Lichens are symbiotic partnerships between fungi and photosynthetic partners, usually algae, and they are famously sensitive to air quality, substrate chemistry and microclimate. Epiphytic lichens, the kinds that grow on tree bark, are therefore excellent biological indicators of environmental conditions, but they are also easily overlooked because many species are small, crust-like and inconspicuous. Although botanical gardens are celebrated reservoirs of cultivated plant diversity, the non-target organisms that colonize their trees spontaneously have received comparatively little attention. The new study set out to close that gap for Pisa, where no published survey of the lichen biota had ever been carried out, apart from a handful of nineteenth-century herbarium specimens.

The garden itself provides an unusually rich substrate. Founded in 1543, when Pisa pioneered the Italian tradition of physic gardens alongside Padua and Florence, the site now covers about 25,000 square metres in seven sectors and hosts more than 2,000 cultivated species in roughly 30 collections. Its Arboretum alone contains about 200 species, including monumental trees planted in 1787, such as a Ginkgo biloba and a Magnolia grandiflora, plus specimens of Myrtus communis and Platanus hispanica that are more than two centuries old. During 2023, the researchers surveyed more than 100 trees representing 61 different taxa, selected to capture the full range of bark variability, and identified the lichens using standard morphological, anatomical and chemical methods. Voucher samples were deposited in the herbarium of the University of Pisa and in the herbarium of the Plant Science and Biodiversity Centre of the Slovak Academy of Sciences.

The tally of 57 lichen taxa and one non-lichenized fungus, belonging to 53 species and subspecies within 40 genera, included several species of conservation concern. According to the Italian red list of epiphytic lichens, three recorded species are considered Vulnerable, one is Near Threatened, two are Least Concern and two are Data Deficient. The most striking discoveries were Arthopyrenia platypyrenia and Coenogonium tavaresianum, both new records for Tuscany, while Lecania cyrtellina is so far known in the region only from the Botanic Garden of Pisa. The record of Arthopyrenia platypyrenia, an inconspicuous fungus that lives inside the bark and shows itself only as a discoloration, is only the second for the whole of Italy; the previous record came from Calabria in 1996.

Each of the rare species tells its own ecological story. Coenogonium tavaresianum, listed as Vulnerable and thought to be declining, formed olive-green granulose thalli abundantly on the bark of the incense cedar Calocedrus decurrens, its distribution in Italy otherwise confined mainly to humid woodlands along the Tyrrhenian coast. Lecania cyrtellina, a crustose lichen with clustered tiny apothecia and narrow, mostly simple spores, was found on a Chilean wine palm, Jubaea chilensis, more than 130 years old; the species was long confused with its relative Lecania cyrtella and only recently distinguished consistently. Lecanographa amylacea, a rare Italian species tied to old trees in ancient woodlands, urban parks and old olive groves, was observed on nine different tree genera in the garden, including the 230-year-old Magnolia grandiflora, and represents only the second known Tuscan locality. Micarea misella, previously known in Tuscany from a single 2016 record in the mountains of Abetone, appeared on the same old Jubaea palm, and Porina borreri, a species of humid old woodlands, had only one earlier Tuscan record near Volterra.

The composition of the lichen community reflects both the garden’s assets and its urban setting. About 70 percent of the biota consists of acidophilous and oligotrophic species, mirroring the availability of acidic bark substrates among the diverse tree collection. At the same time, nitrogen-tolerant species such as Hyperphyscia adglutinata, Physcia adscendens, Candelaria concolor and the familiar yellow Xanthoria parietina were widely distributed, particularly in more open parts of the garden. These eutrophication-tolerant lichens thrive on dust and nitrogen compounds and are characteristic of urban atmospheres. The effect was especially evident on conifers, which in natural environments are typically colonized by acidophilous epiphytes but in the Pisa garden were more often covered by nitrophilous species, a clear signature of the surrounding city’s air chemistry.

