Reviewing the Adverse Climate Change Impacts and Adaptation Measures on Almond Trees (Prunus dulcis)

Abstract

Climate change is one of the most emergent environmental challenges, with rising global temperatures, changes in precipitation regimes, and an increased frequency and intensity of extreme weather events. Climate change impacts on the agrarian sector are being experienced across the world and are expected to be aggravated in the upcoming decades. Almond fruits are highly sought after due to their economic and nutritional interest, which contribute to their spread throughout the world. In 2021, the world almond production was approximately 3.9 × 10⁶ t with upward of 4.9 × 10³ t year⁻¹. Despite being relatively drought- and heat-resistant, this species is also vulnerable to climate change, particularly its production, which is highly dependent on soil water content and air temperature. To address the challenges of climate change, farmers and other stakeholders in the almond industry are increasingly adopting a range of adaptation measures, such as implementing irrigation systems and planting more drought-tolerant almond varieties. This manuscript describes the impacts of climate change on almond cultivation, reviewing the most recent studies on the subject. Furthermore, a comprehensive analysis of possible adaptation strategies against the potentially negative impacts is carried out, which might be of relevance to almond producers and other stakeholders operating in this value chain.

1. Introduction

The almond tree was one of the first cultivated trees in the Old World. The species is native to the warm and arid regions of Southeast Asia and the mountainous slopes of Central Asia, and there are three species of almond trees: Prunus dulcis (sweet almond), Prunus amara (bitter almond), and another group that is a combination of the two previous species [1,2]. The present study is directed to Prunus dulcis Miller (D.A. Webb), which is a member of the Rosaceae family, the Prunoideae subfamily, and the Amygdalus (L.) subgenre [3]. Only sweet almonds are edible, and the growing economic and nutritional interest in this species has contributed to its propagation around the world [4].

The fruit has nutritional components, such as fatty acids, lipids, amino acids, proteins, carbohydrates, vitamins, and minerals [5]. Almond’s chemical and physical composition has anti-depressant, antioxidant, memory-enhancing, and anti-aging effects [1,5,6,7]. It can be consumed in the form of fresh fruits or processed products (i.e., processed milk and flour) [6,8]. Moreover, the fruit by-products (skin) can also be used for other purposes, such as the bio-preservation of food and pharmaceutical products [9]. Over the centuries, through seed propagation, grafting, and cloning reproduction, the species has expanded throughout the world [10]. This dissemination has driven the species to develop physiological, ecological, and genetic characteristics to adapt to the different soil, water availability, and climate contexts [11]. For this reason, there are several almond tree varieties, and each one with different characteristics, of which its main features are the phenological timings, sexual compatibility, vigour and size, ease of pruning, resistance to pests and diseases, productivity, and fruit characteristics [12].

Prunus dulcis is of great interest worldwide in both production and distribution terms. In 2021, the world almond production was approximately 3.9 × 10⁶ t [13]. The continent with the highest production of almonds is America (2 23.5 × 10⁴ t), followed by Asia (62.9 × 10⁴ t), Europe (50.5 × 10⁴ t), Africa (33.9 × 10⁴ t), and Oceania (28.5 × 10⁴ t). In contrast, the largest producer by area is Europe (88.0 × 10⁴ ha), followed by America (54.4 × 10⁴ ha), Africa (53.1 × 10⁴ ha), Asia (27.7 × 10⁴ ha), and Oceania (5.1 × 10⁴ ha) (Figure 1c) [13]. Considering the information obtained by FAO (Food and Agriculture Organization of the United Nations), almond production increased from 1961 to 2021 with a linear trend of 4.9 × 10³ t year⁻¹. In parallel, its cultivation area is also rising with a linear trend of 2.1 × 10³ ha year⁻¹ (Figure 1b). During this period, the maximum production was reached in 2020, exceeding 4.1 × 10⁶ t. In 2021, the maximum area was 2.3 × 10⁶ ha with a productivity of 1.75 t ha⁻¹. This increasing production demonstrates the high interest of the food and cosmetic sectors in this species.

