Natural and Historical Heritage of the Lisbon Botanical Gardens: An Integrative Approach with Tree Collections

Botanical gardens have long contributed to plant science and have played a leading role in ex situ conservation, namely of threatened tree species. Focusing on the three botanical gardens of Lisbon (i.e., Botanical Garden of Ajuda—JBA, Lisbon Botanical Garden—JBL, and Tropical Botanical Garden—JBT), this study aims to reveal their natural heritage and to understand the historical motivations for their creation. Our results showed that these gardens contain a total of 2551 tree specimens, corresponding to 462 taxa, within 80 plant families. Of these, 85 taxa are found in the three gardens, and more than half of the taxa are hosted in JBL (334 taxa), whereas 230 and 201 taxa were recorded in JBT and JBA, respectively. The motivations for the creation of each garden are reflected in the different geographic origins of the trees they host in their living collections. The Palearctic species are dominant in JBA and JBL, and Tropical trees prevail in JBT. With more than 250 years of history, these gardens hold an invaluable natural and historical heritage, with their living collections providing valuable sources of information for the conservation of threatened plant species, at local and global scales.


Introduction
Biodiversity loss is a global phenomenon [1]. Adding to the conservation of species diversity in their natural habitats, the loss and degradation of urban environments made the preservation of urban ecosystems and the promotion of their biodiversity particularly important since the beginning of the 21st century [2]. Many cities still include remains of previous forests and trees that should be actively managed and preserved, namely in urban parks or woodlots [3]. Moreover, botanical and environmental awareness will be better raised among the community by providing education and places to learn about the plants and their conservation, and the botanical gardens play a relevant role in this context [4,5].
Botanical gardens are institutions that keep well-documented collections of living plants and seed banks, and are thus fundamental for conservation, education, and science [6,7]. Since the 1970s, the conservation of species outside their natural habitat (ex situ conservation), namely of threatened tree species, is encouraged by the International [6,7]. Since the 1970s, the conservation of species outside their natural habitat (ex situ conservation), namely of threatened tree species, is encouraged by the International Union for Conservation of Nature (IUCN) and Botanic Gardens Conservation International (BGCI) [8]. As unique urban green spaces, the botanical gardens offer the potential to restore populations of threatened species, as their living collections and seed banks are insurance against the extinction of species in their natural habitats and provide means to reintroduce or reinforce their populations thereby making their survival possible [9,10].
In plant conservation, trees are widely recognized as critically important to the planet and population at the economic, cultural, and ecological levels [7]. However, their numbers have been declining in situ due to, e.g., climate change, overgrazing, agriculture, and logging [11]. According to the IUCN Red List, ca. 10,000 tree species are threatened with extinction worldwide [12]. Several studies have also recognized that trees are key elements in urban nature, as their presence provides numerous services to the urban ecosystem in ecological (i.e., climate, air quality, hydrology, soil, and biodiversity conservation), social, cultural, and aesthetic terms [13].
Lisbon, capital of Portugal, has unique characteristics owing to its location in Southwest Europe, in a transition zone between the Atlantic Ocean and the Mediterranean Sea, and between Africa and Eurasia, which allows it to host a higher average biodiversity than other European cities [14]. According to the municipality, Lisbon has about 148 gardens and parks, among which are three botanical gardens: Botanical Garden of Ajuda (hereafter, JBA), Lisbon Botanical Garden (hereafter, JBL), and Tropical Botanical Garden (hereafter, JBT) (see Figure 1), all of them belonging to the University of Lisbon. Lisbon is the only city in Portugal housing three botanical gardens, and they hold a rich natural and historical heritage, a valuable scientific resource to promote interdisciplinary research activities between the different schools of the University, as well as with worldwide institutions interested in the study of plant diversity and conservation. These three botanical gardens are very different, created at different times but with common overall purposes: research, plant conservation, horticulture, and education. Surely, the study of these gardens and of their collections would enhance the current understanding on Garden Art History and Plant Diversity, but the knowledge of this heritage is often incomplete and outdated. Knowing the botanical and historical heritage of the three Botanical Gardens of Lisbon, and what it represents, is essential to provide the first global measurement of ex situ conservation of living tree collections. Rivers et al. [10] advocate the need to divulge the ex situ collections housed in botanical gardens and arboreta, to plan future collecting efforts and to adopt integrated global approaches to protect threatened tree species.
With the present work, we aimed to contribute to disseminating the heritage represented by the Botanical Gardens of Lisbon, by coupling the history and the characteristics of the tree layer that dominates them. With this approach, we intended to explain why the presence of three Botanical Gardens in Lisbon is not redundant and to highlight their continued relevance for knowledge and for urban sustainability. The first Portuguese botanical garden, Jardim Botânico da Ajuda (Botanical Garden of Ajuda, see Figure 1), was installed on the hill slope of Ajuda after the big 1755 earthquake. Founded in 1768, during the reign of José I (1714-1777) by the indication of his state secretary, Sebastião Carvalho e Mello (1699-1782), the garden was created by the Italian naturalist Domingos Vandelli (1735Vandelli ( -1816, who was its first director. Along with the garden, the Royal Cabinet of Natural History, a Chemistry Laboratory, the Casa do Risco, where designers created scientific illustrations, and a Specialized Library were also created, as well as, after 1777, a cabinet of Experimental Physics. The Royal Cabinet of Natural History became one of the richest European scientific institutions, as a result of the "Philosophical Journeys" to the Portuguese territories overseas, after the 1780s. For this reason, its collection caught great attention in Europe, particularly of naturalists from the Musée National d'Histoire Naturelle de Paris. An example of this was the visit of Étienne Geoffroy Saint-Hilaire (1772-1844), in 1807, who studied and selected specimens from JBA to enrich the collection of the Jardin des Plantes, in Paris. During the French invasions, part of the collections of the garden and of the Royal Cabinet was taken by the Napoleonic troops to the present Musée National d'Histoire Naturelle in Paris. Another part was taken to the Botanical Garden of Rio de Janeiro, when the Portuguese royal family left Lisbon and went to Brazil, in 1807, before the Napoleonic forces invaded Lisbon.
The second director of JBA was Félix da Silva e Avelar Brotero (1744-1828), a famous Portuguese naturalist that Vandelli met in Coimbra, a pioneer in the studies of Botany in Portugal; he wrote the first Portuguese Flora and started the first practical school of Botany of the country at JBA. In 1836, the administration of the garden was assigned to the Academy of Sciences of Lisbon, and its patrimony was once again split up. In January 1837, the Polytechnic School (later, Faculty of Sciences of the University of Lisbon) was created in the center of Lisbon, and its association with a botanical garden was considered indispensable. As a result, some collections of JBA were transferred there in October 1838. Since 1910, the direction of JBA is the responsibility of Instituto Superior de Agronomia, University of Lisbon. In 1944, JBA was listed as Imóvel de Interesse Público ("Building of Public Interest") by the Ministry of Culture.

