Climate Change Indicators and Impacts on Mastic Tree Cultivation in Chios, Greece

Abstract

Climate change, a natural phenomenon driven by anthropogenic interventions, represents a global challenge that requires immediate and effective action. Significant alterations in atmospheric parameters and the occurrence of extreme climatic events have become increasingly evident. To evaluate extreme climate conditions and their variations within a specific region, climate data spanning at least five decades is essential. This study focuses on assessing the impacts of climate change on the island of Chios, with particular emphasis on its influence on the cultivation and harvesting of mastic gum, a unique product derived from the Pistacia lentiscus var. chia tree. Using ClimDex and analyzing 29 climate indices from 1960 to 2024, this paper evaluates historical and current climatic conditions. Key climate phenomena, such as the number of days with very high or low temperatures, extreme rainfall events, daily temperature variations, frost days, and days exceeding 25 °C—widely recognized indicators of climate change—are documented and correlated.

1. Introduction

Climate change is increasingly recognized as a critical threat to agricultural systems worldwide [1,2], particularly in Mediterranean regions, which are already vulnerable to water scarcity, extreme weather events, and rising temperatures [3,4]. The island of Chios, located in the eastern Aegean Sea, is home to the unique cultivation of Pistacia lentiscus var. chia [5,6,7], a plant that produces the globally renowned mastic resin. Mastic is not only economically and medically significant but also deeply ingrained in the cultural identity of southern Chios, where traditional cultivation practices have been preserved and passed down through generations [4,8,9].

1.1. The Mastic Tree

The mastic tree, or mastic shrub (Pistacia lentiscus var. chia, family Anacardiaceae) (Figure 1), is an evergreen plant that typically reaches a height of 2–3 m, grows slowly, and fully matures after 40–50 years, with older specimens reaching up to 5 m [10,11]. Although the tree can live for over 100 years, mastic production begins only after the fifth or sixth year of planting. Yield peaks around the fifteenth year and declines significantly after 70 years. On average, each tree produces 150–180 g of mastic annually, though exceptional cases have been reported where yields range from as little as 10 g to as much as 2 kg [11]. Male trees are predominantly cultivated due to their higher productivity. Cultivation techniques and soil quality are critical factors influencing mastic yield.

The mastic shrub is highly resilient and requires minimal resources, enabling it to thrive in barren, rocky, and nutrient-poor soils [12]. Its moisture requirements are low, supported by a shallow yet wide root system that efficiently utilizes short-duration rainfall and minimal morning humidity [13,14]. However, growth is hindered in highly moist soils due to blocked root aeration. The shrub is also vulnerable to cold temperatures [14]; frost—a rare occurrence in southern Chios—can cause significant damage as it cracks the veins of the trunk. Furthermore, improper harvesting practices may deplete the valuable resin from the outer bark [11]. New plantations are established using cuttings from branches of older, high-yield trees. Older trees are rejuvenated through layering or shoots. A cutting from a proven high-yield tree is planted with only a few leaves visible at the top. Once established, these cuttings require minimal cultivation.

1.2. Uniqueness

Mastic shrubs or trees from this family are essential components of Mediterranean maquis vegetation. However, only in Chios does nature and cultivation combine in a unique way to produce the precious mastic tears, offering pleasure, relief, and healing. As a result, Chios has become synonymous with mastic, and it is even believed that the name “Chios” originates from the Phoenician word for mastic. While mastic shrubs grow throughout the island, mastic production is exclusively confined to southern Chios, specifically in the Mastichochoria (Mastic Villages), where the climate is particularly hot and dry (Figure 2). This region is bordered to the north by a line connecting the villages of Lithí, Agios Georgios Sykousis, and Thymiana—a boundary described by the linguist Pernot as “natural and mysterious”. Attempts to extend cultivation beyond this line have consistently failed. This exclusivity is likely attributed not only to tradition but also to the unique soil and climatic conditions that favor mastic cultivation exclusively in this part of Chios.

The inability of the mastic tree to grow successfully outside southern Chios—even in mainland Greece, other Greek islands, or the neighboring Anatolian coasts—is remarkable. Attempts to transplant the mastic tree to regions such as Attica and other Aegean islands have consistently failed [3,4,15]. Although limited success was observed in Amorgos and Antiparos, these efforts were ultimately abandoned. Further attempts, as noted by Pernot [3,4,5], to cultivate the tree in Rhodes and Lesvos also proved unsuccessful.

The paradox of mastic shrubs being found throughout the Mediterranean while systematic cultivation and mastic production occur exclusively in southern Chios may be attributed to three key factors:

• Unique Microclimate: Chios features an elongated shape with tall, forested mountains in the north that retain moisture and weaken northern winds. The southern hilly region enjoys a distinct climate characterized by mild winters and very dry summers. Rain often falls across the island but rarely in the Mastichochoria. The hot, dry summers in this area are crucial for mastic resin to dry properly; if the resin becomes wet before it “matures,” it is ruined [10,11,15].
• Selective Breeding: Since ancient times, Chian farmers have identified trees with superior resin yields (Figure 3) and propagated them to establish new plantations, preserving the characteristics of the parent trees. Over centuries, this process of selective breeding has resulted in the highly productive mastic variety now botanically recognized as Pistacia lentiscus var. chia [4,11,15].
• Sound Management: From antiquity, the people of Chios have organized mastic cultivation, standardized the product, and effectively promoted it in the market.

These three factors—microclimate, selective breeding, and sound management—have uniquely tied mastic production to southern Chios.

Figure 3. Boles of Mastic trees displaying the characteristic cuts and the resulting mastic tears (Reproduced with permission from Chios Gum Mastic Growers Association. Annual Report 2024) [11].

Figure 3. Boles of Mastic trees displaying the characteristic cuts and the resulting mastic tears (Reproduced with permission from Chios Gum Mastic Growers Association. Annual Report 2024) [11].