Asia, America (particularly California), and the Mediterranean region are the primary regions where the almond industry expanded [1]. Currently, California is still the world’s largest almond producer (2.2 × 10⁶ t) due to its beneficial climatic conditions, intensive almond production, and efficient commercial system, which serve the demands of both domestic and foreign markets [14]. Another production area is the Mediterranean basin, which is broadly defined by mild rainy winters and dry and warm summers that typically favour the sustainability of planting and producing almonds [15,16]. In this region, Spain is the largest European producer, either by tons (36.5 × 10⁴ t) or area (74.4 × 10⁴ ha), which has a major impact on almond exports; Italy ranks second (7.2 × 10⁴ t) and Portugal third (4.1 × 10⁴ t) (Figure 1a). According to the production area, Portugal is the second largest producer by area (5.8 × 10³ ha), followed by Italy (5.4 × 10⁴ ha) and Greece (1.8 × 10⁴ ha). The United States leads in global almond exports, comprising 63% of production in 2021. Spain follows with 13% while Australia and the United Arab Emirates account for 5% and 4%, respectively. The Netherlands and Germany each embrace 3% of the export market [17]. On the import side, Germany takes the lead with a 12% share, followed by Spain, France, and the United Arab Emirates, each with a 9%, 6%, and 6% share, respectively. Italy, Japan, and China include 6%, 4%, and 4%, respectively, in the import market [18].

The climate is a determining factor for the development of the almond tree. Short and cool winters with moderate precipitation followed by long, hot, and dry summers are the most favourable climatic conditions for the development of almond trees. Even more, the occurrence of the different phenological states requires that air temperature and soil water availability match the species’ needs. However, climate change can have detrimental impacts on its development and production. Over the last decades, in the almond tree cultivation areas, which typically feature warm temperate Mediterranean-type climates, there has been an overall increase in temperature accompanied by a reduction in precipitation and an increase in the frequency of occurrence of spring frosts that may significantly influence almond growth and physiological development [19,20,21].

According to some studies, spring frosts can affect ~40% of the flowering stage of extra-early to early varieties, whereas the temperature increase can affect the durability and success of the chilling and forcing phases [2,22]. Moreover, in 2018, the Mediterranean basin countries produced only 29% of almonds under rainfed conditions. With the gradual reduction in soil water content, the implementation of irrigation systems is expected to increase in the next few years [23]. As indicated, climate patterns have been changing, and these changes are expected to be more powerful with the temperature increase in the future, precipitation reduction, and increased frequency of occurrence of extreme climatic events (severe rainfall events, droughts, or heatwaves), which will be more intense and recurrent and can cause perturbations at the ecosystem level by altering species variety, structure, and functional characteristics [24,25]. All these variations could lead to alterations in crop microclimatic environments and implications for the suitability of a given region to grow a specific species [26,27]. Therefore, changes in physiological and reproductive cycles may occur, such as delayed/advanced phenological timings, fruit quality modifications, and production losses.

For the abovementioned reasons, climate projections are useful to evaluate the climatic variables under different future scenarios, which may help identify potential impacts and possible adaptation strategies to minimize risks and maximize new opportunities emerging from climate change [19]. Climate change impact assessments and related adaptation measures are useful not only for stakeholders, but also for guiding decision-makers and policymakers in their planning [26,28]. In the present study, the published literature on the climate change impacts on almond trees are reviewed. It is intended to provide some hints about how climate change may affect the cultivation of almond trees globally as well as an overview of the potential (short- and long-term) adaptation options that are now accessible to growers of Prunus dulcis. Following this introduction, Section 2 discusses the almond tree and its growing conditions. Climatic projections and impacts on the crop are covered in Section 3. Lastly, Section 4 will be devoted to the adaptation measures, split into two sub-sections: short-term and long-term measures.

2. Prunus dulcis Growth and Habitat Conditions

This perennial crop normally shows the first harvest after three years (Figure 2a) and reaches full production at six years, in a traditional almond orchard; for intensive production, these periods may be anticipated [28,29]. The mean productive lifespan of traditional trees is 50 to 60 years. On the contrary, in intensive almond tree cases, their productive lifespan is between 15 and 30 years [28,30]. This deciduous tree can present distinct dimensions (shrubs, low, or tall) with growth up to 4–10 m high and a trunk diameter of up to 30 cm [4,31]. Generally, the leaves are lanceolate, 3–5 cm wide and 4–13 cm in length; the flower (hermaphroditic) is pink or white, with a 3–5 cm dimension; and the fruit is a drupe with a length of 3.5–6.0 cm [1]. The fruit dimension is 25–40 mm, ovoid–oblong, and flattened [4] (Figure 2b,c).