Lisbon Botanical Garden (JBL)
The Jardim Botânico de Lisboa (Lisbon Botanical Garden, see Figure 1) was created in 1873, with many collections brought from JBA. Inaugurated in 1878, it is presently part of the National Museum of Natural History and Science, which is responsible for the garden's management. It was a scientific garden intended to complement teaching and botany research at the former Polytechnic School. The chosen place, Mount Olivete, had already more than two centuries of tradition in the study of botany, with the Jesuit College of Cotovia here established between 1609 and 1759. To install the garden, a draft directive was made in 1843. However, plantations only began in 1873, by the initiative of the Count of Ficalho (1837-1903) and Andrade Corvo (1824-1890), teachers at the Polytechnic School. The collection of the enormous diversity of living plants was first made by the German Edmund Goëze (1838-1929) and the French Jules Daveau , from all the territories under Portuguese rule at the time. Goëze, the first chief gardener, outlined the "Class" and Daveau was in charge of the "Arboretum". Between 1892 and 1909, the French Henri Cayeux, the second chief-gardener, made important contributions to the embellishment of this garden, by introducing more species and creating new varieties of plants. That is the case of the introduction of succulent dahlias and the creation of the hybrid Dombeya × cayeuxii and of the rose cultivar "Belle Portugaise".
The most significant intervention in the garden occurred in the late 1930s and early 1940s. Under director Ruy Telles Palhinha (1871Palhinha ( -1957, the original systematic ordering of the upper terrace of JBL was replaced by ecologically meaningful groups of species. In 2010, JBL was listed as a "National Monument" by the Ministry of Culture.

