Leonardo Costanza1
Francesco Maldera1*
Simone Pietro Garofalo2*
Gaetano Alessandro Vivaldi1
Salvatore Camposeo1- 1Department of Soil, Plant and Food Science, University of Bari Aldo Moro, Bari, Italy
- 2Council for Agricultural Research and Economics, Research Center for Agriculture and Environment (CREA-AA), Bari, Italy
Site selection analysis is a fundamental methodology for the regeneration of Xylella fastidiosa subsp. pauca (Xfp) infected areas, with the introduction of Xfp immune/resistant tree crop species. The diffusion of these species could be assessed by combining ecological optima data, climate and soil attributes of the study area, and GIS tools. The study aimed to evaluate the potential suitability of eight Xfp immune tree crops, including Neglected and Underutilized Species (NUS) drought-resistant and new species, as follows: carob, hawthorn, prickly pear, mulberry, loquat, walnut, persimmon, and avocado. The use of GIS tools allowed the integration of different layers, such as climate and soil, to contribute to the identification of suitable areas for the cultivation of these tree crops helping the policy-makers to define plans for land use at a regional scale. Following the ecological optima, which represents the ideal environmental conditions for each species, this analysis provided valuable insights into the compatibility of the selected tree crops with the prevailing environmental factors in the affected area. Carob revealed its remarkable adaptability and drought resistance, presenting the broadest suitability. Hawthorn and Loquat also exhibited high adaptability, indicating their potential contribution to agricultural diversification and ecological balance. Conversely, crops like Avocado, Prickly pear, and Walnut, despite their economic value, demonstrated limited adaptability due to their specific soil and climate requirements. These findings can potentially contribute to the development of strategies for the policy-makers, aimed at diversifying and enhancing the resilience of agricultural systems, facing the problem of emerging quarantine pathogens and the incoming climate change, and highlighting the possibility of opening new cultivation scenarios in the zones affected by Xfp.
1 Introduction
The recent incursion and widespread devastation caused by Xylella fastidiosa subsp. pauca (Xfp) in the Apulia-South of Italy has significantly impacted the regional olive cultivation and, by extension, its economy and cultural heritage (Calabrese et al., 2012; Scortichini, 2020). Olive trees, the cornerstone of Apulian agriculture, have not only economic but also historical and environmental significance, forming an integral part of the Mediterranean landscape and biodiversity (White et al., 2017; Garofalo et al., 2024a), leading it to be the region with major production of olive oil (Ciervo and Scortichini, 2024). However, the spread of Xfp has led to the death of millions of olive trees across Apulia, necessitating urgent research into new cropping systems, sustainable agricultural alternatives, and land management strategies to mitigate this crisis (Saponari et al., 2019; Maldera et al., 2023). Recent studies have emphasized the importance of exploring alternative crops that could be cultivated in the affected areas as a viable solution to the socio-economic and environmental challenges posed by the Xfp outbreak (Alhajj Ali et al., 2023).
1.1 Minor tree crops
Neglected and Underutilized Species (NUS) are crucial for their multifaceted benefits, particularly in enhancing biodiversity, contributing to food security, and mitigating climate change impacts. Among the wide range of species, two of relevant interest for Xfp-infected areas could be Carob and Hawthorn1. Carob (Ceratonia siliqua L., 1753) thrives in semi-arid conditions, making it a valuable species for arid and poor soils where other crops might fail (Batlle and Tous, 1997). Carob tree drought resistance and low water requirement position it as a critical species for adaptation strategies in increasingly arid environments (Correia et al., 2010; Eshghi et al., 2018), being able to mitigate the stress caused by soil flooding through adaptive mechanisms (Garofalo et. al, 2024b). Carob pods are a rich source of sugars, proteins, and essential nutrients, offering a gluten-free alternative to cocoa that can be used in a variety of food products, hence addressing food security and providing income generation opportunities for rural communities (El Batal et al., 2016). Hawthorn (Crataegus azarolus L., 1753) presents unique advantages: agronomically, this species is valued for its hardiness and ease of cultivation, thriving in a wide range of soils and conditions. Hawthorns are particularly noted for their beneficial effects in conserving biodiversity, serving as host plants for various insect species, and providing food and shelter for wildlife. Food-wise, the fruits of Hawthorn are highly nutritious, and rich in antioxidants, vitamins, and minerals, with traditional uses ranging from jams and jellies to medicinal products that have been linked to cardiovascular benefits (Zhang et