Regarding compatibility, the traditional almond tree is self-incompatible (i.e., “Cristomorto”, “Ferraduel”, “Marcona”), commercial production requires the interplanting of cross-compatible pollinizer cultivars and insects’ pollinators (e.g., honeybees) [32,33]. In the breeding programs, self-compatible cultivars have been developed, such as “Antoñeta”, “Guara”, and “Lauranne” [34,35]. Incorporating self-compatibility varieties allows single-cultivar orchards that simplify orchard management and use fewer agrochemical inputs can be planted, reducing the need for pollinators [33]. In comparison with other species, the almond is the first stone fruit (Prunus) to flower in spring [6]. For this reason, late varieties (“Tardona”) have been developed to reduce the impacts of spring frosts, which affect early varieties (i.e., “Desmayo Largueta”) [36,37]. Moreover, the breeding programs are focused on enhancing productivity, improving the fruit’s quality, and developing tolerance capacities for pests and diseases [38].

The shift from traditional to modern almond orchards has brought about many changes, both in terms of tree density and the maintenance requirements. The traditional system is characterized by less than 350 trees per ha; contrary to this, modern systems may exceed 700 trees per ha [39]. In contrast to traditional almond trees, the modern almond trees require more attention and resources, including increased water availability, optimized harvest mechanization, efficient pruning systems, sustainable management practices, and a higher reliance on pesticides and fertilizers [40]. Moreover, these modern orchards offer other advantages, such as reduced labour costs, improved utilization of natural resources, and decreased workloads, all made possible by better management techniques and the adoption of advanced technology [39].

Almond trees grow in Mediterranean-type climates. These areas are defined by long hot and dry summers and relatively cool and moist winters [32]. Nevertheless, this crop also grows in arid and semi-arid regions, and it develops in many structures, such as mountains or mineral slopes [4,31,41]. Climate conditions are an important physiological and biochemical activity regulator for the plant. Temperature, precipitation, humidity, and luminosity are differential factors for the success of the phenological stage and, consequently, yield production [14]. Almond develops in areas where the annual mean temperature ranges between 15 and 20 °C. In general, the species shows resistance to drought conditions, though it is sensitive to salt stressed environments [41]. Suitable growing conditions are also related to a continentality index ranging from 14 to 20 [16]. Drained and deep soils contribute to healthy root development and greater availability of nutrients and water [32,42]. Regarding precipitation, it is a drought-tolerant species and grows in places with precipitations as low as 300 mm year⁻¹ [43]. The presence of water is closely linked to tree yield [44]. According to Gutiérrez-Gordillo et al. [45], in the irrigated systems, as in California, southern Spain, and southern Portugal, the hydric requirements vary between 8000 and 13,500 m³ ha⁻¹, depending on cultivation practices (such as rootstock, pruning system, almond orchard structure) and variety characteristics. Concerning relative humidity (HR), extreme values induce stomatal closure, perturbing vegetative growth and productivity [46]. The canopy interception of photosynthetically active radiation (PAR) is related to almond potential yields. The maximum yield in the most productive almond orchard is 56 kernel kg/ha per unit PAR [14].

The northern (NH) and southern (SH) hemispheres have opposite seasons, which consequently affects the occurrence of the almond tree’s phenological cycle [47,48,49]. In NH, the dormancy starts in October and harvest happens in September. Contrarily, in SH, dormancy occurs in May, and the harvest starts in February [48]. However, the contrasting phenological cycles and the climatic conditions necessary for almond growth and orchard sustainability are similar in both hemispheres [49,50]. The annual vegetative cycle of the almond tree is divided into two main periods: the vegetative rest period and the vegetative activity period. For the northern hemisphere, the senescence and rest period start after harvest (usually between October and March, depending on the variety) (Figure 3). During the resting or dormancy stage, chill conditions warrant that plants retain the nutrients incorporated to contribute to floral differentiation and allow the branches to harden to be more resistant to frost [36,51]. For this, the tree must be exposed to 100–400 h of temperatures below 7.2 °C, or it must accumulate between 8 (early varieties) and 55 (late varieties) chilling portions, depending on the variety (approximately 41 to 106 days) [2,52,53]. Furthermore, almond trees can withstand temperatures below −15 °C during this stage [32].