Tropical Botanical Garden (JBT)
The Jardim Botânico Tropical (Tropical Botanical Garden, see Figure 1), initially "Colonial Garden", was created by royal decree in 1906, under the rule of King Carlos I and by the initiative of Counsellor Manuel António Moreira Júnior, the Navy and Overseas Minister, and aimed to support the teaching of Tropical Agriculture.
The French landscape gardener Henri Navel , who studied at the National School of Horticulture of Versailles and worked as a gardener in some important gardens (e.g., Royal Botanic Gardens, Kew), had the task to prepare the space for its new function as a colonial garden, between 1910 and 1917. The director position was inherently held by the full professor of the "Economic Geography and Colonial Cultures" chair at the Instituto Superior de Agronomia (University of Lisbon).
The celebrations of the 800th anniversary of Portugal as an independent state and of the 300 years after the restoration of national independence, which took place in Belém with the 1940s "Exhibition of the Portuguese World", left their mark on the morphology and identity of JBT, where the exhibitions concerning the Tropics were concentrated. The program of this section aimed to portray Portugal as the third colonial world power. Buildings, some still existing, and temporary pavilions were built, each dedicated to one of the colonies, and recreating African villages.
In 1944, the "Colonial Garden" was merged with the "Colonial Agriculture Museum", also formally created in 1906, resulting in the Jardim e Museu Agrícola Colonial (Agriculture and Colonial Garden and Museum). Since its creation, the garden was dependent on the Instituto Superior de Agronomia. In 1951, the name changed to Jardim e Museu Agrícola do Ultramar (Overseas Garden and Museum of Agriculture), and in 1974 it became part of the Instituto de Investigação Científica Tropical (Tropical Research Institute). In 2006 it received its current name, and in 2007 it was listed as a National Monument by the Ministry of Culture. In August 2015, JBT became part of the University of Lisbon, under the administration of the National Museum of Natural History and Science.

Tree Collections
The tree layers of the three Botanical Gardens of Lisbon contain a total of 2551 specimens corresponding to 462 taxa (i.e., 448 species, nine subspecies, four varieties, and one form). Of the 462 taxa inventoried, 85 are shared by the three gardens, 130 by JBA and JBL, 146 by JBL and JBT, and 111 by JBA and JBT ( Figure 2). More than half of the taxa recorded in our study were found in JBL (334 taxa), whilst in JBT and JBA 230 and 201 taxa were recorded, respectively (Table 1; Figure 2). The family, habit, taxonomic classification, and native distribution of each taxon are described in Table 1.  Figure 2). The family, habit, taxonomic classification, and native distribution of each taxon are described in Table 1.     There is no clear relationship between the area (ha) (i.e., green spaces) and the number of species in each botanical garden, as shown in Figure 3. Nevertheless, when comparing JBA with JBL, the number of species tended to increase with the garden area, as there are more varied habitat types in JBL able to accommodate a more diverse range of species. Moreover, species diversity can also reflect the path of each botanical garden over time and socioeconomic issues, taking into account the human and financial resources allocated to garden management. However, the greatest difference was caused by the 1941 hurricane, the strongest one affecting Portugal to date: the trees of the JBA, many of them close to 200 years old, were uprooted from the base, while the trees of the JBL were not affected. There is no clear relationship between the area (ha) (i.e., green spaces) and the number of species in each botanical garden, as shown in Figure 3. Nevertheless, when comparing JBA with JBL, the number of species tended to increase with the garden area, as there are more varied habitat types in JBL able to accommodate a more diverse range of species. Moreover, species diversity can also reflect the path of each botanical garden over time and socioeconomic issues, taking into account the human and financial resources allocated to garden management. However, the greatest difference was caused by the 1941 hurricane, the strongest one affecting Portugal to date: the trees of the JBA, many of them close to 200 years old, were uprooted from the base, while the trees of the JBL were not affected.  The Eudicotyledons are the more numerous species in the three botanical gardens (76% in JBA; 73% in JBL; 73% in JBT), followed by the Gymnosperms (including Ginkgophyta and Coniferophyta) (10% in JBA; 12% in JBL; 10% in JBT), the Monocotyledons (9% in JBA; 11% in JBL; 13% in JBT), and finally the Magnoliids (6% in JBA; 4% in JBL; 4% in JBT) (Table 1; Figure 3).
A total of 80 plant families are represented, the Fabaceae being the dominant one (39 taxa), followed by the Myrtaceae (36 taxa). In contrast, 33 families (e.g., Casuarinaceae, Ginkgoaceae and Lamiaceae) are represented by one taxon only ( Table 1). The most dominant families in JBT and JBL are the Arecaceae (ca. 10% and 8%, respectively) and the Fabaceae (ca. 9% and 6%, respectively), whereas the Fabaceae (11%) and the Myrtaceae (7%) are the main ones in JBA.
A total of 80 plant families are represented, the Fabaceae being the dominant one (39 taxa), followed by the Myrtaceae (36 taxa). In contrast, 33 families (e.g., Casuarinaceae, Ginkgoaceae and Lamiaceae) are represented by one taxon only ( Table 1). The most dominant families in JBT and JBL are the Arecaceae (ca. 10% and 8%, respectively) and the Fabaceae (ca. 9% and 6%, respectively), whereas the Fabaceae (11%) and the Myrtaceae (7%) are the main ones in JBA.
In JBA, about 1.0% of the taxa are classified as CR, 1.5% as EN, 4.0% as VU, 2.5% as NT, 51.7% as LC, and 2.5% as DD (Figure 4). In JBT, 0.4% of the taxa are classified as CR, 4.3% as EN, 5.2% as VU, 3.0% as NT, 49.1% as LC, and 2.2% as DD. JBL hosts 1.2% of taxa classified as CR, 3.6% as EN, 3.6% as VU, 2.1% as NT, 50.9% as LC, 2.1% as DD, and 0.3% as EW. A considerable portion of the species (JBA 36.0%, JBT 35.7% and JBL 36.2%) remains unevaluated by IUCN.  Comparing the three Botanical Gardens, 29 tree species stand out in JBL because of their conservation status worldwide, followed by JBT and JBA with 23 and 16 species, respectively ( Table 1). Eleven of these taxa only occur in the JBL collection, namely Brugmansia aurea, while the other six species are exclusive to JBA and JBT collections.