al., 2001). The walnut tree (Juglans regia L., 1753) serves as a multifaceted asset in sustainable agriculture and ecological balance. Walnuts are highly valued for their adaptability and economic contribution, particularly in temperate climates where they thrive. Their deep root systems enhance soil structure and fertility, making them vital in agroforestry systems for intercropping and improving land use efficiency (McGranahan and Leslie, 2012). Moreover, the cultivation of walnuts contributes to carbon sequestration, a critical factor in mitigating climate change. These trees capture atmospheric carbon dioxide, storing carbon in their biomass and soil, thus contributing to the reduction of greenhouse gases (GHGs) (Vinceti et al., 2013). Moreover, walnuts offer significant nutritional benefits, thus they are rich sources of essential fatty acids, proteins, vitamins, and minerals, playing a crucial role in human diets and health (Ros, 2010). The global demand for walnuts has been rising, not only for direct consumption but also for their use in various food products, highlighting their importance in food systems and economies (Martínez et al., 2010). The White Mulberry (Morus alba L., 1753) is highly appreciated for its adaptability to various soil types and environmental conditions (Ercisli and Orhan, 2007). Its leaves serve as the primary food source for the silkworm (Bombyx mori), which is central to the silk industry, thus supporting the livelihoods of millions of people worldwide (Butt et al., 2008). Furthermore, the fruits of Morus alba are rich in vitamins, minerals, and antioxidants, offering health benefits and nutritional value to diets (Ercisli and Orhan, 2007). Prickly pear (Opuntia ficus-indica (L.) Mill., 1768) is exceptionally resilient to harsh environmental conditions, due to its cacti, address them to sustainable agriculture in drought-prone areas and contributing to food security and rural livelihoods by providing fruits, fodder, and even natural fencing (Choukr-Allah et al., 2016). Their minimal water requirements align with sustainable water management practices, essential in combating water scarcity. The nutritional value of prickly pear is significant: the fruits are rich in vitamins, minerals, and antioxidants, offering health benefits such as blood glucose regulation, which is crucial in diabetes management (Feugang et al., 2006). The persimmon (Diospyros kaki L.f., 1782) is a fruit tree of significant relevance, offering multiple benefits from agronomic, and food perspectives. Persimmons are rich in vitamins A and C, minerals, and dietary fibers, contributing to diversified and healthy diets (Castillo et al., 2023). The Loquat (Eriobotrya japonica (Thunb.) Lindl., 1821) is praised for its hardiness and low maintenance requirements. Its resilience to pests and diseases contributes to sustainable agricultural practices by reducing the need for chemical inputs (Lin et al., 2019). The tree exhibits remarkable adaptability to environmental stresses, including drought and temperature fluctuations, making it a strategic crop for regions facing climate variability (Demir, 1987; Freihat et al., 2008). The tree’s evergreen nature and dense foliage offer an effective contribution to carbon sequestration and mitigation of climate change. The fruits are rich in vitamins (notably vitamins A and C), minerals, antioxidants, and dietary fiber, supporting a balanced diet and contributing to human health (Khouya et al., 2022). The Avocado (Persea americana Mill., 1768) stands as a crucial agricultural commodity, not only for its nutritional value but also for its economic implications. Originating from south-central Mexico, the avocado tree has transcended its native boundaries to become a globally new tropical fruit into different diets and cultures. The cultivation of avocados is both a challenge and an opportunity: avocado trees are highly demanding in terms of climatic and soil conditions, preferring well-drained soils and a climate without extreme temperatures. However, when these conditions are met, they can be highly productive for decades, offering substantial economic benefits to growers. Innovations in irrigation, pest management, and breeding have improved yields and resilience, making avocado farming more sustainable and efficient (Sommaruga and Eldridge, 2021). Additionally, avocados contribute to biodiversity, as their cultivation supports a range of organisms, including pollinators crucial for the trees’ flowering and fruit set. Rich in healthy fats, vitamins, and minerals, avocados contribute to a balanced diet, supporting heart health and offering anti-inflammatory properties (Dreher and Davenport, 2013). The demand has also raised concerns about sustainability, water use, viability under water scarcity and salinity conditions, and the social impacts of intensive cultivation (Ramírez-Mejía et al., 2022). These species offer not only economic resilience but also potential benefits to the region’s biodiversity and ecological balance, contributing to the sustainable development of agricultural systems in the wake of Xfp impact.
