Botanical gardens in city centres are green islands, offering a sanctuary for diverse plant and fungal species amidst urban landscapes. Although lichens are not usually a primary focus in botanical gardens, these spaces can serve as important refuges. In this work, the epiphytic lichen biota has been investigated in the Botanic Garden of Pisa, to uncover the hidden elements of biodiversity of botanical gardens in terms of non-target organisms. A total of 57 epiphytic lichen taxa and one non-lichenized fungus were recorded, including several threatened species. Two species, Arthopyrenia platypyrenia and Coenogonium tavaresianum, are new records for Tuscany, while Lecania cyrtellina is so far known in the region as occurring only in the Botanic Garden of Pisa. Lichen colonization reflects the richness of tree species, while the diffused presence of nitrogen-tolerant lichens could be associated with urban environmental conditions. Comments on Arthopyrenia platypyrenia, Coenogonium tavaresianum, Gyalecta liguriensis, Lecania cyrtellina, Lecanographa amylacea, Micarea misella, Porina borreri, and Waynea stoechadiana are given.

Biodiversity, Botanic Garden of Pisa, Epiphytic lichens, Species conservation, Tuscany

Botanical gardens are exceptional reservoirs of biodiversity, hosting not only plants intentionally cultivated and conserved but also a wide range of other organisms (e.g., lichens, fungi, bryophytes), which complement to the ecological complexity and richness of these specific environments (Ravera et al. 1999; Aptroot and Honegger 2006). Often located in town centres, they play a vital role as cultural and ecological hubs, serving as green refuges that mitigate urban heat and provide crucial climate-regulating benefits (Blackmore and Oldfield 2017; Cannon and Kua 2017; Primack et al. 2021; D’Antraccoli et al. 2023a). Botanical gardens support a variety of trees, including rare species (Jackson et al. 2001; Stevens 2007), providing a wide range of physical and chemical properties, which at once, create habitats that encourage the growth of epiphytic lichens.

Italy played a pivotal role in the establishment of botanical gardens, with its rich tradition of medicinal plant cultivation in monasteries and medical schools evolving into more systematic physic gardens during the 16^th Century. According to Chiarugi (1953), Pisa (1543), Padua (1545, July), and Florence (1545, December) pioneered this movement, soon followed by significant botanical gardens in Bologna, Palermo, Naples, Rome, and other cities. These early academic institutions became essential centres for scientific study and the cultivation of medicinal plants, advancing medicine, botany, and global plant exchange, and reflecting Italy’s leadership in botanical innovation during the Renaissance (Spencer and Cross 2017). While lichens are not typically a central focus in botanical gardens, historical records reveal occasional documentation of their presence. For example, notable specimens from the Botanic Garden of Pisa, currently preserved in the Herbarium of the University of Trieste (Martellos et al. 2023) and the Herbarium of the University of Pisa (see also Roma-Marzio et al. 2020, 2023 for details on digitized specimens) date back to 1862 (Arthonia galactites (DC.) Dufour collected by P. Savi and O. Beccari [PI067228, TSB25884]), 1882 (Alyxoria varia (Pers.) Ertz & Tehler [PI067229, TSB25514] and Lecanora horiza (Ach.) Linds. [TSB25512, TSB25513] collected by G. Arcangeli) and 1883 (Alyxoria varia [TSB25535, TSB25536] collected by A. Mori), offer valuable insights into the diversity that was present there in the past. Another herbarium specimen collected in that period in 1882 by G. Arcangeli refers to Lecanora subfusca (L.) Ach. var. diffusa Arcang. [PI067226, PI067227], a variety described from the Botanic Garden of Pisa, which grew on a ca. 100-year-old Ginkgo biloba L., and was also included in a series of exsiccata (Baglietto, Cesati & De Notaris, Erb. Critt. Ital. Ser. II, no. 1267). Early noteworthy observation of lichens was also made in the Botanical Garden of Florence by Cengia-Sambo in 1928, who reported the presence of an unidentified Lecanora species growing on the leaves of a 14-year-old Syagrus coronata (Mart.) Becc. cultivated in a warm and humid greenhouse (Cengia-Sambo 1928). In recent years, the diversity of lichens in botanical gardens across Italy has received increasing attention. Some studies have explored urban environments, such as the Botanical Garden of Rome, Lazio (Bartoli 1990; Ravera et al. 1999, 2003), as well as more natural sites as the Botanical Garden of the Apuan Alps “Pellegrini-Ansaldi” in Tuscany (Loppi and Putortì 2001) and the Garden of the Apennine Flora in Capracotta, Molise (Genovesi and Ravera 2014).