Remarkable Species of JBA
At JBA, worth noting are two splendorous Ficus macrophylla, and the emblematic dragon tree (Dracaena draco) ( Figure 6A), probably one of the oldest specimens in the garden (over three centuries old). Other emblematic trees include the only living specimen of Schotia afra ( Figure 6B) in a European botanical garden [15], Araucaria bidwillii, Zelkova serrata, Ocotea foetens, Quercus faginea and Phytolacca dioica. On the lower terrace, the boxwood hedges enclose some tree specimens which deserve special attention because of their size or shape, namely Lagerstroemia indica, Lagunaria patersonia, Dracaena draco, Araucaria heterophylla ( Figure 6C), and Araucaria cunninghamii. It should be mentioned that Araucaria species are majestic trees, full of symbolism in Portuguese gardens. garden (over three centuries old). Other emblematic trees include the only living specimen of Schotia afra ( Figure 6B) in a European botanical garden [15], Araucaria bidwillii, Zelkova serrata, Ocotea foetens, Quercus faginea and Phytolacca dioica. On the lower terrace, the boxwood hedges enclose some tree specimens which deserve special attention because of their size or shape, namely Lagerstroemia indica, Lagunaria patersonia, Dracaena draco, Araucaria heterophylla ( Figure 6C), and Araucaria cunninghamii. It should be mentioned that Araucaria species are majestic trees, full of symbolism in Portuguese gardens.

Remarkable Species of JBL
The collection of plants here is the most diverse among the three gardens. For instance, the outstanding diversity of palms ( Figure 7A), brought from all continents, provides unexpected tropical scenarios at several points in the garden [16,17]. This garden holds some collections particularly worth mentioning and some species that stand out because of their conservation status worldwide, namely: Pinus torreyana, Metasequoia glyptostroboides, Chrysophyllum imperial ( Figure 7B), Brahea edulis, Afrocarpus mannii, Taxodium distichum var. mexicanum or Dracaena draco ( Figure 7C), among other species with conservation interest.