1.2 GIS applications in site selection analysis and aim of the work
Choosing the right location for an industrial tree plantation is essential. This decision should be based on sustainable exploitation of the soil’s productivity and the suitability of local climate conditions to ensure the best possible growth of the selected tree species (Muñoz-Flores et al., 2011). Geographic Information Systems (GIS), as noted by Zhu, 2016, are crucial in advancing planning and land management strategies, facilitating the collection, organization, analysis, and visualization of spatial data. Pan and Pan (2012) emphasize the essential goal of land suitability assessments in agriculture to evaluate the capabilities and limitations of land for crop cultivation. In modern agricultural practices, land management, GIS plays a key role (Bill et al., 2012; Shadeed et al., 2017); this approach includes evaluating land suitability for crops using Boolean mapping techniques, as well as leveraging remote sensing (RS) imagery to enhance the accuracy and efficiency of these assessments (Hoobler et al., 2003; Wahba et al., 2007; Lanya and Manalu, 2021). Furthermore, concerning agroecological zoning, the integration of satellite remote sensing allows for a comprehensive analysis that considers topographical, vegetation, climate, and soil data (Akpoti et al., 2019). This multidimensional approach to land site selection analysis ensures that agricultural land use is optimized, promoting sustainable and productive farming practices.
Given the urgent need for sustainable agricultural alternatives in Apulia, this paper aims to evaluate the potential ecological suitability of eight different tree crops, including NUS, drought resistant, and new species, in the Apulian areas infected by Xfp. Utilizing GIS-based site selection analysis, this study seeks to provide comprehensive insights into the feasibility and potential impacts of introducing these alternative crops, thereby contributing to the diversification and resilience of the Apulian agricultural landscape in the face of ongoing and future Phyto pathological challenges. This approach provides valuable insights into the compatibility of these species with the environmental conditions in Xfp-affected areas, offering innovative strategies for agricultural diversification, and opening new cultivation scenarios.
2 Materials and methods
2.1 Study area
The Apulia region, South of Italy (Figure 1A), serves as a vital agricultural hub renowned for its extensive olive groves. These groves contribute significantly to the local economy through olive oil production and play a crucial role in biodiversity conservation and landscape heritage. In this study, we focused on the areas where Xfp is present and is constantly monitored [16.922 E, 39.789 N; 18.520 E, 41.082 N] (Figure 1B); The administrative limitations, in terms of the single province and region at national scale, were defined using the shapefiles of the Regional Territorial Landscape Plan – PPTR (updated on 15 November 2021) and Italian National Statistics Institute (updated on 2023), respectively. Additionally, the boundaries of the infected area (infected, containment, and buffer zone) were demarked using the regional Web Map Service (WMS) (emergenzaxylella.it, 2024a); the points show the distribution of infected olive trees considering a period from 2015 to 2023 (Figure 1C). These data were retrieved until 2021 using the online available dataset (Raparelli and Bajocco, 2023). Furthermore, to include 2022 and 2023 data in the analyses, we digitalized the data of the official regional monitoring authority by downloading each report from the Xylella emergency official site (emergenzaxylella.it, 2024b). These data confirm the capability of the vector P. spumarius to widespread OQDS against olive orchards (Figure 1C; EFSA, 2015).
Figure 1. (A) Apulia region at national scale; (B) location of the four infected provinces (Bari and Taranto are partial); and (C) distribution of olive orchards tested positive to Xfp and boundaries of infected area. For each area different restriction measures are applied (OpenStreetMap Contributors, 2024).
