton were indicated as erroneous or doubtful reports for Apulia in the Portal to the flora of Italy 2024.2 and were found by us in Apulian territory. A scattered distribution in Italy is noted for Agave sisalana Perrine (Apulia, Sicily, and Sardinia), Asparagus africanus Lam. (Tuscany and Apulia), Polygala myrtifolia L. (Apulia, Liguria, Sicily, and Sardinia), Quercus trojana Webb and Syagrus romanzoffiana (Cham.) Glassman (Apulia and Basilicata), Prunus webbii (Spach) Vierh (Apulia and Sicily), Vachellia farnesiana (L.) Wight and Arn. (Apulia, Calabria, Sicily, and Sardinia), and Aloe arborescens Mill. and Cestrum parqui (Lam.) L’Her. (distributed only in south Italy).

Viburnum rhytidophyllum Hemsl., previously reported only for northeast Italy, and Pyrus calleryana Decne, so far known only for Emilia-Romagna, represent new reports for Apulia that extend the distribution of these taxa to southern Italy as well.

Finally, it is confirmed that only in Apulia is there the presence of Cereus repandus (L.) Mill. and Cistus ×purpureus Lam.

3. Discussion

More recent botanical studies have neglected the investigation on ornamental plants despite the fact that many plants introduced within Botanical Gardens over the past centuries have subsequently spread throughout the Italian regions such as, for example, Cedrus deodara (Roxb. ex D. Don) G. Don, Chrysojasminum fruticans (L.) Banfi, Citrus reticulata, Eriobotrya japonica (Thunb.) Lindl, and Parthenocissus quinquefolia (L.) Planch, and some others have become spontaneous such as Agave americana L., Robinia pseudoacacia L., and Ailanthus altissima (Mill.) Swingle, coming to characterize the landscape of vast areas of Italian territory.

Our study highlights the abundant presence of ornamental plants in comparison with native species (Figure 8 and Figure 9) not only within urban centers but throughout the region of Apulia. The checklist is an unpublished contribution to the knowledge of ornamental plants in an Italian southern region such as Apulia, which, due to its geographical location and land characteristics, hosts a rich contingent of ornamental species from different geographical origins.

Figure 8. Ornamental plants surveyed in private gardens, hospitals, and streets of the city of Bari. (a) Abies cephalonica, (b) Aesculus hippocastanum, (c) Brachychiton rupestris, (d) Melaleuca citrina, (e) Chamaecyparis lawsoniana, (f) Camphora officinarum, (g) Cupressus cashmeriana, (h) Erythrina crista-galli, and (i) Ficus maclellandii.

Figure 9. Ornamental plants surveyed in private gardens, hospitals, and streets of the city of Bari. (a) Lagunaria patersonia, (b) Ligustrum ovalifolium, (c) Metrosideros excelsa, (d) Ulmus parvifolia, (e) Phytolacca dioica, (f) Prunus domestica, (g) Prunus cerasifera subsp. pissardii, (h) Quercus suber, and (i) Rhaphiolepis indica.

The high percentage of alien species (more than 50%) detected among those surveyed is another important finding to assess the role of such species used as ornamentals, especially in cities. In fact, with the funding of the National Recovery and Resilience Plan (NRP), the focus on alien species is back on the agenda, and efforts are being made to raise awareness of the risks of the introduction and spread of invasive alien species in our country with a focus on the most correct practices, both in terms of production and gardening activities, to limit/contain the risk.

Apart from Acacia saligna (Labill.) H.L. Wendl., Ailanthus altissima (Mill.) Swingle, and Robinia pseudoacacia L., by now invasive in most of the Italian territory, among the taxa surveyed in this survey, there are no cases that would let us assume with certainty, to date, the transformation into invasive aliens. Some doubt is actually left by Parkinsonia aculeata L., a species rapidly expanding in the Apulian territory as highlighted by Pardi et al. [20] and is already reported as an invasive alien also in Sicily and Sardinia by the Portal to the flora of Italy.