Remarkable Species of JBL
The collection of plants here is the most diverse among the three gardens. For instance, the outstanding diversity of palms ( Figure 7A), brought from all continents, provides unexpected tropical scenarios at several points in the garden [16,17]. This garden holds some collections particularly worth mentioning and some species that stand out because of their conservation status worldwide, namely: Pinus torreyana, Metasequoia glyptostroboides, Chrysophyllum imperial ( Figure 7B), Brahea edulis, Afrocarpus mannii, Taxodium distichum var. mexicanum or Dracaena draco ( Figure 7C), among other species with conservation interest.

Remarkable Species of JBT
Among the most notable specimens in JBT is the collection of rare palm trees, the exuberant Ficus macrophylla (one of the largest specimens in Europe) (Figure 8A), and a two-century-old Yucca gigantea ( Figure 8B). Palms are a dominant element in the garden, as immediately perceived upon entrance, in the main avenue flanked by Washingtonia robusta and Washingtonia filifera, inspired by the Botanical Garden of Rio de Janeiro ( Figure 8C). Additionally remarkable are the Dracaena draco, Ficus sycomorus and Afrocarpus mannii, among others, e.g., [18].

Natural and Historical Heritage of the Botanical Gardens of Lisbon
The three Botanical Gardens of Lisbon represent an invaluable legacy, and their different scientific and educational roles reflect on their plant collections. Over the centuries, Lisbon's climate has allowed the coexistence of plant species from many different biogeographical origins, which greatly diversified its gardens. This survey of the tree species (sensu lato) present at the three botanical gardens highlighted that the diversity and tree richness of each garden is linked with its historical background. Despite their different ages and histories, the three Botanical Gardens of Lisbon share the same purposes: research, education, and conservation. Their value goes far beyond their roles as simple gardens, as they can contribute to urban forestry and resilient landscapes, to support and advance urban agriculture, and to conserve urban biodiversity [19].
Although the diversity of species might be expected to depend on the area of the garden due to the potentially higher availability of space and/or of varied habitat types [20], this was not clear in our study. In fact, species diversity can also reflect the development of each garden over time, as well as socioeconomic issues, taking the human and financial resources allocated to garden management into account [21]. In the case of the Botanical Gardens of Lisbon, the historical background and, in particular, the specific mission of each garden cannot be disregarded when trying to understand the existing