1.3. Care of the Mastic Tree

Plantations: The location of mastic plantations is determined by local factors, covering a total area of approximately 25,000 stremmas (2500 hectares). Trees are planted in rows, spaced 2–3 m apart. While the canopies of the trees touch one another, sufficient space is maintained between trunks to ensure healthy growth. Direct sunlight on the trunk may affect resin output. With the assistance of farmers, the trunk branches into 2–4 lateral shoots, forming a dense crown. The shape of the crown is critical as it influences access to the trunk.

Cultivation: Mastic cultivation requires a temperate and cool climate, as the trees are destroyed at temperatures below 0 °C [14]. They thrive in coastal, well-ventilated areas with dry, calcareous soils of moderate fertility. However, in fertile soils with low moisture, both the lifespan and resin yield of the trees are higher [15,16].

Propagation: Propagation is carried out using cuttings with visible buds. Large cuttings are planted in their final position at a depth of 40–50 cm and at a slight tilt, from late autumn to early spring. These cuttings are pre-selected the previous year and planted in February or March. Rows are spaced 3–4 m apart to facilitate plowing and digging. Expansion of cultivation is regulated by law to maintain a balance between supply and demand. In 1940, 1.5 million trees were recorded, increasing to 2,184,684 by 1986. Today, approximately 1,150,000 mastic trees are cultivated in southern Chios, covering an area of 20,000 stremmas (2000 hectares) [11,12,15].

Pruning: Pruning begins in the tree’s third year. Light pruning is performed annually to remove dry branches, while more systematic pruning occurs every 5–6 years to shape the tree. This practice enhances aeration, optimizes sunlight exposure, and facilitates tasks such as digging and fertilizing. After pruning, cuts must be treated with tar or other drying substances to prevent disease.

Watering: Young mastic trees require watering, especially during their first summer, with 4–6 waterings depending on climate and soil conditions. In the second and third years, additional watering improves results. Mature mastic trees are drought-resistant, but excessive moisture can damage the trees, reduce resin quality, and increase the risk of infections [11,12,15,17].

Fertilization: Although mastic trees thrive in rocky, poor soils, systematic fertilization significantly enhances growth and yield. Chemical fertilizers such as ammonium sulfate and potassium nitrate (6-8-8 N-P-K) yield excellent results, with ammonium sulfate (21-0-0 N-P-K) being particularly suitable for poor soils. Fertilization is applied annually in January or February, using 0.5–1 kg per tree. An alternative green fertilization method involves sowing legumes, such as broad beans, in October. These legumes are plowed under when they flower, enriching the soil with nitrogen essential for tree growth [11,12].

Diseases: Mastic trees are generally resistant to diseases, and no systematic control program is implemented. The most serious threat is a fungal disease (white rot) caused by Polyporus species, which damages roots and branches. Another fungus, Eutyra armeniaca, causes complete drying of the tree, necessitating uprooting, burning, and disinfecting [18].

Sensitivity to Weather Conditions: Mastic cultivation is highly sensitive to weather conditions [14,19,20,21,22,23]. The resin is harvested through incisions in the bark, relying on specific microclimatic conditions to solidify and be collected effectively [6]. Excessive rainfall during the harvest season can damage the resin or make collection impossible, while extremely high temperatures can inhibit solidification. For instance, in September 2024, extreme rainfall caused significant damage, with an estimated loss of around 40 tons of mastic production in the Mastichochoria region [11]. Recent years have also seen prolonged heatwaves negatively affecting resin exudation and drying [15,24,25].

Cultivation of Mastic: The cultivation of mastic (Pistacia lentiscus var. chia) is a highly specialized agricultural practice exclusive to southern Chios, Greece, in the region known as Mastichochoria. This area comprises traditional villages such as Pyrgi, Mesta, Vessa, and Kalamoti, where generations of farmers have perfected the art of producing mastic resin. Recognized with a Protected Designation of Origin (PDO) by the European Union [10], mastic cultivation is labor-intensive and sensitive to environmental conditions. The process begins in early spring with pruning and soil preparation. By mid-summer, typically between July and early September, small incisions (kentimata) are made in the bark, allowing resin to exude slowly. Over several weeks, the resin hardens into tear-shaped drops, which are manually collected, cleaned, and sorted (Figure 4 and Figure 5). Success depends on a precise set of climatic conditions.

1.4. Mastic Gum Cultivation in Chios and Its Climatic Sensitivities

The crop is particularly sensitive to weather conditions. Mastic resin is harvested through incisions in the bark and relies heavily on specific microclimatic conditions to solidify and be collected effectively [7,8]. Excessive rainfall during the harvest season can damage the resin or make collection impossible, while excessively high temperatures can inhibit solidification and reduce effective working days. Recent climate trends show increased interannual variability and a rise in the frequency and intensity of extreme events [2,19,23,24,26,27]. These challenges are particularly acute during the harvest season:

September 2024: A powerful rainstorm affected multiple mastic-producing areas, including Vessa, Pyrgi, Ionia, and Mesta, with localized rainfall totals exceeding 100 mm in 24 h. The Chios Mastic Growers Association estimated the loss of approximately 45 tons of mastic [28,29,30,31].

Heatwaves in 2021, 2022, and 2023: Prolonged heatwaves extended into early September, causing the resin to fail to solidify. In some cases, it remained sticky and vulnerable to microbial attack [32,33].

Phenological changes: Local farmers have reported changes in phenology, including earlier resin exudation or erratic flow patterns, which complicate the incision and collection process [11].

Given the global significance of mastic and its economic importance to the region—spanning applications in medicine, pharmacology [33,34,35], aesthetics, perfumery, and cooking [7,8]—it is vital to examine how evolving climate trends and extremes in Chios may impact this traditional and specialized cultivation [6,7,8].

2. Materials and Methods

2.1. Climate Extremes Indices

To assess trends in climate extremes, the study utilized indices developed by the Expert Team on Climate Change Detection and Indices (ETCCDI) [1,36,37,38]. These indices [see Appendix A], part of the CLIMDEX project supported by the World Meteorological Organization (WMO), are widely recognized for their consistency and comparability in analyzing extreme temperature and precipitation events.