2.2 Climate dataset
Thermo-pluviometric data (with hourly frequency) considering a long-term period (1994–2023), have been provided by Regional Agency for Irrigation and Forestry Activities (ARIF). For the investigated zone, we considered 44 agro-climatic stations (Figure 2A) to process, compute, and interpolate the key climate variables for site selection analysis as rainfall (mm) and temperature (°C) threshold. In particular, Chilling Hours (CH) and Growing Degree Days (GDD) have been obtained using fruitclimpad and ChillR package (R software CRAN), respectively (Luedeling and Brown, 2011; Luedeling et al., 2011; Luedeling, 2012a, 2012b; Miranda et al., 2021). Inverse Distance Weighting (IDW) interpolation, a widely used deterministic method for spatial interpolation, was employed to illustrate the spatial variability of climatic patterns across the study area, emphasizing the significance of understanding climatic gradients in determining areas suitable and non-suitable from a climatic perspective (Supplementary Figure S1). IDW interpolation operates on the principle that points closer to the prediction location have a greater influence on the predicted value than points further away, with the degree of influence determined by the power parameter, p. Fine-tuning of this p parameter through Leave-One-Out Cross Validation (LOOCV) ensures the optimal balance between local detail and overall trend, enhancing the accuracy of spatial predictions (Figure 2B). This methodology is particularly pertinent in the context of the middle-south and south of the Apulia region, where climatic variability – characterized by gradients in temperature, precipitation, and other climatic factors – plays a crucial role in delineating areas conducive to specific agricultural practices, conservation efforts, and urban planning. By employing IDW interpolation fine-tuned with LOOCV, this study provides a nuanced understanding of the region’s climatic diversity, facilitating informed decision-making for land use and resource management based on climatic suitability. Lu and Wong’s exploration of the effectiveness of LOOCV in IDW interpolation (Lu and Wong, 2008) offers a methodological framework for optimizing the p parameter, ensuring the precision of spatial predictions. Voronoi polygons were employed for variables counting the days when temperatures exceeded specific thresholds (Supplementary Figure S1). This approach, which divides a plane into regions based on distances to a specific set of points, ensures that each location within a given Voronoi polygon is closer to its generating point than to any other, allowing for an accurate representation of temperature distribution across the study area. The use of Voronoi diagrams for spatial analysis in climatology, as demonstrated by Sugihara and May (1990), provides a robust framework for mapping and analyzing temperature exceedances over thresholds. This methodology is particularly relevant in regions with significant climatic gradients, such as the Apulia region, where the spatial distribution of temperature-related variables can significantly influence agricultural practices and biodiversity conservation (Figure 2B).
Figure 2. (A) The positions of agro-climatic stations, managed by Region al Agency for Irrigation and Forestry Activities (Regional Agrometeorological service – ARIF Puglia), used for gathering climate data. (B) Maps show the Voronoi diagrams, and the spatial variability of key climate variables obtained with IDW interpolation. (OpenStreetMap Contributors, 2024). (C) Map shows the distribution of yearly average temperature in the selected area.
2.3 Soil dataset
The regional soil database was sourced from the ACLA 2 project (ACLA 2; Agroecological characterization of the Apulia region as a function of production potential; Steduto and Todorovic, 2001) is dedicated to mapping the agroecological features of the Apulia region with an emphasis on its agricultural productivity potential (Figure 3). This comprehensive project facilitated the extraction of critical soil data, which included information on soil pH, texture (following the USDA classification guidelines), and salinity levels (ECe, dS/m), all of which were meticulously compiled into shapefiles for detailed analysis (Alhajj Ali et al., 2023). These data layers are instrumental in understanding the soil’s suitability for various types of crops. The maps provide a visual representation of these essential soil parameters, showcasing maps that detail the distribution of soil pH, salinity, and texture throughout the surveyed regions. This graphical illustration aids in the immediate identification of areas with optimal or challenging conditions for agricultural practices, serving as a valuable resource for researchers and practitioners alike in making informed decisions regarding land use and crop management.
Figure 3. Maps displaying key soil characteristics such as texture, salinity (ECe, dS/m), and pH across the studied areas in investigated zone, utilizing the ACLA2 database.
2.4 Alternative tree crops and ecological optima
The site selection analysis focused on the suitability of different tree crop species, like NUS (carob and hawthorn), drought-resistant (prickly pear), and high-water requirements species, such as mulberry, loquat, persimmon, and avocado. These species were considered because they are not included in the list, due to their resistance to Xfp (emergenzaxylella.it, 2024c). Ecological optima for each species were found through extensive bibliographic research, focusing on climate and soil conditions for these tree crops (Table 1). Values may also vary among cultivars because the response to these optima is strongly cultivar-dependent (Wang et al., 1992): the ones better adapted to high and low temperatures evolve their root structures to enhance water and nutrient uptake efficiency, aiming to maintain stable yields in these challenging conditions (Calleja-Cabrera et al., 2020). Furthermore, the suitability of this species in a specific region can vary continuously also based on what new species and cultivars breeders propose each year to the farmers (Kopeć, 2024).
Table 1. Ecological optima extracted from bibliographic research for the selected fruit tree crops.
Rainfall (mm/year) and chilling hours (hours) further contribute to defining the ecological optima for these tree crops, with specific requirements that ensure successful cultivation and yield. The gathered bibliographic data underscore the complex interplay between climate and soil conditions in cultivating these tree crops, highlighting the necessity of targeted research and management strategies to optimize growth conditions across the different regional areas.