Natural and Historical Heritage of the Botanical Gardens of Lisbon
The three Botanical Gardens of Lisbon represent an invaluable legacy, and their different scientific and educational roles reflect on their plant collections. Over the centuries, Lisbon's climate has allowed the coexistence of plant species from many different biogeographical origins, which greatly diversified its gardens. This survey of the tree species (sensu lato) present at the three botanical gardens highlighted that the diversity and tree richness of each garden is linked with its historical background. Despite their different ages and histories, the three Botanical Gardens of Lisbon share the same purposes: research, education, and conservation. Their value goes far beyond their roles as simple gardens, as they can contribute to urban forestry and resilient landscapes, to support and advance urban agriculture, and to conserve urban biodiversity [19].
Although the diversity of species might be expected to depend on the area of the garden due to the potentially higher availability of space and/or of varied habitat types [20], this was not clear in our study. In fact, species diversity can also reflect the development of each garden over time, as well as socioeconomic issues, taking the human and financial resources allocated to garden management into account [21]. In the case of the Botanical Gardens of Lisbon, the historical background and, in particular, the specific mission of each garden cannot be disregarded when trying to understand the existing living collections.
The role of JBA was particularly relevant in the late 18th century as a sponsor of the "Philosophical Journeys", the first scientific expeditions to the Portuguese territories overseas [22]. Later, the importance of this garden and of its plant collection decreased in favor of JBL, created in the late 19th century to support teaching at the Polytechnic School.
Standing out as a support for botany learning [23] and well-adjusted to the site and the mild climate of the city, the Lisbon Botanical Garden (JBL) holds the most diverse collection (ca. 334 taxa) of the three studied gardens, in terms of both species and geographic origins (see Figure 5). In the heart of Lisbon, it constitutes a reference in urban biodiversity.
Created in the early 20th century, the Tropical Botanical Garden (JBT) is noteworthy for its exotic and economically valuable plants (e.g., spices, fruits, medicinal, stimulants, and ornamentals) especially from tropical and subtropical regions [18,24], related to the educational role in the study of Tropical Agriculture and the introduction of economic and exotic plants [25]. The importance of JBT in this context was made clear in 1940, when it hosted the "Colonial Section of the Portuguese World Exhibition". Nowadays it allows the general public contact with the tropical sciences, thus contributing to promoting scientific culture among the Community of Portuguese Speaking Countries [26].
Comparing the diversity of species that occur in the three Botanical Gardens of Lisbon, some trees stand out for their biological and iconic character; such are the cases of Ginkgo biloba, the only representative of the Ginkgoaceae family at present [27] and considered a relict taxon dating from the early Jurassic period, a most well-represented species in botanical gardens worldwide [10], and of the Vulnerable Dracaena draco, the most emblematic tree of the Macaronesian region, where it is endemic. Its exuberant growth-form (i.e., arborescent and pachycaulous) and its red resin, known as "dragon's blood", are of great interest both at the ornamental and ethnobotanical levels, and the species is widely cultivated despite its rarity in the wild, e.g., [28]. The Endangered Sequoia sempervirens, native to the Pacific coast of the USA, and the only surviving species of the Sequoia genus [29], is found in the collections of JBL and JBT; and the Critically Endangered Metasequoia glyptostroboides, found only in JBL, is also a relict taxon, endemic from a very restricted region of central China [30], and the second best-represented tree species in botanical gardens worldwide [10] where it is successfully cultivated, although the environmental requirements in the wild remain poorly known [31].
The historical imprint is clear in the collections of the three Botanical Gardens of Lisbon. For example, 11% of taxa are from the Afrotropical biogeographic region, reflecting the Portuguese colonization of some very floristic-rich African regions (e.g., Angola or Mozambique) [32]. Whereas Palearctic species are dominant in JBA and JBL, the distinctive tropical character prevails in JBT. The numerous fruit trees from South America (namely Brazil), as well as the numerous useful species of Asiatic origin, account for these values. The flora of south-western Indian Ocean territories (Australia, New Guinea and New Zealand) is the least represented, except for the typical presence, in all the gardens, of some Australotemperate elements, including species of the genera Araucaria, Brachychiton, Eucalyptus, Ficus, and Melaleuca.
The introduction and acclimatization of new exotic plant species in Lisbon, from different geographical regions and with economic and ornamental interest, were not restricted to the botanical gardens. In fact, private gardens also played important roles in the dissemination of such plants, and the taste for these botanic "novelties" was transposed to the streets and public gardens of Lisbon. Private gardens, such as the Parque Monteiro-Mor and Tapada das Necessidades (both from the 18th century), have played a prominent role in the dissemination of such plants through "art" and supported by landscape gardeners, horticulturists, and nurseries.
The future of the botanical gardens, as spaces of knowledge about the plant world, will certainly depend on the reinforcement of research, education and cultural activities. This, in turn, requires the continuous and careful maintenance of the botanical heritage of these living museums. Accordingly, the three Botanical Gardens of Lisbon are expected to: (i) enhance the visibility of their collections and heritage, and to strengthen bonds, partnerships and joint work between different collections; (ii) manage biological collections using modern software; (iii) create a DNA Bank associated to the collections; (iv) disseminate results to the scientific community, and promote innovative education actions and seminars; and (v) promote ex situ conservation initiatives, including maintenance of the seed bank and identification of living collections of rare and threatened species, particularly from the Portuguese flora.

Final Remarks
All the green areas of Lisbon constitute a very important heritage for our ex situ plant diversity conservation, however, the Botanical Gardens of Lisbon must be seen as unique spaces for tree conservation and their articulation with the surroundings, in ecological, aesthetic, cultural, historical, social, and economic terms, is fundamental to the urbanized space of Lisbon [14].
The preservation of trees, as well as of green urban areas which encompass the botanical gardens, is a current purpose aiming at making cities more resilient to climate change [40,41]. This was one of the main objectives of the "European Green Capital 2020" for Lisbon [42], promoting sustainability, biodiversity, and preservation of threatened species; it should also be a common goal to all cities, since more than half of the world's population currently lives in cities and predictions indicate that more than two thirds will live there by 2050 [43].
In this context, botanical gardens play a very important role in urban sustainability: adding to botanical knowledge, they also store years of practice in horticulture and arboriculture, which is extremely useful to improve urban green spaces in fields such as tree selection and planting, urban forest management plans and restoration [4]. Furthermore, this kind of knowledge can help in the implementation of sustainable environmental management practices, urban biodiversity [44], and the maintenance of urban trees [45], as well as to promote ecosystem services, preserving and valuing biodiversity [46].