Developed in collaboration with international agencies such as Environment Canada and the Australian Bureau of Meteorology, the ETCCDIs are based on daily observations and account for the frequency, intensity, and duration of extreme events. Unlike fixed-threshold approaches, many indices use percentile-based definitions, making them suitable for different climatic contexts and for long-term trend analysis.

These indices have been extensively applied in regional studies and global assessments, including those conducted by the Intergovernmental Panel on Climate Change (IPCC). They provide a robust framework for identifying changes in climate extremes such as heatwaves, cold spells, droughts, and intense rainfall.

Key indices used in this study include TXx (annual maximum of daily maximum temperatures), SU (number of summer days) [27,36,37], CDD (maximum number of consecutive dry days), RX1day (highest 1-day precipitation), and R95p (very wet days above the 95th percentile). This study evaluated specific indices associated with the mastic tree and its gum to derive conclusions. Climate change is increasingly recognized as a critical threat to agricultural systems worldwide, particularly in Mediterranean regions that are already prone to water scarcity, extreme weather, and rising temperatures [1,13,14,23]. Mastic is a product of both cultural heritage and environmental uniqueness and has been recognized with a Protected Designation of Origin (PDO) by the European Union [11,12,13], where cultivation practices have been passed down for centuries [11,14,15,31,39].

2.2. Optimal Climatic Conditions for Mastic Gum Production

The ideal climate for mastic gum cultivation is Mediterranean semi-arid, characterized by mild, wet winters and hot, dry summers [10,11]. The following climatic conditions are considered optimal:

• Temperature: Maximum daily temperatures of 30–36 °C during July–August promotes proper resin flow and solidification.
• Rainfall: Minimal summer rainfall (<10 mm/month) is crucial to prevent resin wash-off and microbial contamination.
• Humidity: Low relative humidity facilitates the drying and hardening of the resin.
• Diurnal temperature range: Significant night–day temperature differences support plant stress recovery and enhance resin production.
• Wind conditions: Calm winds during resin exudation minimize physical damage to the resin drops and reduce contamination from soil.

Table 1. Climatic stressors and their effects on Mastic Gum production.

Climatic Stressor	Critical Period	Observed or Potential Impacts on Mastic Gum Cultivation
High temperatures (>38–40 °C)	July–September	Delayed or incomplete resin solidification; reduced quality; stress on plant metabolism, fewer working days
Heatwaves (>3 consecutive hot days)	July–September	Inhibition of resin hardening; increased evapotranspiration; damage to young bark tissue
Intense rainfall (>50 mm/day)	August–September	Resin washed off or contaminated; muddy soil prevents collection; heightened risk of fungal rot
Rainfall during incision period	Late July–early August	Delayed resin exudation; increased risk of wound infection
High nighttime temperatures (>25 °C)	July–August	Lower diurnal temperature variation reduces physiological recovery; negative effects on resin biosynthesis
Low rainfall in winter (<150 mm total)	December–February	Reduced soil moisture recharge; weak vegetative growth during spring
Strong winds during harvest	August–September	Physical harm to resin drops; increased contamination from soil particles.
Unseasonal temperature fluctuations	Spring and autumn months	Disrupted resin flow timing; early or late resin exudation outside the collection schedule.

summarizes the key climatic stressors and their corresponding impacts on mastic gum production, highlighting the challenges faced by farmers in maintaining optimal yield and quality.

2.3. Meteorological Data and Station in Chios

The analysis covers data from 1960 to 2024 including rainfall, extreme weather indices (developed by the ETCCDI), and temperature variability. The data sources include the Hellenic National Meteorological Service (HNMS-ΕΜΥ) [40] and the National Observatory of Athens (NOA) [41].

The NOA data, spanning the years 2014–2024 (with 2023–2024 data used), were collected from a Davies II station located on the roof of the 1st Chios Senior High School (Lyceum). In contrast, EMY data (1960–2022) were obtained from a station situated at Chios Island Airport, approximately 1 km away from the NOA station (Figure 2). The geographical coordinates of Chios airport are 38.343° N latitude and 26.141° E longitude. A comparison of measurements from 2017 reveals relatively good agreement across three physical quantities, as demonstrated below in the results station.

3. Results

This section provides an integrated analysis of long-term climatic trends on the island of Chios, with a focus on their implications for mastic cultivation and harvest.

3.1. Comparison of HNMS (EMY) and NOA Measurements (2017)

A comparison of meteorological data from EMY and NOA for the year 2017 is presented, showcasing their alignment and supported by error bars that illustrate the monthly absolute differences between the two sources.

Maximum Temperature (°C): The average absolute difference for ‘Maximum Temperature (°C)’ is 1.10 units, with a standard deviation of 0.97.

Minimum Temperature (°C): The average absolute difference for ‘Minimum Temperature (°C)’ is 1.68 units, with a standard deviation of 1.53.

Precipitation (mm): The average absolute difference for ‘Precipitation (mm)’ is 0.16 units, with a standard deviation of 0.25.

Table 2. Quantitative Assessment of EMY-NOA Agreement.

Variable	Mean Abs. Diff.	Std. Dev.	Min Diff.	Max Diff.	% (Months) Diff ≤ 0.1	% (Months) Diff ≤ 0.5	% (Months) Diff ≤ 1.0
Max Temp (°C)	0.57	0.45	0.03	1.33	33.3	50.0	75.0
Min Temp (°C)	0.66	0.34	0.25	1.52	0.0	41.7	83.3
Precipitation (mm)	4.86	7.52	0.00	26.60	25.0	50.0	50.0

provides a summary of statistical measures that demonstrate the closeness of monthly observations between EMY and NOA for each variable.