2.5 Site selection analysis
The application of GIS and programming language software (R software 4.3.3 version) in site selection analysis enabled the creation of overlay raster and distribution maps of suitability area. Leisz et al., 2005 highlighted the effectiveness of GIS-based approaches in evaluating land suitability. The LTR (Long Term Release) version of QGIS software (3.34.4), employing the WGS 1984 UTM/ZONE 33N (EPGS: 32633) coordinate reference system, was used for cartographic and geospatial analyses. The process involved integrating various parameters to produce a comprehensive suitability map. For each ecological optima considered in site selection analysis for different crops, binary raster layers were created. In these sets, the raster layers pixel has been marked as 0.0 to indicate a no-suitable zone and 1.0 to indicate a suitable area, leading to the creation of the final suitability map that identified areas meeting all specified ecological criteria. Once individual raster layers were prepared, they were merged with a shapefile of agricultural zones identified by Corine Land Cover (CLC18v.20). This method, which is straightforward yet effective, is a commonly employed algorithm in GIS spatial analysis and has been utilized in prior studies (Alhajj Ali et al., 2023). It is of notable attention for the resulting suitable map for hawthorn, loquat, and avocado where rainfall (mm/year) optima have not been considered because they exceeded the regional average rainfall (mm/year). The reason why they were still considered is because of the use of irrigation for these orchards.
3 Results
The Xfp-resistant tree species responded significantly to the different pedo-climatic conditions. In Table 2 the growing areas suitable for each tree crop in the Xfp infected areas are reported. This detailed analysis encapsulated the potential diffusion of these minor tree species.
Table 2. Suitable areas (ha) of the selected fruit tree crops Xfp-infected areas: Bari (BA), Brindisi (BR), Lecce (LE) and Taranto (TA).
The results revealed nuanced insights into the matching of the ecological optima for each crop species with the pedological and climatic conditions of the assessed areas. The aggregated data, culminating in the total suitable area for each crop species, showed a comprehensive overview of the agricultural potential and regional suitability landscapes for these tree crops. This information is invaluable for stakeholders in agriculture, including farmers, policy-makers, and researchers, as it guides strategic planning, resource allocation, and the development of sustainable farming practices.
A map for each species was made, highlighting which zones are the most suitable (Figures 4, 5). This distribution highlighted the species-specific considerations required when planning for agricultural diversification and the relevance of aligning crop ecological requirements with the pedoclimatic characteristics of each area.
Figure 4. Distribution maps of the suitable areas for avocado, carob, hawthorn, and loquat within the agricultural zones affected by Xfp obtained. The different colored zones highlight areas that are suitable for each species, based on a combination of the different ecological optimal considered (binary site selection analysis).
Figure 5. Distribution maps of the suitable areas for mulberry, persimmon, prickly pear, and walnut within the agricultural zones affected by Xfp obtained. The different colored zones highlight areas that are suitable for each species, based on a combination of the different ecological optimal considered (binary site selection analysis).
On a total of 487,383 ha available in agricultural zones evaluated, avocado showed notable suitable areas in the province of BR (22,652 ha) and LE (34,528 ha), indicating that those areas could provide favorable climatic and soil conditions (Figure 4). However, in BA avocado cultivation was not suitable. Carob showed the highest number of hectares available for cultivation (263,362 ha), with almost half of them localized in Lecce (125,968 ha). Hawthorn exhibited widespread suitability, with significant areas in all provinces, particularly in BR (64,391 ha) and LE (59,570 ha). Loquat exhibited a widespread suitability across all surveyed provinces, with a significant presence especially in LE (140,462 ha), followed by BR (68,628 ha), TA (30,688 ha), and BA (13,166 ha). The total suitable area for loquat cultivation amounted to 252,943 hectares (Table 2; Figure 4). Mulberry, although showing a more limited distribution of suitability, with a notable absence of data for BA and a relatively small suitable area in TA (774 ha), identified pockets of high suitability in BR (15,792 ha) and LE (14,913 ha) (Figure 5). Persimmon showed a focused area of suitability exclusively in the province of LE, with a total of 13,114 hectares (Figure 5). Walnut presented a rather limited distribution, with a notable suitability only in LE (8,260 ha) and a minimal area in BR (416 ha), totaling 8,677 hectares. Prickly pear has identified suitable areas primarily in LE (85,229 ha), with smaller areas in BR (1,557 ha) and TA (949 ha), leading to a total of 87,734 hectares.