The checklist of ornamental trees, shrubs, and succulents of Apulia becomes also a useful tool for municipal governments to direct future choices within cities with particular reference to the health of inhabitants. For example, many of the ornamental plants are found near schools and within public gardens frequented by children and the elderly, two of the categories most exposed to the growing problem of allergies.

The most well-known ornamental plants considered as responsible for pollinosis are Cupressus sp. pl., Hesperocyparis sp. pl., Pinus sp. pl., Olea europaea L., Quercus ilex L., and Populus sp. pl., all plants that are widely used within Apulian cities. Also of wide use are some poisonous plants such as Melia azedarach L., Nerium oleander L., Nicotiana glauca Graham, and Thuja occidentalis L., as well as plants causing irritation to the eyes and hands due to the presence of latex in the leaves and stems (Euphorbia sp. pl. and Ficus sp. pl.). In addition, the wide use as ornamental of Brachychiton spp. and Lagunaria patersonia (Andrews) G. Don, due to the presence of stinging hairs inside the fruits, exposes citizens to damage on the mucous membranes of the eyes and mouth.

For what has been stated above, we can say that in most cases the choices made by the Administrations in term of ornamental plants are dictated by the aesthetics of the plants and the availability of them in nurseries rather than by a careful evaluation carried out with the professional support of botanists.

4. Materials and Methods

Periodic observations on the presence of ornamental trees, shrubs, and succulents in street trees, parks, and private and historic gardens of the six provinces of Apulia region, i.e., Bari, Barletta-Andria-Trani, Brindisi, Foggia, Lecce, and Taranto, have been carried out. Each province was visited twice in spring and autumn between 2021 and 2024. The names of the plants are listed alphabetically, and the presence within the province is indicated with an asterisk. The checklist comprises also hybrids and cultivars. The binomials and trinomials follow the database Plants of the World Online (https://powo.science.kew.org/) of the Kew Royal Botanical Gardens. The occurrence status of each taxon refers to Galasso et al. [17]. Recorded taxa are arranged in Appendix A Table A1 which comprises the binomial and trinomial, the family to which it belongs, and the presence/absence in the investigated provinces indicated, respectively, with an asterisk (★) and with a hyphen (-). An additional Appendix A Table A2 is present in which each census taxon is matched with its geographic origin, biological form, and alien or native species status. All of this information was obtained from the database Plants of the World Online (https://powo.science.kew.org/) of the Kew Royal Botanical Gardens. In Appendix A Table A1, in addition to species in sensu strictu, the forms, varieties, hybrids, and cultivars are included. Among the latter, as highlighted in Appendix A Table A2, artificial hybrids are also indicated as AH. The Portal to the flora of Italy 2024.2 https://dryades.units.it/floritaly/ (accessed on 15 June 2024) was used to compare the occurrence of the taxa we surveyed with other Italian regions. Taxa that are either new to Apulia in the above-mentioned Portal or erroneously reported for Apulia previously but confirmed by us, and other taxa that have so far escaped reporting due to the scarce exploration of the Apulian territory, or because they have not yet begun the process of spontaneization, have thus been marked with the symbol (●) in Appendix A Table A1.

5. Conclusions

In conclusion, our study makes a relevant contribution to the advancement of botanical studies in the Apulia region, being the first census on ornamental species. Even at the national level, such a type of census is so far very limited The data reported in this study are useful in noting the high rate of alien species in the Apulian territory resulting from often illogical choices made by municipal administrations that, without taking into account the possible problems related to biological invasions and the health of the most fragile citizens such as children and the elderly, leveraging only the material readily available in regional and extra-regional nurseries, they introduce species of all types and origins without consulting botanical experts. Our hope is that this census can be a useful basis for discussion with local authorities for a future public green policy based on rational choices also shared by citizens.