**4. Discussion**

The olive tree is a rustic species able to live on different types of soil. Over the centuries, it has been widely spread in all of the provinces of Sicily, from sea level to wherever the climatic conditions have allowed it (about 900 m asl). It has become, over time, the tree that dominates and is one of the most expressive, if not the most expressive, tree of the Sicilian agricultural landscapes, together with the marginally determined manna ash (*Fraxinus angustifolia* Vahl), another species of the same family [48,49].

Traditional olive groves with monumental specimens host a high number of varieties. The importance of this latter genetic resource and the potential it holds for agriculture is also a common element in other European and Mediterranean contexts, as shown by the recent studies on centenary olive trees conducted in several Mediterranean countries [50–55]. The cataloging, characterization, and in situ conservation of secular olive trees is, in fact, a priority to safeguard their genetic, natural, and agricultural value and to protect ancient genotypes threatened with extinction [56,57].

The varietal diversity of olive germplasm in Sicily is represented by 25 cultivars with different features in terms of the morphology, production, and quality of the oil [58].

The surveyed monumental olive trees belong to 11 different cultivars, including the cv. Fastucara, Biancolilla, and Cerasuola in the province of Agrigento, the cv. Ogliarola Messinese in Caltanissetta, the cv. Santagatese and Nocellara Etnea in Catania, and mainly the cv. Moresca in Enna.

In the province of Messina, the monumental specimens mainly belong to the Santagatese cultivar and, to a lesser extent, to the Ogliarola Messinese. In the province of Palermo, in addition to cv. Ogliarola Messinese and Santagatese, there are also some individuals belonging to the cv. Nerba. In Southern Sicily, monumental olive trees are of cv. Tonda Iblea and Moresca in the province of Ragusa and cv. Virdisa or Pizzuta in the

Syracuse area. In the western sector of the island, in the province of Trapani, in addition to the Nocellara del Belice, there are olive trees of cv. Cerasuola, Santagatese, and Tonda Iblea.

These are ancient cultivars grafted onto the wild olive trees that have given rise to irregularly planted olive groves, favoring the natural lay of the soil; several centuries-old plants still survive, cultivated extensively, with generally hollow trunks or with numerous cavities, which constitute microhabitats for vertebrate and invertebrate fauna. To preserve these trees in the best vegetative condition, as far as possible, it is necessary to ensure adequate crop care, including rational pruning, aimed at removing dying branches and aerating the foliage, monitoring and fighting against the agents of wood decay, and contrast, in some cases, to attacks by harmful insects and fungi. For some specimens, located in the Nebrodi mountains, it is also necessary to proceed with the removal of *Viscum album* L. and *Loranthus europaeus* Jacq., two semi-parasitic species that often develop abundantly on the canopy of centuries-old olive trees causing a vegetative decay and subsequently a worsening of the phytosanitary conditions

However, *Olea europaea* is a species characterized by remarkable longevity and by an extraordinary resistance to drought, due to different forms of anatomical and physiological adaptation, so much that it can survive and provide a certain fruit production even in conditions of low rainfall. It also resists fires very well due to its remarkable ability to emit vigorous suckers from the buds present on the stump after the passage of the fire. Therefore, the olive tree can be defined, with good reason, as "a plant that never dies". In this regard, Morettini [59] assigns the qualification of perennial species to the olive tree, observing that "the aerial portion is not perennial [, but] the underground portion it is; it is the base which, expanding into the bases formed by the new trunks that have succeeded over the centuries, replacing [ ... ] the oldest ones, retains the vitality and a set of generations of other younger olive trees".

The antiquity of olive growing in the Sicilian territory is testified by several archaeological and historical finds, artifacts, laws, deeds, and inventories, as well as by the numerous toponyms of various places that recall the olive tree (Ogliastro, Marcatagliastro, Alivazza, Ogliastrillo, Madonna dell'Olio, etc.) and, above all, from the discovery of ancient plants and/or stumps. The oldest olive trees found in the Sicilian territory are spread within the areas where the wild variety of the olive tree lives or can potentially live. The latter is an important element of the Mediterranean scrub of which it constitutes particularly expressive formations, settled on the semi-rupestrian stations, both on calcareous and quartzarenitic soils, along the coasts and in the hinterland where the temperature almost never drops below 0 ◦C [11].

In Sicily, as it is in the rest of the Mediterranean, olive trees are relevant landscape elements because of their considerable economic, cultural, and environmental roles. According to Cancila [60], in the fourteenth century, the olive trees, with the exception of the provinces of Palermo and Catania, did not constitute wide groves; they were spread elements of a mixed agricultural system, very common in the island country. At the end of the fifteenth century, olive growing was also quite widespread in the province of Messina, and considerable development began in other areas of Sicily in the sixteenth century.

Currently, the island's olive trees are mainly grown in semi-intensive or traditional dry systems, both in specialized and mixed cultivation. The landscapes characterized by the olive tree are widespread and diversified in the regional territory in relation to the history of the places and the pedological and microclimatic characteristics and, above all, to the position, which also influenced the cultivation methods (Figure 4).

In hilly and coastal areas of Western and Southern Sicily, in favorable geopedological conditions, specialized olive groves are widespread and, until the mid-twentieth century, constituted a real cornerstone of the rural economy of the island. In addition to having considerable importance for production and a strong identity value for the landscape, the olive grove plays a fundamental role in protecting the soil, especially in areas with higher slopes where older plants are present. It is in these areas that most of the monumental olive trees surveyed in this work have been preserved.