Precipitation shows moderate agreement between the two datasets, with a mean absolute difference of approximately 4.9 mm and monthly discrepancies ranging from 0 to 26.6 mm. In contrast, air temperature exhibits substantially higher agreement. Maximum temperature differences are small, with a mean absolute difference of 0.57 °C, and 75% of the months differing by ≤1 °C. Minimum temperature also shows good consistency, with a mean absolute difference of 0.66 °C, although individual months exhibit deviations of up to 1.52 °C. These patterns of agreement and variability are illustrated in Figure 6, Figure 7 and Figure 8 through the inclusion of monthly error bars.

3.2. Climate Indices Results

The results for several key indices, essential for the growth of mastic trees and the production of mastic gum (

Table 3. Important indices for mastic tree and mastic gum corp (data from EMY for years 1960–2022).

Index	Description	Trend Direction	Impact on Mastic Cultivation
PRCPTOT	Annual total precipitation	↓ (slight)	Potential water stress
RX1day	Max 1-day precipitation	↑	Harvest damage in September 2024
R95p	Very wet days	↑	Increased erosion/flood risk
TXx and relative indices	Annual maximum daily temperature	↑	Resin solidification is hindered
WSDI	Warm Spell Duration Index	↑	Inhibits mastic formation
DTR	Diurnal Temperature Range	↓	Signals night-time warming

), are outlined here [11,21,22,38]. All trends were tested using the Mann–Kendall test. Trends with p < 0.05 were considered statistically significant.

The indices related to extreme precipitation and high-temperature extremes—RX1day, R95p, TXx, and WSDI—exhibit statistically significant trends (p < 0.05).

In contrast, although the DTR index shows a long-term decreasing tendency, the Mann–Kendall test did not identify this trend as statistically significant (p > 0.05). Likewise, the trend in PRCPTOT was not statistically significant.

The regional climate has exhibited signs of intensification in extreme rainfall events and prolonged heatwaves, particularly in the southern part of the island where mastic trees are predominantly cultivated. High summer temperatures, especially when sustained over several days (WSDI), along with reduced diurnal temperature ranges (DTRs), can hinder the solidification of mastic resin. On the other hand, intense rainfall events during harvest-sensitive periods, such as September, have led to direct production losses [28,29,30,31].

Observed trends indicate a warming climate and a more extreme precipitation regime in Chios, which could pose significant challenges to the sustainability of mastic cultivation unless effective mitigation measures are implemented. In Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20 and Figure 21, solid black straight line represents the trend line, dashed black line the polynomial regression curve illustrating long-term non-linear variation and thin grey line with open circles the actual measurements.

3.3. Climate Indices Relevant to Mastic Production in Chios (Period 1960–2022)

3.3.1. Temperature

• TX10P—Cool Day Frequency

TX10P represents the percentage of days with maximum temperatures below the 10th percentile. In Chios, this indicator has shown a statistically significant decline (slope: −0.101, p-value: 0.00), reflecting a reduction in cooler days throughout the year (Figure 9). This trend points to an overall warming environment, which increases the challenges for mastic resin to solidify properly, especially during the harvesting season [1,11,39].

During colder periods, Pistacia lentiscus experiences significant biomass reduction when grown at low temperatures [14]. Furthermore, the distribution of this species across Greece’s mainland, where winters are colder than in Chios, has been reported to be strongly influenced by low temperatures [14,15]. While heat-stressed plants demonstrate only minor developmental differences, cold-stressed plants show significant growth retardation. It appears that summer stress is manageable for these plants, whereas the brief yet severe chilling conditions during winter remain a persistent challenge for Mediterranean plant life [22,23,42].

Figure 9. TX10P—Cool Day Frequency and trend line.

Figure 9. TX10P—Cool Day Frequency and trend line.

• TX90P—Hot Day Frequency

The TX90P index represents the percentage of days in a given year during which the daily maximum temperature exceeded the 90th percentile of the reference distribution (data period 1960–2022). In this study, the 90th percentile was calculated using data from the entire available period (1960–2022), providing a stable climatological baseline for annual comparisons. An upward trend in TX90P signifies an increasing frequency of unusually hot days relative to the historical climate, indicating a shift toward more frequent extreme heat events (Figure 10). This trend aligns with broader global and regional warming signals.

Figure 10. TX90P—Hot Day Frequency and trend line.

Figure 10. TX90P—Hot Day Frequency and trend line.

Heat stress poses a significant challenge under projected global warming scenarios [1,25,26,39], with the frequency and intensity of heat waves expected to become more severe. Expanding our understanding of ecosystem responses and the adaptive strategies plants have developed to cope with heat stress could lead to innovative methods for enhancing crop resilience to extreme heat events [23,27,36]. Addressing this issue represents a critical scientific challenge with implications for the future of global agriculture.

Since the percentile thresholds remain fixed throughout the study period, observed increases in TX90p reflect genuine changes in the upper tail of the temperature distribution rather than natural variability. Such conditions can delay or inhibit the crystallization of mastic resin, particularly during the critical summer months.

• TXn—Minimum of Daily Max Temperatures

TXn represents the annual minimum value of daily maximum temperatures. An upward trend in TXn suggests that even the lowest maximum temperatures are increasing, reinforcing the broader observation of rising overall heat levels (Figure 11). This trend reduces the range of thermal recovery periods, which in turn impacts the drying behavior of mastic resin.

Figure 11. TXn—Minimum of Daily Max Temperatures and trend line.

Figure 11. TXn—Minimum of Daily Max Temperatures and trend line.

• TXx—Annual Maximum of Daily Max Temperatures

TXx represents the highest maximum temperature recorded each year. Although the trend is less pronounced, the frequent occurrence of extreme values around or exceeding 40 °C highlights the ongoing risk of extreme heat events (Figure 12). Such conditions can adversely affect both the quality and yield of mastic [15,19,23].

Figure 12. TXx—Annual Maximum of Daily Max Temperatures and trend line.

Figure 12. TXx—Annual Maximum of Daily Max Temperatures and trend line.

