MAP Intercropping in the Mediterranean Basin: Comparison
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The Mediterranean basin (MB), a “climate hotspot”, is experiencing faster than average increases in global temperature and water deficit, as well as soil degradation, with detrimental impacts on food crop yield and pest/pathogen incidence. Hence, there is an urgent requisite for sustainable crop diversification strategies to promote crop resilience, soil quality conservation and pest/pathogen control. Intercropping is a strategy that has yet to be widely adopted. Presently, cereal–legume combinations represent the most common intercrops. Of relevance, a large number of medicinal and aromatic plants (MAPs), native to the MB, serve as potentially profitable indigenous resources for intercropping with food crops. Environmentally sustainable benefits of MB MAP intercropping with food crops have ironically been reported largely from research outside the MB.

  • Mediterranean basin
  • medicinal and aromatic plants (MAPs)
  • intercropping

1. Introduction

The Mediterranean basin (MB), coined a “climate change hotspot” [1], is experiencing a 25% faster than average increase in global temperatures coinciding with a predicted 30–40% reduction in precipitation, particularly in the southern portion of the MB [2][3]. Climate-related changes in the MB also involve increases in the frequency and severity of extreme weather events, including heat waves, dry spells, drought, unexpected flash floods and frost [4][5][6]. Given the environmental heterogeneity of the MB, climate-related events occur with differing magnitudes from one region to another [3][6][7]. Together with climate change, unsustainable agricultural management practices have rendered the MB more susceptible to physical (erosion and desertification), chemical (reduction in soil fertility) and biological (biodiversity loss) soil degradation processes [8], with knock-on effects on pest/pathogen and disease incidence [9][10][11][12]. Collectively, these multi-faceted challenges pose a major threat to both production yield and quality in the entire MB agricultural sector [5].
Focusing on perennial nut and fruit tree crops, there is no return in investment until several years after planting. Hence, climate change and non-sustainable management effects on phenology, physiological processes, disease–pest frequency, yield and product quality specifically represent major challenges to producers of this agricultural sector within the MB [9][10][13][14][15][16][17][18]. Important staple cereal and legume field crops in the region, intrinsically linked to food security, are similarly subject to yield fluctuations [19][20] and increasing pathogen/pest pressure [5][21][22][23]. The spread of broomrape (Orobanche spp.), a parasitic achlorophyllous herbaceous weed, also represents a major constraint to legume production in rainfed cropping systems of the MB and Middle East [24]. Hence, there is an urgent requisite to increase food crop resilience to climate change impacts, to shrink the agricultural carbon footprint of unsustainable management practices and to enhance soil and biodiversity conservation whilst guaranteeing crop yield, weed and pest/pathogen control. In response, various adaption strategies, including the selection of more resilient plant material, have been extensively covered in recent reviews and meta-analyses for perennial fruit and nut crops [13][16][17][25][26][27], including grapes [16][28][29][30][31], and for annual field crops [21][24][32].
Of the adaption strategies, crop diversification, defined as the increase in crop diversity through the implementation of practices such as crop rotations, cover crops and intercropping, represents an important approach for sustainable agricultural development [10][33]. Of parinticular relevanceerest to the discussionpresent review is intercropping, the agronomic practice of simultaneously growing two or more crop species in the same field in close proximity for a considerable proportion of the growing season [34]. This practice promotes crop resilience, product stability and environmental security though agro-ecosystem benefits (soil quality conservation, biodiversity and pest control) and provides alternative products to increase farmer profitability [34][35][36][37]. Common intercropping typologies include, row intercropping (specific row patterns with varying row ratios), strip intercropping (cropping in wide strips to facilitate machine operations), alley intercropping (growing crops in-between trees and bushes), mixed intercropping (sowing two crops on one terrain with no distinct row arrangement) and relay intercropping (growing two or more crops simultaneously during part of the life cycle of each crop).
Throughout the history of agriculture, intercropping has been a key element of traditional, smallholder, farming systems, but has largely been neglected in both scientific-based research and industrialized production [35][36][38]. As such, intercropping has yet to be widely adopted [37]. From a scientometric analysis of global intercropping research between 1992 and 2020, a significant upward trend in research publications was evident from 2015 [34]. Cereal grain–legume combinations constitute the most common intercropping strategy in annual field crops, whereas in agroforestry systems, the intercropping of orchards with legumes, legume mixes, cereals and grasses is reported [26][34][35][37]. Intercropping with medicinal and aromatic plants (MAPs), as a climate change adaptation strategy, was not featured in the aforementioned meta-analyses or reviews for fruit and nut perennials or annual field crops. In a recent meta-analysis published specifically for Mediterranean climate regions in 2020 [26], only a single reference was made to the use of MAPs as an intercrop [39].
A large number of MAPs, comprising anise, basil, caper, caraway, chamomile, chive, coriander, cumin, dill, fennel, fenugreek, lavender, lemon balm, lemon grass, licorice, marigold, marjoram, mint, parsley, rosemary, saffron, sage and thyme, are native to the MB [40]. Sustainable agricultural advantages of MAP cultivation include, improved soil health (soil organic nitrogen and carbon, soil water content, microbial activity and biomass), bio-pesticide and bio-herbicide control by allelopathic secondary metabolites, adaptability to diverse ecological conditions, including semi-arid conditions, with an added advantage to farmer profitability (essentials oil with high economic value) [41][42][43][44][45]. The potential benefits of intercropping with MAPs have been highlighted from studies, conducted mostly on vegetable crops, which have been subject to various reviews [42][43][46][47]. From these latter reviews, as well as from emerging MAP intercropping studies widely available on internet search engines, environmentally sustainable benefits of native MB MAPs have ironically been published in countries that are predominantly located outside the MB, such as China, India and Iran (major producers of MAPs), as well as parts of Africa [43][45][47][48]. To address desirable sustainable agriculture policy objectives in the MB such as improvements to soil quality and biodiversity [6], as well as to take into consideration climate change and unsustainable management risks to the MB, the present study aims to review published literature from 2003 to 2023 on intercropping perennial nut and fruit crops, as well as annual field crops, with native MB MAPs within the MB. Scientific research on MAP intercropping with perennial fruit trees and annual field crops in the MB is shown to be extremely scarce and warrant attention. Nonetheless, where possible, the objective was to provide an overview of benefits from the intercropping of various selected crops and MAPs in land use efficiency (LUE, involving increased yields, economic returns and weed control), soil health (improved physical, chemical and biological properties), bio-control (pathogen/pest and improved fauna biodiversity in natural predator populations and ecosystem services) and product quality (improved yield quality).