A focus was made only in olive tree zones (Supplementary Table S1) with a total area of 240,335 ha. Olive tree emerges as the most extensive crop in terms of suitability and area coverage (240,335 on a total of 487,383 ha) (Grotti et al., 2019). Such extensive cultivation is reflective of the olive’s historical, cultural, and economic significance, especially in the production of olive oil. Still, the Carob tree, with a total suitability of 127,468 ha, predominantly in the BR and LE provinces, highlights the great suitability of this crop. Similarly, the loquat, covering 116,885 ha with a significant presence in the LE province, illustrates the market’s demand for its fruits, fostering its cultivation in subtropical climates. The data also reveals the cultivation of hawthorn and mulberry, with 75,047 and 14,648 ha respectively, pointing towards their specific use. Emerging crops like the avocado and persimmon, with 23,885 ha and 8,156 ha respectively, signify diversification in agricultural practices. Their cultivation, although limited compared to traditional crops, points to a trend towards the integration of more diverse and high-value crops into the agricultural landscape. The analysis of prickly pear and walnut, with their distinct ecological niches, underscores the importance of understanding and leveraging environmental conditions for agriculture. Prickly pear’s adaptation to arid conditions, especially in the LE province, and walnut’s specific requirements reveal the potential for niche markets and the importance of biodiversity in agriculture.
4 Discussion
The potential diffusion results of the examined minor fruit tree species, shown in Table 2 and in Figures 4 and 5, were strictly related to the essential ecological parameters for optimal growth, reported in Table 1. However, it’s fundamental to understand which ecological parameters played a key role in assessing the suitable areas for each species.
The avocado trees have a broad soil pH tolerance, ranging from 5.0 to 7.0, which allows them to thrive in slightly acidic to neutral soil environments. For this, it can be grown in diverse soil conditions, with a preference for well-drained soils to avoid issues like root rot (Salgado and Cautín, 2008). The salinity tolerance is less than 2 dS/m, indicating sensitivity to high soil salinity levels; thus, they are best suited to areas with low to moderate salinity in irrigation water or soil, like the zone in the north of Brindisi or the south of Lecce ( (Nektarios et al., 2021). Temperature requirements are notably specific: they cannot withstand temperatures that fall to −5°C or lower for more than 4 hours in the entire year, highlighting their vulnerability to frost. The median temperature suitable range isn’t explicitly provided, but given their sensitivity to cold, avocados thrive in regions with mild, frost-free winters (Lomas, 1988; Bayram et al., 2014). Rainfall requirements range from 530 to 650 mm from April to October, suggesting a preference for a moderate amount of water distributed throughout the growing season (Nektarios et al., 2021). Considering the avocado rainfall threshold, all the water requirements not satisfied by rainfall will be reached using irrigation. Avocados have a Growing Degree Day (GDD) requirement of over 2200 GDD (Wolstenholme, 2013). This high requirement limited the suitability, excluding all the northern parts of Salento and a consistent part of the Adriatic coast. Finally, the specific requirements for temperature and water make them less adaptable than other species, particularly regarding cold tolerance and soil salinity (Supplementary Figures S1, S2). Only 13% of the total agricultural zones can be planted as avocados, with more than 50’000 hectares in the provinces of Brindisi and Lecce (Table 2).
The carob trees showed an environmental condition that supports the cultivation of this drought-resistant and economically valuable crop (Figure 4). In terms of soil pH, it can adapt in a range from 6.2 to 8.6 and can withstand higher salinity levels up to 6.8 dS/m (Correia et al., 2010). This adaptability suggested that carob trees could be cultivated across a wide range of soil types, except in waterlogged conditions and very high pH soils like in Torchiarolo and Carovigno. Temperature thresholds indicated a tolerance for temperatures below −4°C during the winter and a requirement for temperatures equal to or above 40°C during the summer (Batlle and Tous, 1997). The average rainfall ranged between 250 and 500 mm/year, making all the area suited. Chilling hours and GDD are still not known for this species, so no information could be given (Santos et al., 2019). The major part of the excluded areas for Carob is related to clay soils, not suited for almost every fruit tree species. With this data, the Carob tree demonstrated the broadest adaptability in the selected zone, with more than 260’000 hectares vocated to its cultivation (Supplementary Figures S1, S2).