Studied Areas
The three Botanical Gardens of Lisbon (JBA-Botanical Garden of Ajuda; JBL-Lisbon Botanical Garden; and JBT-Tropical Botanical Garden) are characterized in Table 2, and their location in Lisbon (see Figure 1). JBA is located in Calçada da Ajuda (Figure 1) opposite to the Palácio Nacional da Ajuda and is the oldest Botanical Garden in Portugal. It was established on two terraces, separated by a limestone balustrade, and connected by a central and two lateral staircases authored by the architect Manuel Caetano de Sousa (1730-1802). The upper terrace hosts the botanical collection, and the lower terrace is composed of a central fountain and 4 km of hedges of boxwood and myrtle, in geometric forms around basins and sculptures from the stone's school of Machado de Castro (1731-1822), possibly conceived as a recreation space for the royal family. JBA combines several styles and epochs, with a predominant line of baroque influence [47].

Lisbon Botanical Garden (JBL)
JBL is located in the center of Lisbon ( Figure 1) and occupies the core of the block bordered by the main avenue (Avenida da Liberdade) and a large garden square (Jardim do Príncipe Real). It is structured in two parts, the "Class" and the "Arboretum" [48,49]. The "Class" develops at the same level as the main building of the National Museum of Natural History and Science/University of Lisbon; the "Arboretum" extends downwards the slope, as an organic composition of flowerbeds, streams, waterfalls, lakes, and passages, surrounded by a pathway [50].

Tropical Botanical Garden (JBT)
JBT is situated in the monumental area of Lisbon, next to Mosteiro dos Jerónimos and to the Palácio de Belém and covers 800 m along a south-facing hill slope, overlooking the Tagus River (Figure 1). Favored by a privileged microclimate, JBT has plenty of water and has been enriched with a great diversity of exotic plant species. JBT encloses historical buildings such as the Palace of Condes da Calheta (17th century), and marble sculptures of different aesthetic concepts that span over several historical periods, from the 17th to the 20th centuries [18].

Tree Layer Inventory
The surveys in the three botanical gardens focused on tree specimens and were carried out in the framework of the "LX GARDENS" research project (2014-2017, targeting Lisbon's historic gardens). The following methodology was used for the botanical study: inventory, location (with geographic information systems) and specimen identification. All data were recorded in a relational database built on a SQL server. Location data recorded included the following items: (1) Specimen ID number; (2) Garden code; (3) Species code; (4) Family; (5) Species; (6) Species classifier; (7) Geographic origin of the taxon; (8) Naturality Status in Portugal (i.e., native, non-native and/or invasive); (9) Growth form (meaning the plant's physiognomy); (10) ETRS 1989 coordinates; (11) Extinction risk assessment using the IUCN Red List [12]. The data of the three botanical gardens were continuously updated until May 2021.
Tree species were chosen for the study due to their perenniality and longevity, which seemed appropriate and important traits for the intended historical approach. Tree definition followed the one proposed by the IUCN's Global Tree Specialist Group (GTSG): "a woody plant with usually a single stem growing to a height of at least 2 m, or if multistemmed, then at least one vertical stem 5 cm in diameter at breast height". Several habit types were considered: trees sensu stricto, shrubs, rosette trees, and stem-succulents. For simplicity, all these types are here considered, in a broad sense, as "trees", and belonging to the "tree layer" of the gardens. To support species selection, the GlobalTreeSearch [15] was also consulted.

Database of the Tree Layers
Plant data of each Lisbon botanical garden, including species names, families, growth form, and native distribution, are summarized in Table 1. The native distribution follows Morrone's [75] Biogeographical Regions (i.e., Afrotemperate, Afrotropical, Andean, Australotemperate, Australotropical, Nearctic, Neoguinean, Neotropical, Neozelandic, Oriental, and Palearctic) and online databases, namely the Plants of the World Online [73] and the GBIF platform [76]. The conservation status of each species was retrieved from the IUCN Red List of Threatened Species [12].

Data Treatment
All analyses were carried out in the RStudio program version 1.1.456 [77]. In order to detect the diversity patterns of the tree layers of the three Botanical Gardens of Lisbon, the following analyses were performed: scatter pie plot, Euler diagram, and heatmap. All the plots were visualized by ggplot2 [78].