**Figure 4.** Distribution of the monumental olive trees in the olive Sicilian landscapes.

In coastal areas, especially in Southern and Eastern Sicily, the olive tree represents a characteristic element of mixed agricultural systems together with the carob, almond, and pistachio trees. These systems have been maintained in marginal areas, with often uneven morphologies, where the olive trees are unevenly distributed in the plots. In the traditional groves of the foothills, the individual trees are sometimes protected from erosion thanks to drystone lunettes. In other areas (e.g., in the Etna area), the olive tree can also be found within terraced polycultural systems.

Many olive groves in Northern Sicily, located in the province of Messina and Palermo, within the ancient marquisate of Geraci, date back to the early sixteenth century when the Marquis Ventimiglia allowed the farmers to graft the wild oleasters that grew spontaneously in his fields, letting them become owners of the single trees: this gave origin to a promiscuous property, as the land, where the inhabitants could exercise civic uses, remained in the hands of the feudal lord [60].

Cases of mixed ownership, although less frequent than in the past, are still known both on the Madonie mountains, especially in the territory of Castelbuono, and on the Nebrodi mountains, in the municipality of Tusa. A tangible sign of this anachronistic form of ownership is the engraving on the stem of the initial letter of the tree owner's surname (Figure 5), often highlighted in recent decades with a red painting [43]. The promiscuous property in Sicily concerns only the olive tree and no other cultivated tree species; as a matter of fact, because of its longevity, the olive tree is considered as real estate and therefore transmissible by inheritance.

Furthermore, on the northern side of the Madonie mountains, there is an extraordinary landscape created by the promiscuity of ancient olive trees within the relict manna ash tree grove. In this context, there is no lack of fruit species, such as pear, almond, fig, rowan, apricot, and plum.

In Eastern Sicily, on the slopes and on the Iblean plateau, olive trees spot the fields of wheat or pastures. The landscape of this arbored arable land is marked by an extensive system of fences built with dry stone walls. Inside these, there are ancient and isolated olive, almond, and carob trees, many of which are of extraordinary size and with limited productivity. In this context, the presence of several monumental olive trees is very frequent.

**Figure 5.** The Olivo di Zimmari (Tusa) engraved on the stem of the initial letter of the tree owner's surname is different from the landowner.

The traditional olive agroecosystems with monumental olive trees constitute peculiar tiles of the expressive Sicilian landscape consisting of a heterogeneous mosaic of agricultural systems and semi-natural systems, more or less fragmented and interspersed with hedges, dry stone walls, and narrow bands of forest species, which result in a high taxonomic and landscape diversity. The presence of monumental olive trees can be considered an indicator of the high biocultural diversity in the landscape and its agroecosystems. These last ones, given their diffusion in Sicily, define territorial mosaics with a high "widespread naturalness" [61], in which extensive agricultural practices are applied with a low environmental impact, compatible with the sustainable managemen<sup>t</sup> of natural resources. They host rather diversified residual habitats, characterized by species of high conservation interest (e.g., [62–67]) and by a rich flora of ethnobotanical interest that is used for food purposes by local populations [68,69] or for its dyeing properties [70]. Along the margins, there are frequent residues of the ancient *Rhus coriaria* L. cultivation, recently re-evaluated for the antioxidant properties of the fruits [71]. According to Blasi et al. [72], the olive grove is a cultivation that maintains a dynamic link with potential natural vegetation.

The recognition of the value of these landscapes in terms of biodiversity conservation has led Biondi et al. [73] to sugges<sup>t</sup> their inclusion in Annex I of the EEC Directive 92/43, called "Centuries-old olive groves with evergreen *Quercus* spp. and arborescent mattoral" (code 6320), as a priority habitat, considering them peculiar to the Mediterranean region.

The olive tree, with its unmistakable persistent foliage, glaucous green on the upper page and silvery-gray on the lower page, gives itself and its landscape a particular and unusual beauty. In this context, numerous monumental specimens, with their particular shapes and extraordinary dimensions, testify to the slow passing of the centuries and the succession of human generations. For this reason, monumental olive trees can represent the destination or one of the major attractions of naturalistic itineraries within agroecosystems and the "oil routes". These green patriarchs, in fact, can alone constitute the reason for a trip, since they always have something special to offer: their shape, their size, and their age make each of them unique, and visitation of them is a memorable event [44].

From the data reported in this work, it emerges that, throughout the Sicilian territory, the relationship between man and the olive tree is historicized, as can be inferred both from the different signs of material culture, of which centuries-old olive trees constitute an extraordinary testimony and from the historical sources. Their longevity, the longest among cultivated plants, allows for a stratified reading of the contemporary landscape in which these "plant monuments" are the silent witnesses of the transformations of the environment that characterized the Anthropocene.

The monumental olive trees represent the bulwarks of a Mediterranean agricultural landscape that, over the last millennium, in many parts, has undergone limited alterations, at least up to about 60 years ago. In recent decades, this precious heritage, of incomparable historical, landscape, and scientific value, is seriously in danger of disappearing due to the abandonment of cultivation, especially in the hilly and foothills areas on sloping soils and by fires. Therefore, a renewed attention and protection toward these monuments of nature is crucial.