After sufficient chill accumulation, the reproductive activity starts with inflorescence emergence [49]. In the Prunus species, the flower buds began to develop earlier than the leaf buds [49]. Bloom begins from late January (early varieties) to April (ultra-late varieties) [53,54]. Moreover, the flowering occurrence is directly influenced by climatic conditions, particularly air temperature and the occurrence of spring frosts [20]. At the beginning of this stage, temperatures of −3 °C over 30 min are damaging while temperatures of −2 °C and −0.5 °C are destructive to flowers and fruits, respectively [6]. In a study reported in California, it was indicated that low night-time temperatures (Tmin) in February negatively affect yields by 10% [19]. Conversely, heat accumulation has a positive correlation with flower buds’ growth, which can be classified by Growing Degree Hour (GDH) values ranging between 5.5 × 10³ (early) and 9.0 × 10³ (late varieties) [36,55]. However, temperatures around 20–26 °C reduce flowering durability and, consequently, productivity [35,56].

The pollinators are very vulnerable to low temperatures and windy and rainy conditions while the maximum activity occurs at optimum temperatures between 15 °C and 30 °C [34,46]. Temperatures between 10 and 12 °C and precipitation occurrence affect pollination; in particular, bees slow down or cease activity [50]. For the fruit development stage (Figure 2c), the optimal temperature for photosynthetic activity is 20–30 °C [32,46]. At temperatures below 15 °C and above 35 °C, the trees are under stress and vegetative growth may be interrupted [32,57]. During fruit maturation, in good climatic conditions, the mesocarp separates from the shell of the kernel and reveals the stone [4,32]. Depending on the variety, the harvest takes place from the end of August to the beginning of October. It is important to highlight that the time of flowering and ripening are independent. For instance, early varieties can be the first to flower and the last to ripen [49].

Figure 3. Prunus dulcis phenology stages according to the extended BBCH general scale in the northern hemisphere, adapted from: [48,49,58].

Figure 3. Prunus dulcis phenology stages according to the extended BBCH general scale in the northern hemisphere, adapted from: [48,49,58].

Branch dieback is one of the diseases that affect almond orchard viability and is caused by Botryosphaeria dothidea, Neofusicoccum mediterraneum, Diaporthe Neotheicola, Diaporthe Rhusicola, and Cytospora cedri [59]. Phomopsis amygdali, Fusicoccum amygdali Delacr., and Diaporthe amygdali Delacr. are a few agents that cause twig canker and shoot blight in almonds, which causes necrosis and the death of plant organs [59,60]. According to León et al. [60], in coastal areas with higher humidity and milder temperatures, the proliferation of these fungi is recurrent. Other species were reported, including Colletotrichum truncatum and Taphrina deformans, which propagate under high precipitation and high humidity conditions, and Stigmina carpophila is associated with warm, rainy springs [32,61]. Armillaria mellea is responsible for root rot and Phytophtora spp. for tree necrosis [32]. Grapholita molesta affects the almond fruit. Capnodis tenebrionis L., Cossus cossus L., and Zeuzera pyrina can cause the tree’s death [62].

In California, Cytospora spp., Botryosphaeria dothidea, Neofusicoccum mediterraneum, Neofusicoccum parvum, Macrophomina phaseolina, and Diplodia seriata were identified from band cankers, and B. dothidea, Neof. mediterraneum, Neof. parvum, and Dothiorella sarmentorum were identified from canopy cankers [59,63,64]. In addition, Diaporthe australafricana and Diaporthe novem were observed in almond orchards [60]. In Spain, brown rot and blossom blight (Monilinia spp.) are considered diseases of high economic impact [65]. Other fungi were observed: Botysphaeria ribis Gross. and Dugg., Botysphaeria dothidea, which caused cankers, stem dieback, fruit rot, trunk cankers, and death of the parts distal [65,66]. Moreover, Dia. neotheicola, Dia. rhusicola, and Dia. amygdali that cause branch cankers and D. mediterranea as an agent of twig cankers and shoot blight were reported in Spain [59,60,67]. Additionally, trunk diseases are caused by Collophora hispanica, Phaeoacremonium amygdalinum, and Phaeoacremonium iranianum [68]. Diaporthe eres and Diaporthe/Phomopsis species were reported on P. dulcis in Portugal, and Diaporthe foeniculina is present on almonds in Italy [60,67]. The diseases of the aerial part (i.e., floral organs) are instigated by Colletrotichum acutatum and Monilinia laxa Honey [69].