The hawthorn tree is the third most suitable species, with 36% of the total agricultural zone that could be dedicated to this crop (Supplementary Figures S2, S3). While there wasn’t a specific texture preference, it indicated a pH range of 6.9–7.7 and salinity tolerance between 1.88 to 4.6 dS/m (Radha and Khwarahm, 2022). Although temperature preferences were not detailed, the specified average temperature range of 11 to 17°C and rainfall requirements of 920–1030 mm annually underscored Hawthorn’s preference for temperate climates with adequate precipitation (Radha and Khwaramh, data not published). Considering the very few numbers of published information about this crop, the average temperature played a relevant role in land suitability. However, Hawthorn reflected its adaptability and the broad ecological niches it could occupy (Figure 4).
The loquats demonstrated the second higher adaptability, with more than 50% of suitable zones. In terms of soil pH, this crop tolerates a range from 5.0 to 8.0(Table 2). They can withstand salinity levels up to 4 dS/m, indicating a moderate tolerance to saline conditions (Wu and Dodge, 2005). Temperature requirements suggest loquats cannot endure temperatures below -3°C, while the optimal maximum mean summer temperature is above 35°C (Polat, 2007). In the center of the Salento (a relevant Voronoi polygon caused the subtraction of all the orchards in that zone due to very low temperatures in some days of the winter (Supplementary Figure S3). Moreover, they need a rainfall range of 1000–1200 mm/year and 100–200 chill hours, positioning them as suitable for a variety of temperate to subtropical environments (Polat, 2007). The high water demand made the use of irrigation to fill the gap between rainfall and water necessary (Supplementary Figures S2, S3).
The mulberry was identified to prefer clay loamy soils, with a pH range of 6.5 to 7.5 and a low salinity tolerance of below 1.0 dS/m, indicating the need for soils with good moisture retention and low salt content (Jones and Costello, 2007). These thresholds were very strict for the study area, leading to a suitability of only 31,500 hectares (Figure 5). Temperature thresholds highlighted robustness against low winter temperatures (no lower than −30°C) and a requirement for summer temperatures to reach or exceed 40°C (Jones and Costello, 2007; Rohela et al., 2020) (Supplementary Figures S2, S3). Rainfall adaptability was broad, ranging from 600 to 2500 mm annually, with a chilling requirement of 400 hours, suggesting versatility in various humid climates (Rohela et al., 2020).
The persimmon tree indicated a very localized potential for persimmon cultivation, suggesting that the environmental conditions in LE are particularly conducive to this crop (Figure 5; Supplementary Figures S2, S3). This crop could fit in soils with pH levels ranging from 5.5 to 6.5, indicating a preference for slightly acidic to neutral soil conditions (Buesa et al., 2013). They are relatively sensitive to salinity, with a tolerance of less than 1.2 dS/m, suggesting they are less adaptable to saline conditions compared to species like carob (Visconti et al., 2017; Intrigliolo et al., 2018). Also here, the proposed thresholds lead to a very low number (13,000 ha) of suitable orchards, exclusively concentrated in the lower part of the Salento. Temperature-wise, persimmons could not withstand extreme cold, with tolerance to temperatures not falling below −13°C to −16°C (Chujo et al., 1973). They require a median temperature range of 20–25°C during fruit growth and adequate rainfall between 700–900 mm/year (Perucho, 2015). The chilling hours requirement was not limiting, with only over 200 hours.
Particular behavior was observed for prickly pears. Only a wide stripe in the northern part of Lecce province was found to be suitable (42%), while only 1% of hectares could be used in BR and TA (Table 2; Supplementary Figures S2, S3). The crop exhibited a wide soil pH tolerance from 5.1 to 8.1, making it the most tolerant to acid soils and one of the most adaptable to various soil conditions (Le Houérou, 1992; Prisa, 2021). They have a salinity tolerance of less than 2 dS/m, indicating they can handle slightly saline environments (Bakewell-Stone, 2012). With a minimum mean winter temperature requirement of above 5°C, prickly pears are adapted to warmer climates and cannot withstand cold winters. The most important and the most limiting threshold resulted in the annual mean temperatures, which needed to be 18–23°C (Prisa, 2021). Furthermore, it required 350–500 mm of yearly rainfall, which positions them as well-suited to semi-arid to arid climates, given their lower water requirement compared to the other discussed species.