The Botanic Garden and Museum of the University of Pisa (hereafter Botanic Garden of Pisa) currently hosts more than 2,000 species in cultivation, grouped in about 30 collections (D’Antraccoli et al. 2024). The localization and the metadata associated with each plant specimen are accessible through the public version of the botanic garden’s plant documentation system, U-plant DISCOVER (https://uplantdiscover.sma.unipi.it/). One of the most representative collections of the botanic garden is the Arboretum, consisting of about 200 species in cultivation (D’Antraccoli et al. 2023a). The oldest trees are a specimen of Ginkgo biloba L. and one of Magnolia grandiflora L., both planted in 1787 by Giorgio Santi (Savi 1828). The garden also hosts other noteworthy trees, including specimens of Myrtus communis L. and Platanus hispanica Mill. ex Münchh. (both over 200 years old), as well as specimens of Araucaria bidwillii Hook., Chamaerops humilis L., Cinnamomum camphora (L.) J.Presl, Jubaea chilensis (Molina) Baill., Quercus virginiana Mill., Taxus baccata L., Tilia platyphyllos Scop., Washingtonia filifera (Gloner ex Kerch., Burv., Pynaert, Rodigas & Hull) de Bary, and Wisteria sinensis (Sims) DC. These trees, introduced during the 19^th century, range from 100 to 200 years (D’Antraccoli et al. 2022).

The aim of this study is to explore the epiphytic lichen diversity in the Botanic Garden of Pisa, to uncover the hidden biodiversity of botanical gardens in terms of non-target organisms. No published studies have specifically addressed the lichen biota in the Botanic Garden of Pisa, aside from the historical herbarium specimens mentioned earlier. By investigating this overlooked aspect, we also aim to contribute new and interesting data on the distribution of lichens in Tuscany.

A total of 57 epiphytic lichen taxa and one non-lichenized fungus (Table 1), belonging to 53 species and subspecies within 40 genera were recorded.

Epiphytic lichens were surveyed in both indoor and outdoor environments of the Botanic Garden. However, so far, no lichens were found on trees cultivated in greenhouses, despite several of them being examined carefully (namely Brachychiton acerifolius (A.Cunn. ex G.Don) F.Muell., B. discolor F.Muell., B. rupestris (T.Mitch. ex Lindl) K.Schum., Cycas taiwaniana Carruth., C. circinalis L., Ficus elastica Roxb. ex Hornem., Strelizia nicolai Regel & K.Koch, Sterculia balanghas L.). Only a colonization by green algae was noticed on some of the barks (especially on Cycas taiwaniana and Ficus elastica). All recorded lichens were found in outdoor conditions with varying levels of colonization across different trees. Sparse lichen growth, with thalli appearing sporadically or in minimal coverage, was observed on trees of the genera Camellia, Cedrus, Citrus, Sequoia, Viburnum, and Wisteria. No lichen colonization was observed on the bark of Araucaria bidwillii, a tree that has stood in the garden for 152 years. In contrast, species like Aesculus pavia, ×Chitalpa tashkentensis, Melia azedarach, Phillyrea angustifolia, Pyrus communis, and Vitex agnus-castus exhibited a high degree of colonization, particularly in terms of diversity, with more than ten lichen species identified from the individual trees.

According to the red list of epiphytic lichens of Italy (Nascimbene et al. 2013), three lichen species are considered Vulnerable, one is included in the category Near Threatened, two in Least Concern and other two in Data Deficient IUCN categories (Table 1). Two species, Arthopyrenia platypyrenia (Nyl.) Arnold and Coenogonium tavaresianum (Vězda) Lücking, Aptroot & Sipman, are newly recorded for the lichen biota of Tuscany, and Lecania cyrtellina (Nyl.) Sandst. is so far known only from the study area (Ravera et al. 2024). Additionally, the record of Arthopyrenia platypyrenia is the second for Italy.