Almond xylem disease (ALS) is caused by the bacterium Xylella fastidiosa (Xf), which prevents the flow of fluids. Consequently, photosynthesis stops, and transpiration reduces, preventing evaporative cooling, which causes the rising of leaf temperature [70,71,72]. ALS was reported in areas characterized by higher availability of water [73]. The Philaenus spumarius and Xanthomonas arboricola pv. Pruni bacteria were detected in almond orchards [32,74]. The mites Panonychus ulmi and Tetranychus urticae affect photosynthetic activity [69]. As an example, the aphid Myzus persicae Sulzer causes leaf curling, and Hyalopterus amygdali Blanchard and Hyalopterus pruni Geoffroy reduce photosynthetic efficiency [12]. Anarsia lineatella Zeller and Monosteira unicostata Mulsant and Rey cause fruit damage, premature leaf fall, and weakening of the tree, respectively [32,69]. Effectively, the almond crop has a wide variety of pathogens that may affect it, but the cultivated variety, management, and climate conditions are determining factors in the viability of the almond orchard [14,75].

3. Climate Projections and Climate Change Impacts

Climate variability is a determinant factor in the agriculture sector in terms of food production, ecological diversity, population growth, and economic sustainability [76]. Several methodologies have been applied to evaluate climate evolution over the centuries [77,78,79,80]. Since records began, the climate has been constantly changing. From the latter half of the 1800s onward, human activity has played a determinant role in the elevation of the average worldwide temperature by approximately 1.0 °C [78]. Accompanied by this change, extreme weather events have been changing (e.g., heat waves), namely their duration, frequency, and intensity [81]. Regarding precipitation, heavy events have increased in frequency, and extremely low temperatures have decreased [79]. Moreover, the increasing greenhouse gas (GHG) concentrations have been affecting crops [82].

According to Fraga et al. [81], the recent trend for annual total precipitation reductions remains in the Mediterranean region. Additionally, in North America and Europe, the frequency of occurrence of extreme precipitation events, such as short-duration storms, has increased, which has devastating consequences for natural and man-made ecosystems [24]. Heat waves and droughts have an impact on the survival of species of plants, animals, and human beings [50]. Temperature increases, allied with precipitation decreases and evapotranspiration increases, are determinants of drought events and heat waves, which are becoming more recurrent, mostly in semi-arid regions [83,84].

For the future, it is necessary to understand the evolution of the impacts of climate change, and for this, several procedures and tools have been developed. Climate projections are one of these tools that support the understanding of possible future climate conditions and the occurrence of extreme events (i.e., disaster prevention and reduction), contribute to biodiversity variability recognition, and assist in the development of adaptation measures for the short- and long-term [25,76]. These projections are based on the analysis of various factors (historical data and real-time information) that influence the climate, such as socioeconomic development pathways and corresponding greenhouse gas emissions. Global Climate Models (GCMs) and Regional Climate Models (RCMs) are key tools to develop climate projections.

GCMs and RCMs are based on computer simulations that aim to predict how the global climate will be affected by changes in factors such as greenhouse gas emissions, land cover changes, and solar activity [85]. To promote research on the effects of climate change and potential policy responses, the Representative Concentration Pathways (RCP) were developed to take into account a set of GHG concentrations and emission trajectories. The Shared Socioeconomic Pathways (SSP), an RCP optimization, consider determinants of socioeconomic development (such as population characteristics and economic growth), greenhouse gas concentration, and potential mitigation actions [86,87]. With the help of these advanced scenarios, it is possible to make objective and integrated management decisions in the agricultural sector, considering projections of changes in crop productivity and trade policy regimes as an example [85,88]. Moreover, in agriculture, the combination of distinct models (such as phenological stages) and climate projections enables studies on the interaction of climate variables and crop characteristics for assessing the impacts and developing adaptation strategies to future climate conditions [35]. For almond California growers, the climate projections for the representation of long-term trends or shifts in crop-specific agroclimatic metrics are useful tools for their decision-making [89].