2. MAP Intercropping in Perennial Woody Nut and Fruit Crops

Perennial woody fruit and nut crops are among the most bio-diverse agricultural systems in the MB, and include a wide range of deciduous (almond, chestnut, hazelnut, pine nut, pistachio, pecan, walnut, grape, cherry, apple, apricot, peach, plum, pear, fig, persimmon and pomegranate) and evergreen (olive, date palm and citrus fruit) species. Deciduous and evergreen perennials can be further classified into climatic zones which include warm temperature to subtropical (pistachio, pecan, persimmon, fig, olive, citrus fruits and date palm) and temperate (warm to cool temperate, chestnut, almond, grape, peach, hazelnut, cherry, apple and pear).
Climate change and non-sustainable agricultural management practices impact various aspects of perennial cultivation. Phenology in perennial fruit and nut crops is driven by temperature which regulates dormancy periods. Endodormancy is induced under cold temperatures rendering the trees, bushes or vines cold hardy, and chilling units (the number varying between different species) are then employed by the plant to track the passage of time over winter. Upon the completion of the required chilling units, the plants enter ecodormancy (requiring species-specific levels of heat), necessary to resume growth and production which also coincides with increased sensitivity to colder temperatures [17][49]. Rising temperatures in the MB are currently compromising the chilling unit requirements, leading to developmental abnormalities affecting yield and quality specifically in the temperate tree crops [7][17][49]. Since fruit trees in the MB are often grown in marginal and unfertile lands with low levels of soil organic matter [13] and are also traditionally cultivated at a low plant density [25][39], increased temperatures combined with variable and unpredictable precipitation events (drought, flooding) have accelerated water erosion and worsened soil degradation [18][26]. Moreover, the implementation of non-sustainable agronomic practices such as tillage to remove weeds on exposed land between trees, as well as the use of chemical fertilizers have collectively contributed to anthropogenic greenhouse gas (GHG) emissions and reduced agro-ecosystem fauna services [10][13][26]. Reduced ecosystem services and climate change (more specifically rising temperatures) aggravate pest outbreaks, which are currently spreading in Mediterranean orchards [9][10].
Considering the diversity of nut and fruit crops cultivated in the MB, published research on intercropping with MAPs as a crop diversification strategy under field conditions within this region is particularly scarce and warrant attention as a new potential sustainable strategy. The research will be covered in the following subsections for deciduous perennials and evergreen perennials, respectively. However, given that among the deciduous fruit crops, grapes have the largest area and the highest economic importance globally [15], grape was reviewed in a separate subsection from the remaining deciduous perennials.

3. MAP Intercropping in Annual Field Crops

Field crops cultivated in the MB are very diverse and include the cereal crops with durum wheat, bread wheat and barley being the most cultivated cereal crops, traditionally grown under rainfed conditions [20][50]. Durum wheat represents around 6% of the total wheat cultivation but has an important economic and cultural relevance in the MB, where it represents a staple crop that is increasingly threatened by drought and insect pests [23]. Additional field grain crops produced in the MB include rye, oats and maize, rice, sorghum and triticale. Sunflower is the most widely cultivated oilseed crop, whilst industrial crops include rapeseed, cotton and tobacco. Alternative, innovative field crops introduced into the MB as contributions to climate change mitigation, in compliance with the EU (European Union) Green Deal objectives, include teff, quinoa, camelina, black cumin, chia, emmer and flax [51]. Dry legume pulses (lentils, chickpeas, peas and beans) are representative field crops that form the backbone of the Mediterranean agro-ecosystems from ancient times, yet the unique and broad biodiversity of legumes has not been sufficiently valorized in the MB [52]. Among the legume crops, faba bean (Vicia faba L., also known as faba bean or broad bean) represents an excellent, untapped, source of sustainable and quality dietary proteins, with potential as a functional food [53]. In Egypt, the faba bean represents an important source of food and feed protein but remains an underutilized crop in Western countries [53].

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