The lichen biota in the Botanic Garden is mainly composed by acidophilous/oligotrophic species (about 70%), overall reflecting the availability of acidic (bark) substrates. Nevertheless, the wide distribution of species tolerant to eutrophication, or even enhanced by the deposition of dust and nitrogen compounds (such as Hyperphyscia adglutinata, Physcia adscendens, Candelaria concolor, and Xanthoria parietina, early colonizers of eutrophicated substrates), well reflect the influence of urban atmospheric conditions, especially in more open conditions. This phenomenon is particularly evident on the bark of conifers, which in their natural environment are generally colonized by acidophilous epiphytes, but in the Botanic Garden of Pisa are more often colonized by nitrophilous lichens. Lichen colonization is influenced by various interacting factors, such as light availability, relative humidity, bark chemistry, water-holding capacity, health status of the substrate and environmental pollution (Hauck 2005; Loppi and Baragatti 2011; Ellis 2012). Some of them are directly shaped by the characteristics of the individual tree hosting the lichen, as well as the composition of the surrounding vegetation (Petersson et al. 2022 and references therein). Consequently, a condition with a greater diversity of tree species and structural features (such as in even aged forests) often supports higher lichen species richness (Petersson et al. 2022). The same could apply also to botanical gardens. In general, the diverse range of trees, with their physical and chemical properties, creates various habitats that support the growth of epiphytic lichens thriving on their surfaces (e.g. Käffer et al. 2016). In fact, there is evidence that the lichen biota in a botanical garden is proportionally richer than that of the surrounding environment, especially in urban contexts (Aptroot and Honegger 2006; Wilkie 2020).

On the other hand, along with nitrophilous species commonly found in urban areas, the Botanic Garden of Pisa hosts several species, whose presence is typically associated with more natural environments. Among them, Coenogonium tavaresianum, Gyalecta liguriensis, Lecanographa amylacea, and Porina borreri are also recognized as indicators of undisturbed forests and ancient woodlands (Nimis 2024) and were observed on some of the garden’s monumental trees, including Ginkgo biloba, Magnolia grandiflora, Cinnamomum camphora, and Quercus virginiana. Additional species like Catapyrenium psoromoides, Micarea adnata, M. misella, M. prasina, Opegrapha niveoatra, Porina aenea, Pyrenula chlorospila, and Waynea stoechadiana exhibit affinities for natural and semi-natural habitats (Nimis 2024). Notably, some of them, including C. tavaresianum, G. liguriensis, L. amylacea, M. prasina, O. niveoatra, and W. stoechadiana, have also been recorded in the Botanical Garden of Rome (Ravera et al. 1999), highlighting the role of botanical gardens as alternative habitats for species with specific ecological demands. Botanical gardens in city centres serve as vital green islands, offering a sanctuary for diverse plant and fungal species amidst urban landscapes. Their importance as refugia for biodiversity can be also exemplified by the New Botanical Garden of the University Zürich: despite a relatively small area (53,000 m²), it hosts approximately 10% of all lichen species recorded in Switzerland (Aptroot and Honegger 2006).

In the following section there are specific notes on several species of concern found within the Botanic Garden of Pisa.

Botanical gardens can contribute to biodiversity conservation not only for taxa in cultivation (see for instance D’Antraccoli et al. 2023b), but also in the case of non-target spontaneously growing organisms such as lichens, which can benefit indirectly from habitats and conditions provided by botanical gardens, such as in urban environments. Although lichens are not usually a primary focus in botanical gardens, these spaces can serve as important refuges for their diversity. Indeed, the lichen biota of the Botanic Garden of Pisa includes also rare species according to the red list of epiphytic lichens of Italy (Nascimbene et al. 2013), and for three taxa, this is currently the only known locality in Tuscany. Lichen colonization (especially by acidophilous/oligotrophic species) well reflects the richness of tree substrates, while the distribution of nitrogen-tolerant lichens is likely associated with urban environmental conditions.

Financial support by Fondazione per la Flora Italiana for the publication fee is gratefully acknowledged.