## 2 Materials and Methods  
### Study Area and Olive Trees Selection  
Naxos, the tenth-largest Greek island and the eighteenth-largest in the Mediterranean, is located in the central Aegean Sea within the Cyclades complex. It covers an area of 429.79 km\({}^{2}\) and is characterized by a mountainous terrain, boasting three prominent peaks: Zas (1004 m a.s.l. ), Koronos (998 m a.s.l. ), and Fanari (888 m a.s.l.). Naxos experiences a climate marked by hot summers and a prolonged dry period lasting nearly seven months. Annual precipitation ranges from 300 to 400 mm, primarily concentrated between November and March, with peak rainfall occurring in December and January. Mean monthly temperatures vary from approximately 12 \({}^{\circ}\)C in winter to 25 \({}^{\circ}\)C in summer [95]. The island's landscapes showcase remarkable diversity, with extensive slopes and terraces dominating the eastern and north-northeastern mountainous regions, facilitating the existence and cultivation of centennial traditional olive groves. In these mountainous areas, forest cover is limited and the predominant features are pastures and forest land characterized by shrub vegetation (Figure 1). Figure 1: Distribution of the main land cover types of the island of Naxos, Greece, along with the study site in Laoudis region (dotted rectangle in the main map). Traditional olive groves on the island host over 270,000 olive trees of the variety \"Throubolia\" (Municipality of Naxos and Small Cyclades, personal communication, 23 November 2018), spanning an area of 9813.8 ha, with the majority concentrated in the central region. We selected a low-input traditional olive plantation in the northeastern part of the island, specifically in the Laoudis region (\(37^{\circ}4^{\prime}45.05^{\prime\prime}\) N, \(25^{\circ}34^{\prime}35.80^{\prime\prime}\) E). Covering an area of 2.4 ha and situated at an altitude of 60 m a.s.l., this olive grove epitomizes the traditional character that aligns it with the concept of High Nature Value Farmlands, exhibiting the typical features of traditional olive cultivation found in the Mediterranean. It retains the old-age \"Throubolia\" variety, which is known for its historical significance and cultural heritage [96; 97], while the centennial olive trees have been cultivated using time-honored, low-input practices that prioritize sustainable land management, emphasizing traditional harvesting techniques ([PERSON], personal communication, 18 December 2022). Additionally, the presence of terraces and dry-stone walls within this grove underscores its traditional character. These handcrafted stone structures serve both practical and ecological functions, reflecting a commitment to preserving the natural landscape and supporting biodiversity [27]. In parallel, the grove hosts a diverse array of flora and fauna, with the surrounding landscape characterized by its low-intensity land cover and semi-natural vegetation (pers. obs. ), further emphasizing its role as a High Nature Value farmland. In order to ascertain the centennial nature of this olive grove, we employed a combination of qualitative and quantitative methods. Classical indicators of tree age, including visual characteristics such as overall appearance, structural attributes, and trunk thickness [98], were employed alongside a quantitative approach utilizing an allometric equation proposed by [PERSON] et al. (Age = 2.11 \(\times\) Diameter (cm) + 88.93; R\({}^{2}\) = 0.80) [99]. The application of this allometric equation confirmed that the olive trees in this grove indeed qualify as centennial, with a calculated mean age of 218.27 \(\pm\) 51.6 years. It is worth noting that we opted for this quantitative method due to inherent difficulties associated with measuring annual growth rings, primarily stemming from the presence of cavities and decaying tissues within the inner sections of the tree trunks [99; 100].