• WSDI—Warm Spell Duration Index

WSDI measures the annual count of days that occur within warm spells—defined as periods of at least six consecutive days with maximum temperatures exceeding the 90th percentile. In Chios, the trend shows a statistically significant increase (slope: 0.13, p-value: 0.032), indicating more frequent and prolonged heat waves (Figure 13). These warm spells pose a significant threat to mastic production, as they can result in extended periods during which the resin fails to solidify, thereby increasing the risk of widespread losses during the harvest phase [11].

Figure 13. WSDI—Warm Spell Duration Index (thin solid line with circles). Precipitation and trend line.

Figure 13. WSDI—Warm Spell Duration Index (thin solid line with circles). Precipitation and trend line.

• DTR—Diurnal Temperature Range

The diurnal temperature range (DTR) exhibits a statistically significant decreasing trend (p = 0.024, slope = −0.007), likely driven by rising nighttime temperatures (Figure 14). The narrowing of the Tmax—Tmin range aligns with other findings associated with the nighttime “greenhouse effect” [1,36].

Figure 14. DTR—Diurnal Temperature Range and trend line.

Figure 14. DTR—Diurnal Temperature Range and trend line.

• TNn and TNx—Lowest Tmin and Highest Tmin during the period

The analysis of temperature indices TNn and TNx over the period 1960–2022 reveals distinct patterns in cold and warm extremes. The TNn index (coldest minimum temperature of the year) shows a strong and statistically significant warming trend of about +0.069 °C per year. This indicates that extremely cold nights have become notably less severe, reflecting a substantial reduction in cold extremes.

In contrast, the TNx index (warmest minimum temperature of the year) exhibits only a weak, statistically insignificant upward trend (+0.015 °C per year). This suggests that the hottest nights of the year have not changed consistently over time, with considerable interannual variability masking any clear long-term signal.

Overall, the results highlight a pronounced warming at the lower tail of the minimum temperature distribution (fewer very cold nights), while the upper tail (very warm nights) remains relatively stable.

Figure 15. TNn—Lowest Tmin during the period and trend line.

Figure 15. TNn—Lowest Tmin during the period and trend line.

Figure 16. TNx—Highest Tmin during the period and trend line.

Figure 16. TNx—Highest Tmin during the period and trend line.

• SU25—Days with Tmax ≥ 25 °C and FD0- Days with Tmin < 0 °C

The analysis of the SU25 index, which represents the number of days with maximum temperatures above 25 °C, reveals a slight but consistent increasing trend over the period 1960–2022 (slope = +0.126 days/year, p = 0.073). Although the trend is only marginally significant, the data suggests a gradual rise in the frequency of warm days, particularly after the 1990s, which aligns with broader warming patterns observed in the eastern Mediterranean region. Prolonged periods of elevated temperatures could have direct implications for local ecosystems and agricultural practices, especially in crops sensitive to thermal stress.

Figure 17. SU25—Days with Tmax ≥ 25 °C and trend line.

Figure 17. SU25—Days with Tmax ≥ 25 °C and trend line.

In contrast, the FD0 index, representing the number of frost days (Tmin < 0 °C), exhibits a statistically significant decreasing trend (slope = −0.122 days/year, p < 0.001). Frost events have become increasingly rare in recent decades, with many years after 2000 recording no frost days at all. This reduction in cold extremes indicates a substantial shift towards milder winters, which can benefit certain crops by reducing frost-related damage. However, combined with the observed rise in warm days, it may also exacerbate challenges associated with heat stress, water availability, and phenological changes.

Figure 18. FD0—Days with Tmin < 0 °C and trend line.

Figure 18. FD0—Days with Tmin < 0 °C and trend line.

For the island of Chios, where mastic tree cultivation (Pistacia lentiscus var. chia) plays a vital socioeconomic role, these climatic shifts are particularly relevant. Higher temperatures during the summer months may affect the resin exudation process, potentially delaying or reducing mastic production, while milder winters may alter dormancy cycles and pest dynamics. Thus, the combined evidence from both indices highlights an emerging trend towards a warmer local climate, carrying significant implications for sustainable agricultural management and long-term adaptation strategies.

3.3.2. Precipitation

Rainfall plays a vital role in any type of cultivation, particularly in Chios Island where mastic gum harvesting is a significant activity. Rainfall patterns are typically categorized as either “beneficial” or “harmful.” Mastic trees require rain during their early developmental stages (up to their fifth year), except during late summer. However, rainfall in late summer and September can be detrimental to the trees and the harvest. On the other hand, rain from October to March is highly beneficial and considered a blessing for the trees. Based on the three precipitation indices below, we can observe an almost steady state of rainfall patterns for Chios Island (Figure 19, Figure 20 and Figure 21). The primary challenges, however, arise from heavy rainfall during the harvest period, which can disrupt the process and negatively impact the yield.

• Annual Total Precipitation (PRCPTOT)

Figure 19. Annual Total Precipitation (PRCPTOT) and trend line.

Figure 19. Annual Total Precipitation (PRCPTOT) and trend line.

• R95p Very Wet Days

Figure 20. Very Wet Days (R95p) and trend line.

Figure 20. Very Wet Days (R95p) and trend line.

• RX1day Maximum 1-Day Precipitation

Figure 21. Maximum 1-Day Precipitation (RX1day) and trend line.

Figure 21. Maximum 1-Day Precipitation (RX1day) and trend line.

3.3.3. Development and Analysis of the Mastic Phenology Suitability Index (MPSI): 1960–2024

To evaluate the potential of each year to support optimal mastic yield, a composite climatic index, the Mastic Production Suitability Index (MPSI_year), was utilized. This index integrates critical temperature and precipitation variables during the resin development period (July–September), capturing both heat accumulation and moisture availability. By correlating annual MPSI values with historical mastic production records, the sensitivity of the crop to seasonal climate fluctuations can be assessed.