The scarcity of walnut suitable areas highlighted the selective nature of this tree species regarding climatic and soil requirements (Supplementary Figures S2, S3). Walnut trees were found to thrive in a few soil textures, with a pH preference ranging from 6.5 to 7.2 (Table 2) (Ponder, 2004). In particular, optimal soil conditions are identified in loams and silty compositions (Scedei et al., 2020). This parameter showed the highest relevance as a threshold, minimizing the suitable areas at a very low number of hectares, mostly in the lower part of Salento. Rainfall requirements were specified as 650–700 mm annually, with a chilling hour requirement of 650 (Cisneros et al., 2009; Prevenda, 2020; Yilmaz, 2021; Aslamarz et al., 2009). These two climatic parameters were the most relevant, compromising the suitability of a wide range of zones and excluding a relevant number of hectares, reducing at only the 2% of the agricultural zones.
5 Conclusion
The research addressed the lack of information on introducing Xylella fastidiosa subsp. pauca immune/resistant tree crop species in Apulia Region. A site selection analysis through a GIS-based approach was useful to investigate the potential suitability of minor fruit tree crops in the Xylella fastidiosa subsp. pauca infected-areas. The analysis revealed that Carob, with its remarkable adaptability and drought resistance, presented the broadest suitability, offering a viable alternative for sustainable agricultural development in the area where the problem is not only the bacterium but also the availability of water for agricultural uses. Hawthorn and Loquat also exhibited high adaptability, indicating their potential contribution to agricultural biodiversity and ecological balance. Conversely, Avocado, Prickly pear, and Walnut, despite their economic value, demonstrated limited adaptability due to specific soil and climatic requirements, underscoring the importance of matching crop selection to local environmental conditions. The adoption of alternative crops, that fit the ecological optima in the affected regions, stands as proof of the resilience and adaptability of agricultural systems dealing with pathogens and climate threats. These outcomes highlighted the significance of targeted research for management strategies essential for policy-makers, to optimize growth conditions, ensure the sustainable development of agricultural practices in regions affected by plant pathogens, open new cultivation scenarios, and give new hopes for the olive growers that had suffered the impact of this bacterium into their orchards.
Data availability statement
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.
Author contributions
LC: Conceptualization, Data curation, Investigation, Methodology, Software, Visualization, Writing – original draft. FM: Data curation, Investigation, Resources, Visualization, Writing – original draft, Writing – review & editing. SG: Data curation, Investigation, Methodology, Visualization, Writing – review & editing. GV: Writing – review & editing. SC: Funding acquisition, Resources, Supervision, Validation, Writing – review & editing.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This project was funded by Progetto DAJS “Rigenerazione Sostenibile dell’agricoltura nei territori colpiti da Xylella Fastidiosa” (J89J21013750001).
Acknowledgments
We want to thank prof. Alessandra Gallotta for the help in the research and in receiving the climatic data. Furthermore, we also want to thank to Dr. Nicola Puletti (CREA, Research Centre for Forestry and Wood, Arezzo) help us in the research and in provide the table with the suitable areas resulting by an intersection against our shapefiles and olive tree zones shapefile (OLIVEMAP project).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fagro.2024.1421627/full#supplementary-material
Supplementary Figure 1 | Voronoi polygons highlighted in yellow show the case when the Tmin and Tmax threshold counting has been over zero defining the exclusion of the corresponding zone.
Supplementary Figure 2 | Inverse Distance Weight (IDW) interpolation map referring to climate parameters.
Supplementary Figure 3 | ACLA2 map referring to soil parameters.
Footnotes
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Keywords: NUS crops, drought tolerance, land suitability, site selection analysis, decision-making
Citation: Costanza L, Maldera F, Garofalo SP, Vivaldi GA and Camposeo S (2024) Ecological optima show the potential diffusion of minor tree crops in Xylella fastidiosa subsp. pauca-infected areas through a GIS-based approach. Front. Agron. 6:1421627. doi: 10.3389/fagro.2024.1421627
Received: 22 April 2024; Accepted: 30 July 2024;
Published: 22 August 2024.
Edited by:
Stéphane Cordeau, INRAE, UMR Agroécologie, FranceReviewed by:
Rehana Sardar, Emerson University Multan, PakistanSaber Hussain, University of the Punjab, Pakistan
Copyright © 2024 Costanza, Maldera, Garofalo, Vivaldi and Camposeo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Francesco Maldera, francesco.maldera@uniba.it; Simone Pietro Garofalo, simone.garofalo@crea.gov.it