Short- and long-term studies are available on different crops [24,90]. For instance, recent studies have highlighted that climate change has different impacts on agricultural systems across different parts of the world. In northern Europe, productivity is projected to increase, and the range of crop species will grow. In contrast, Southern Europe is expected to be adversely affected with a decrease in harvestable yields and a reduction in areas for traditional crops due to the higher temperatures and recurrent droughts [91,92,93]. In Mediterranean-type regions, there is projected heightened warmth and dryness, elevated water requirements, and pronounced fluctuations in rainfall. Moreover, for the same region, there is a downward trend in precipitation (−15%) and an annual temperature increase (0.5 °C) [94]. Another example is California, where changes in weather patterns are observed with negative effects on crops. According to Pathak et al. [95], a 2 °C increase may contribute to lower almond yields in California.

For Prunus dulcis, among the other parameters, orchard sustainability, yield viability, and production of high-quality fruits are susceptible to climate conditions [22,50,96]. The phenological stages may be affected by insufficient chill accumulation, heat waves, drought, irregular rainfall, and spring frost. Air temperature directly influences the endodormancy, and ecodormancy phases, which may affect the success of almond production. [21]. Therefore, several studies have been conducted to understand the evolution of chill and heat accumulation in almond trees in various scenarios [6,36,37,52,53,55,97].

During the endodormancy, when the chill requirement has been fulfilled, the success of flowering yield production is guaranteed [36]. However, with the increase in warmer temperatures, especially in the future, the chill reduction may cause disturbances in the plant by delaying the development of the different phenological stages, abnormal flower growth, reduction in fruit set, changes in kernel quality, and productivity decreases [35,37]. According to Gitea et al. [40], with the increased temperature during the vegetative rest period, vegetative activity may start earlier in the spring (approximately 10 days), which affects fruit development. Mediterranean regions may be affected in the future by excessive delays in chilling fulfilment [98]. Moreover, when comparing early and late flowering varieties, early flowering varieties tend to be more vulnerable to spring frosts while late flowering varieties are more susceptible to a reduction in chill accumulation [20,21,99].

To flowering and fruit development and maturation stages’ heat accumulation is essential to the sprouting and flowering buds, fruit ripening, and vegetative growth [21]. However, excessively warm temperatures are associated with the occurrence of heat waves and droughts, which can cause a reduction in pollinator activity, inhibition of photosynthetic activity, the synthesis of photosynthetic pigments to be negatively affected, and reduced water availability [44,50]. Furthermore, species with long heat accumulation periods, such as “Marcona”, are not recommended for warmer environments because they are more exposed to heat stress conditions [20]. In general, insufficient chill accumulation can affect bud and flower development, and heat accumulation can affect fruit development [55].

Another stress factor is spring frost during the flowering stage and fruit set, where mean temperatures under −2 °C are very likely to affect the viability of flower and fruit development and, consequently, the crop’s economic sustainability [22,35,54]. Spring frosts can affect 40% of the flowering stage of early varieties. For this reason, it is important to develop late, extra-late, and ultra-late varieties to delay flowering until late spring, when frost is less likely to occur [2,22,98].

Due to climate change, heat waves, droughts, and heavy precipitation events are becoming more frequent and severer [80]. In the Mediterranean-type climate regions, where an important fraction of almond trees grows, the mean annual precipitation tends to decrease, and consequently, the water availability for the crop decreases. In this region, water scarcity is common where extended dry spells occur frequently, which may result in low flower and fruit-setting, photosynthesis activity reduction, flower abortion, and yield decline [81]. To reduce the harmful effects of these events, several studies have been conducted to understand the ability of almond trees to adapt to different irrigation strategies [44,100,101,102]. According to the study by Prgomet et al. [44], when water application reduces to 35% of crop evapotranspiration (ETc) during the fruit-filling stage, it does not affect yield. Another study concluded that effective irrigation water savings may be feasible, though these responses differ according to the cultivar [101]. It stands out that the almond tree shows a high capacity to tolerate drought conditions due to its positive response to deficit irrigation [23].