Years with high MPSI values typically indicate favorable growing conditions, leading to increased resin output and improved yield quality. Conversely, years with low MPSI values are often associated with adverse climatic conditions, such as extreme heat, excessive rainfall, or drought. These factors can disrupt resin solidification, diminish its quality, or hinder production altogether.

The MPSI serves as a valuable tool for evaluating the climatic suitability of mastic tree phenology in Chios during the critical harvest season (July–September) [10,11].

It is calculated as follows:

$$\mathrm{MPSI}=\mathsf{\alpha }·{\displaystyle \frac{{\mathrm{G}\mathrm{D}\mathrm{D}}_{JAS}}{\mathrm{m}\mathrm{a}\mathrm{x}\left({\mathrm{G}\mathrm{D}\mathrm{D}}_{JAS}\right)}}+\mathsf{\beta }·(1-{\displaystyle \frac{R_{JAS}}{\max \left({\mathrm{R}}_{JAS}\right)}})$$

(1)

where:

• α = 0.6, β = 0.4 are non-dimensional weighting empirical coefficients emphasizing thermal over moisture conditions.
• GDD_JAS: Sum of Growing Degree Days (T_G—15 °C) for July–September

  $${GDD}_{JAS}={\sum }_{i=1}^n\mathrm{m}\mathrm{a}\mathrm{x}(T_{G,i}-T_{base}, 0)$$

  (2)
• where GDD_JAS is the sum of Growing Degree Days for the July–September period, T_G,i is the daily mean air temperature on day i, T_base is the base temperature (15 °C), and n is the total number of days in the July–September period.
• R_JAS: Total rainfall in mm for July–September
• maxGDD_JAS (in days) and maxR_JAS (in mm) are the maximum values in the 1960–2024 dataset [40]

The GDD_JAS index represents the cumulative Growing Degree Days (GDDs) during the July–September period, calculated using a base temperature of 15 °C [43,44]. This index captures the total thermal energy available to support plant growth during late summer and early autumn, a critical phase for many Mediterranean crops. The chosen base temperature of 15 °C reflects the thermal threshold above which active growth and physiological processes occur in many thermophilic Mediterranean species. For crops such as mastic (Pistacia lentiscus var. chia), olives, and grapevines, this threshold is a reasonable baseline, marking the onset of productive development during summer [16,21].

Using a lower threshold, such as 10 °C, would overestimate thermal accumulation during cooler periods that may not significantly contribute to growth. Conversely, a higher threshold, like 18 °C, would restrict the calculation to only the warmest days, shifting the focus from general development to potential heat stress conditions. Therefore, 15 °C strikes a balance, providing an accurate estimate of the effective growing energy available during the critical July–September period.

In this study, GDD_JAS values are used to evaluate seasonal warming trends and their potential impacts on crop development, phenology, and harvest timing. An increasing GDD_JAS trend may signify a longer or more intense growing season, but it also raises the risk of heat-related stress during key production stages. For mastic trees, which rely on stable summer temperatures for proper resin formation and solidification, elevated GDD_JAS values could influence both yield and resin quality [8,22,45].

Table 4. MPSI Values (1960–2024)—Years with Lower Values Indicating Unsuitable Conditions for Mastic Harvest.

Year	JAS GDD	JAS Rain (mm)	MPSI
1978	801.4	239.8	0.406
2002	925.5	94.6	0.473
1996	838.6	53.7	0.549
2024	1185.1	101.6	0.576
1976	699.6	0.0	0.590
1964	864.4	32.7	0.630
2009	902.1	31.5	0.661
1971	857.4	18.4	0.668
2006	959.7	40.2	0.674
1968	904.0	27.1	0.677
2023	1157.6	40.4	0.812

highlights years with lower values, indicating unsuitable conditions for mastic harvest.

Figure 22 presents the temporal evolution of MPSI values from 1960 to 2024, illustrating the suitability of climatic conditions for mastic production across the study period. The dashed red line represents the suitability threshold (MPSI = 0.75), which is identified in this research as the minimum value necessary for optimal resin yield and quality. The threshold value of MPSI = 0.75 was empirically determined based on the correspondence between historical resin yield records and the calculated index values (see below in the selected years).

This value represents the minimum climatic suitability required for optimal mastic resin production, reflecting both adequate temperature conditions and moderate moisture stress. Years with MPSI values above the threshold are considered favorable for mastic production, while years below it reflects suboptimal growing conditions. This figure underscores the sensitivity of mastic trees to climatic fluctuations, as analyzed in the context of this study.

Figure 22. MPSI Values (1960–2024)—Dashed Red Line Represents Suitability Threshold (MPSI = 0.75).

Figure 22. MPSI Values (1960–2024)—Dashed Red Line Represents Suitability Threshold (MPSI = 0.75).

Selected Years and Notes on Mastic Production Conditions

Year 1978

GDD_JAS: 801.4

Rainfall (JAS): 239.8 mm

MPSI: 0.406

1978 experienced the highest rainfall during July–September (239.8 mm), which significantly suppressed the MPSI. Despite moderate heat accumulation (GDD = 801.4), the excessive rainfall resulted in an MPSI of 0.406, marking extremely poor conditions for mastic production.

Year 2024

GDD_JAS: 1185.1

Rainfall (JAS): 101.6 mm

MPSI: 0.576

2024 recorded the highest GDD on record (1185.1), reflecting extreme summer heat. However, rainfall totaling 101.6 mm, particularly in September, suppressed the MPSI to 0.576. This led to one of the worst harvests, with an estimated loss of 40 tons of mastic due to excess moisture. Additional comments about 2024 can be found in Section 5.

Year 2023

GDD_JAS: 1157.6

Rainfall (JAS): 40.4 mm

MPSI: 0.812

2023 featured very high GDD (1157.6) paired with low rainfall (40.4 mm), resulting in a high MPSI of 0.812. This has made it one of the most favorable years for mastic production in recent decades.