Colonization intensity varied dramatically from tree to tree, and the reasons are a textbook illustration of how lichen ecology works. Lichen establishment depends on interacting factors including light availability, relative humidity, bark chemistry, water-holding capacity, the health of the substrate and pollution levels, many of them shaped by the individual tree and the surrounding vegetation. Some trees, such as Araucaria bidwillii, which has stood in the garden for 152 years, showed no lichen colonization at all, while sparse growth appeared on Camellia, Cedrus, Citrus, Sequoia, Viburnum and Wisteria. In contrast, Aesculus pavia, the hybrid Chitalpa tashkentensis, Melia azedarach, Phillyrea angustifolia, Pyrus communis and Vitex agnus-castus each hosted more than ten lichen species. Notably, no lichens were found on any of the trees grown in the greenhouses, where only green algae colonized some barks, confirming that outdoor conditions are essential for this biota.

The study’s broader message is that botanical gardens act as refugia for species with demanding ecological requirements, not merely as showcases for cultivated plants. Alongside the urban nitrophiles, the Pisa garden hosts species normally associated with undisturbed forests and ancient woodlands, including Coenogonium tavaresianum, Gyalecta liguriensis, Lecanographa amylacea and Porina borreri, several of which were found on the garden’s monumental trees. Six of these woodland-affinity species, including C. tavaresianum, G. liguriensis, L. amylacea, Micarea prasina, Opegrapha niveoatra and Waynea stoechadiana, have also been recorded in the Botanical Garden of Rome, reinforcing the idea that such gardens provide alternative habitats across cities. The pattern is not unique to Italy: the New Botanical Garden of the University of Zurich, despite covering only 53,000 square metres, hosts roughly ten percent of all lichen species recorded in Switzerland.

There is also a historical dimension to the discovery. Herbarium specimens collected in the Pisa garden in the nineteenth century, now preserved in Trieste and Pisa, document lichens gathered there as early as 1862 by P. Savi and O. Beccari, and in 1882 and 1883 by G. Arcangeli and A. Mori. One variety, Lecanora subfusca var. diffusa, was even described from the garden itself, having grown on a roughly hundred-year-old Ginkgo biloba. In Florence, the pioneering lichenologist Cengia-Sambo reported in 1928 an unidentified Lecanora growing on the leaves of a palm in a warm greenhouse. These fragments of history show that lichens have quietly inhabited Italy’s botanical gardens for at least 160 years, even when nobody was systematically counting them.

For conservation, the implications are concrete. The lichen biota of the Botanic Garden of Pisa includes rare species listed in the Italian red list, and for three taxa the garden is currently the only known locality in Tuscany, which gives a single urban green space an outsized role in regional biodiversity. The authors conclude that botanical gardens contribute to conservation not only through the taxa they cultivate but also through non-target, spontaneously growing organisms such as lichens, which benefit indirectly from the habitats and conditions the gardens provide. As cities expand and intensify, the diverse tree collections of botanical gardens, with their wide range of physical and chemical bark properties, create habitat mosaics that surrounding environments cannot match. The hidden richness clinging to the bark of a two-century-old magnolia or a Chilean wine palm is a reminder that biodiversity hotspots can hide in plain sight, on the oldest trees in the middle of town.

Subject of Research: Epiphytic lichen diversity and conservation value of the Botanic Garden of Pisa in an urban environment

Article Title: Islands of lichen diversity in urban environments: a hidden richness in botanical gardens

Article References: Fačkovcová, Z., Spagli, G., D’Antraccoli, M., Roma-Marzio, F., Peruzzi, L., Paoli, L., & Guttová, A. (2024). Islands of lichen diversity in urban environments: a hidden richness in botanical gardens. Italian Botanist, 18, 245-258. https://doi.org/10.3897/italianbotanist.18.144373

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.18.144373

Keywords: lichens, botanical gardens, urban biodiversity, epiphytes, Pisa, Tuscany, biodiversity conservation, red list species, air quality indicators, monumental trees, nitrophilous species, refugia