The increase in the occurrence of extreme rainfall events may be more frequent, contributing to pest and disease proliferation [50]. For example, post-harvest sicknesses promote a decrease in almond anther dehiscence, a delayed harvest period, and productivity losses [35,103]. Furthermore, the increased humidity content in the crop could be associated with excessive watering and intensive production [32]. However, low humidity and high temperatures protect plants from fungal diseases. Other climate variables, such as cold temperatures, windy speed/direction, and/or precipitation occurrence, may reduce the pollinator activity, which is essential for self-sterile varieties of the almond tree [14,33]. The events associated with climate change can indeed have consequences on the entire ecosystem surrounding almond trees, e.g., changes in the physical and chemical characteristics of the soil, instability in water availability, and biotic stress, such as the presence of diseases, pests, and plants competing for space [40]. Climate change affects the sustainability of soil by causing erosion, compaction, and reduced fertility, which in turn alters the chemical and physical composition of the soil. This, in turn, affects the production of trees and the viability of crops, thus posing a threat to food security [104,105]. Additionally, climate change can contribute to the spread of pests by expanding their dispersal areas and enabling them to develop resilience to adverse conditions, such as winter weather. It can also diminish the effectiveness of biological control methods and lead to an increase in pest populations [106]. However, heat waves and heavy precipitation can act as limiting factors for pests and diseases, hindering their ability to propagate and reproduce [107].

4. Mitigation Policies and Adaptation Measures

Climate change may have varying degrees of impact on almond trees. Therefore, mitigation policies and adaptation measures are key strategies to deal with the impacts of climate change, involving changes in infrastructure or technology to improve crop resilience and adaptive capacity [80,108]. Defining these measures is the joint responsibility of farmers and policymakers [80]. However, it is essential to analyse each situation at the local level, focusing on crop and economic management parameters. The potential benefits, technical feasibility, and associated costs must be considered when implementing any solution, taking into account local particularities and regional climate projections [81,108]. It is also crucial to recognize that the impacts of climate change are more pronounced in underdeveloped countries with fewer resources, limiting the potential of various adaptation mechanisms [25]. Short- and long-term mitigation policies and adaptation strategies depend on the specific application period, and the most important will be presented below.

4.1. Short-Term Strategies

Short-term adaptation methods aimed at addressing specific threats can be seen as the initial protection against climate change. Short-term adaptation techniques, such as orchard interventions that can be implemented within a year or two, require adjustments to producer management practices. Several of these solutions are briefly outlined in the subsequent sections [84].

4.1.1. Increase/Maintain Water Availability and Use Efficiency

The reduction in water availability, resulting from downward trends in precipitation and increasing water consumption, is a limiting factor in most of the almond tree cultivation areas. During these periods of drought and heat waves, low water availability can affect the sustainability of the almond orchards, with intense defoliation and dehydration of the tissues and plant mortality [41]. Resources are becoming increasingly scarce nowadays, so optimizing the use of water resources is a priority. The almond tree is a drought-tolerant crop, thereby capable of adapting to water-deficient irrigation systems defined by hydro-sustainable strategies.

One of the most popular techniques is deficit irrigation (DI), which applies less water than necessary to meet the tree’s full needs [44]. Among the possible strategies, regulated deficit irrigation (RDI) and sustained-deficit irrigation (SDI) are the most recurrent. However, partial root-zone (PRD) and low-frequency deficit irrigation (LFDI) techniques are also options according to the requirements of each case [101]. The DI implementation can contribute to advancing almond development under water stress conditions, adjust the water resource availability, minimize the loss production, have a constructive effect on the decrease of pest and disease occurrence and damages, and help to control the plant vigour [23,45,69,109]. Besides the plant benefits, an improvement in the nut quality in terms of unit weight; kernel thickness, length, and colour; almond flavour (concentrations of volatile compounds increase); and biochemical characteristics (total phenolic compounds, organic acids, and sugars increase) can be achieved [23]. Furthermore, accurate knowledge of crop phenological development and its physiological response to water stress is essential for DI to succeed [109]. However, uncontrolled water stress can have negative consequences, as it may change plant and fruit characteristics, affect mineral nutrients, modify fruit compounds (quality and kernel characteristics), and the wet soil volume [45,69,100]. For Portugal and Spain, the suggested DI for almond orchards is between 1300 and 6500 m³ ha⁻¹ of water [69,101].