Year 1996

GDD_JAS 838.6

Rainfall (JAS): 53.7 mm

MPSI: 0.549

1996 experienced relatively low GDD (838.6) and moderate rainfall (53.7 mm), culminating in an MPSI of 0.549. This indicates a moderately unsuitable year, primarily constrained by limited thermal conditions.

However, in certain years—such as 2024—the original formulation overestimates suitability because extreme temperatures or localized intense rainfall events are not adequately captured by seasonal averages. To address this, we introduced a penalty term to account for unfavorable stress conditions:

$$\mathrm{MPSI}=\mathsf{\alpha }·{\displaystyle \frac{{\mathbf{G}\mathbf{D}\mathbf{D}}_{\mathit{J}\mathit{A}\mathit{S}}}{\mathbf{m}\mathbf{a}\mathbf{x}\mathbf{\left(}{\mathbf{G}\mathbf{D}\mathbf{D}}_{\mathit{J}\mathit{A}\mathit{S}}\mathbf{\right)}}}+\mathsf{\beta }·(\mathbf{1}\mathbf{-}{\displaystyle \frac{{\mathit{R}}_{\mathit{J}\mathit{A}\mathit{S}}}{\mathbf{max}\mathbf{\left(}{\mathbf{R}}_{\mathit{J}\mathit{A}\mathit{S}}\mathbf{\right)}}})-\mathsf{\gamma }·\mathrm{Penalty} \mathrm{year}$$

(3)

• Penalty year accounts for negative stress events such as extreme daily temperatures, sudden rainfall events (e.g., September storms), or erratic climatic variability.
• α, β, and γ are weighting factors, calibrated to ensure that their sum approximates 1 (e.g., α = 0.5, β = 0.4, γ = 0.1).

Penalty year is a normalized indicator (ranging from 0 to 1) representing the intensity of climatic stress conditions for a given year. It can be defined using normalized ETCCDIs that capture extreme heat and precipitation events, such as TXx (maximum daily temperature), R95p and RX1day (extreme rainfall events mainly during harvest period), and CDD (consecutive dry days). The formulation can be expressed as:

Penalty year = w₁ × (TXx_year/TXx_max) + w₂ × (R95p_year/R95p_max) + w₃ × (RX1day_year/RX1day_max) where w₁ + w₂ + w₃ = 1, and each w coefficient reflects the relative importance of each climatic stress factor. Specifically, we used w₁ = 0.40, w₂ = 0.35, and w₃ = 0.25, following the relative influence of temperature stress, water deficit, and extreme events on mastic tree physiology. The higher the Penalty year value, the stronger the negative climatic impact on mastic production.

This formulation ensures that the MPSI captures both the favorable seasonal averages and the detrimental effects of short-term climatic extremes that can significantly affect mastic resin productivity. Below are the results derived from the modified MPSI and the corresponding mastic gum production data (provided by the Chios Mastic Producers Association) [11] versus year (1994–2024) (Figure 23). For earlier years, the records are less accurate due to the absence of systematic monitoring of mastic trees. Over the last 30 years, however, records indicate a relatively stable number of trees [10,11].

4. Correlation of Climate Indices with Mastic Tree Growth and Gum Production

The relationship between climate indices and the growth, harvest, and gum production of mastic trees can be clearly demonstrated. Notably, the modified index (MPSI) shows a strong correlation with gum production trends over the past 30 years.

The correlation analysis (

Table 5. Pearson and Spearman correlation analysis for the MPSI vs. GDD_JAS vs. JAS vs. yield and other parameters.

Variables	Pearson r	p-Value	Spearman ρ	p-Value
MPSI vs. GDD_JAS	0.787	2.5 × 10−7	0.898	1.6 × 10−11
MPSI vs. Rainfall_JAS	−0.798	1.3 × 10−7	−0.611	3.3 × 10−4
MPSI vs. Yield	0.444	0.014	0.355	0.054
GDD_JAS vs. Rainfall_JAS	−0.256	0.173	−0.338	0.068
GDD_JAS vs. Yield	0.421	0.021	0.342	0.064
Rainfall_JAS vs. Yield	−0.286	0.125	0.031	0.870

) reveals that MPSI is strongly and positively correlated with summer GDD, while exhibiting a strong negative correlation with summer rainfall. This indicates that warm and dry summers enhance the MPSI signal. Additionally, the correlation between MPSI and actual mastic yield is moderate but statistically significant, highlighting the potential of MPSI as an integrative climatic suitability indicator. Furthermore, GDD_JAS shows a positive and statistically significant relationship with mastic yield, whereas summer rainfall does not display a significant correlation. These findings underscore the critical role of thermal conditions during the summer months as a key driver of year-to-year variability in mastic gum production.

We report both Pearson and Spearman correlations to assess linear and monotonic climate–yield relationships. Spearman’s ρ is more effective in capturing the monotonic response of MPSI to climatic variability, while Pearson’s r highlights the strongest linear relationship between MPSI and actual mastic yield. This combined approach offers a more comprehensive and robust validation of the index.

Nevertheless, it is important to acknowledge that additional climatic factors also play a significant role in influencing the growth and productivity of mastic trees.

4.1. November–December Rainfall

An essential aspect of climate change, which is not captured by the ETCCDI model indices, is late autumn rainfall. This specific rainfall period is critical for the growth and harvest of various tree species, including olives and citrus, which are cultivated on Chios Island [11,45].

High interannual variability in late-autumn rainfall has been observed. The wettest year on record was 1960, with a total of 435.5 mm, whereas 2015 experienced the lowest value, only 23.4 mm (Figure 24). These extremes highlight the growing unpredictability of precipitation, which can significantly impact early plant development and overall water availability. For instance, 2019 was an exceptional year for olive oil production, with yields increasing by nearly 40% compared to the previous year.

4.2. Extreme Rainfall and Climate Impact on Mastiha Cultivation—Chios, September 2024

4.2.1. Rainfall Geographical Distribution Across Southern Chios September 2024

In September 2024 (Figure 25), the island of Chios experienced localized intense rainfall episodes that severely impacted the southern mastic-producing regions. Data collected from five meteorological stations (Chios EAA, Pyrgi, Vessa, Chios Town, and Ionia) revealed significant variations in total precipitation, ranging from 21.8 mm in Vessa to 131.2 mm in Chios Town. The heaviest rainfall occurred in the central and eastern parts of the island, while the western and inland areas received considerably less precipitation [11,28,29,30,31].

4.2.2. Impact on Mastic Production

As reported by multiple sources [11,36,37,38,39], the intense rainfall on 10–11 September caused damages exceeding 4.5 million euros, with an estimated loss of approximately 40 metric tons of mastic due to flooding, contamination, and crop degradation. Local producers emphasized that the rainfall occurred during the critical harvesting period, leading to resin losses and difficulties with solidification. These events underscore the vulnerability of the mastic sector to climate extremes, particularly given the product’s uniqueness and its reliance on dry and stable conditions during the summer and early autumn.

The limestone soil, combined with the mastic tears, underwent complete dissolution and degradation due to the intense rainfall and flooding, highlighting the vulnerability of the resin and its surrounding environment to extreme weather conditions (Figure 26).

4.3. Historical Context—September Rainfall (1960–2024)

Historical data from the Hellenic National Meteorological Service (HNMS) indicate that the average September rainfall in Chios from 1960 to 2023 was approximately 29 mm (Figure 27). In sharp contrast, the September 2024 rainfall of 131.2 mm recorded in Chios Town represents one of the most extreme events in recent decades [40]. Figure 27 highlights that the years 1978 and 2024 recorded the highest total rainfall values for September in Chios, emphasizing the rarity and extremity of these events within the observed period (1960–2024).

5. Discussion

This study examines the impacts of climate change on the island of Chios, with a focus on understanding the shifts affecting local agricultural production and the community. Overall, the results suggest a warming trend accompanied by a rise in the frequency and intensity of extreme temperature and precipitation events, rather than substantial changes in mean climatic conditions. Τhis is likely attributed to the island’s geographical features, including its elongated shape and proximity to the Aegean Sea. The strong winds prevalent throughout the year in the Aegean appear to act as a stabilizing factor for climatic conditions. Nevertheless, various ETCCDI climate indices revealed subtle yet significant changes that have already begun to impact the local community and agriculture [43,44,46].

Specifically, the study underscored the increasing sensitivity of Pistacia lentiscus var. chia cultivation to evolving climatic conditions, particularly in the southern region of Chios, where mastic gum production is concentrated. By integrating ETCCDI climate indices with a customized Mastic Production Suitability Index (MPSI), the study assessed the influence of recent changes in temperature, rainfall, and extreme events on mastic gum yield. Findings indicated that the increased frequency and intensity of high-temperature events, as reflected in indices such as TX90p, TXx, and WSDI, likely disrupt the physiology of mastic trees, negatively affecting resin exudation and solidification.

Our findings align closely with broader Mediterranean research. Moriondo et al. [43] emphasize the significant impact of heat extremes on crop responses, which supports our observation that exceptionally warm summers can adversely affect mastic productivity. The variability patterns we identify are consistent with the regional climatic fluctuations described by Xoplaki et al. [44]. Furthermore, the warming trend observed in Chios mirrors the broader temperature increases across the Eastern Mediterranean, as reported by Luterbacher et al. [47], thereby underscoring the regional relevance of our results.

Moreover, intensified rainfall episodes during the critical harvest period, such as those in September 2024, had devastating consequences, resulting in crop losses of approximately 40 tons. While the original MPSI underestimated the impacts of such extreme events in certain years, including 2024, it was adjusted by introducing a penalty term for extreme conditions. The modified MPSI demonstrated a strong correlation with actual production records over the past 30 years, proving its potential as a predictive and strategic planning tool. Data showed that years with high summer growing degree days (GDDs) and low rainfall generally favor resin production, up to a certain threshold. Long-term trends reveal increasing GDD and reduced diurnal temperature range (DTR), both of which likely influence plant physiological responses and resin formation.

6. Conclusions

This study highlights the critical impacts of climate change on the cultivation of Pistacia lentiscus var. chia in Chios, Greece, emphasizing the increasing vulnerability of mastic production to evolving climatic conditions. The findings reveal a clear warming trend, along with a rise in extreme weather events, such as heatwaves and intense rainfall. These changes are shown to directly disrupt key processes of mastic cultivation, including resin exudation and harvesting.

The introduction of the MPSI has proven effective in linking climatic variables with mastic yield, offering a valuable tool for assessing long-term trends. The enhanced version of the MPSI, which incorporates penalties for extreme events, further strengthens its predictive capacity by aligning closely with historical production data. This refined approach provides a more accurate representation of the challenges posed by climate variability.

The study emphasizes the need to develop adaptive strategies that mitigate the adverse effects of climate change on mastic cultivation. These strategies may include optimizing irrigation systems, improving soil management practices, and enhancing the resilience of mastic trees to heat and drought. Moreover, further research into the effects of additional environmental stressors—such as wind patterns and soil moisture dynamics—is essential to gaining a more comprehensive understanding of climate change impacts on mastic production.

Ultimately, the findings highlight the necessity of integrating climate resilience into agricultural management practices to safeguard the unique cultural and economic significance of mastic production in Chios.

In future research, we aim to broaden our analysis by integrating additional climatic parameters and methodological approaches, as suggested by recent reviews and peer feedback. These enhancements will include a detailed investigation of extreme weather indices, higher-resolution spatial modeling, and the application of advanced statistical techniques—such as generalized additive models and multivariable regression—to better quantify the combined effects of overlapping climatic drivers. Furthermore, we plan to assess the impact of local microclimatic conditions on mastic tree productivity. These extensions will provide a more comprehensive understanding of the climatic factors influencing environmental and agricultural conditions in Chios.