It is important to highlight that the diversity among almond varieties is a determining factor in response to drought as well as DI strategy. A study developed by Fernandes de Oliveira et al. [41] indicated that the varieties “Arrubia” and “Texas” have physiological characteristics that are adapted to drought conditions in comparison to the “Cossu” and “Tuono” varieties [41]. Another study indicates that “Guara” was the most sensitive cultivar concerning growth and fruit numbers per tree under SDI conditions in comparison with “Lauranne” and “Marta” [23,101]. The regulated deficit irrigation (RDI) applications also show positive results in almonds compared with full-irrigated (FI) and over-irrigated (150% ETC) treatments [102].

Further, as a complementary measure, the combination of deficit irrigation with biostimulants can be beneficial to the crop in terms of physiological characteristics and production [45]. Planting almond cultivars on drought-tolerant rootstocks (grafting) to reduce the negative consequences of water scarcity [110]. Moreover, rootstocks may also confer resistance to different biotic and abiotic stresses in the soil [111]. Several studies have been conducted to evaluate the potential of rootstock to mitigate climate change impacts [111,112,113]. Another measure is the use of remote sensing tools (i.e., UAV), which may help monitor the water stress of the almond tree with the Crop Water Status Index (CWSI) and Normalized Difference Vegetation Index (NDVI) [100,114]. Studies carried out on almond trees show that thermal sensors are an aid to water management and the physiological monitoring of the plant, which allows quick and easily accessible results to be obtained [115,116]. Further, using alternative water sources for crop irrigation is a sustainable option, such as treating wastewater, promoting circular economy solutions, and the sustainability of increasingly threatened water resources. Several countries, such as Australia, Greece, and Morocco, have used treated and untreated wastewater for irrigation [117]. Lastly, water availability is strongly connected to the type of soil and soil management [118].

4.1.2. Improved Soil Management

Soil sustainability is a decisive factor for the viability of any crop. Deficient crop management, agriculture intensification, and climate change have contributed to soil degradation with increased soil erosion and decreased soil fertility. According to Cárceles Rodríguez et al. [119], land degradation affects ~30% of global land areas. In Europe, the loss of agricultural productivity due to soil erosion is ~0.43% per year. Thus, it is a priority to find measures to maintain soil quality and, in more serious cases, to restore soil conditions.

Biomass conservation in soil is a potential strategy to reduce climate change impacts. As carbon (C) increases, there are improvements in soil fertility, with increased microbial and enzymatic activity in the soil, consequently improving the efficient use of irrigation water and orchard productivity [42]. Soil management strategies (SMSs), such as cover crops, may improve the soil quality at physical (such as available water content and bulk density), chemical (pH, nutrients, and conductivity), and biological (microorganisms) levels and provide better almond nut characteristics [119]. Cover crops in orchards have a higher impact on soil carbon stocks (SCS). Moreover, for almond trees, legume cover contributes to an increase in total polyphenol content and antioxidant activity [119]. Interest in cover crops that increase soil organic carbon is particularly high because they can support adaptation to climate change while also reducing global warming by capturing atmospheric carbon and thus also contributing to climate change mitigation [120]. Almond orchards in steep sloping terrains are advantageous for retaining soil carbon and reducing soil erosion [121]. Concerning the water retention capacity in the soil, which is essential in the context of climate change, the mulching application reduces evaporation and protects against soil erosion and degradation. Cover crops help to reduce the shear stress of runoff, strengthening soil water infiltration, and enhancing the uptake of atmospheric carbon [93].

In some almond orchard soils, one of the elements in deficit is nitrogen (N). Soil fertilization may be a way to protect soil characteristics and replenish this compound [122]. However, inappropriate mineral or organic fertilizers (nitrates) may cause disturbances in the original soil dynamics as well as pollute the surrounding systems (e.g., water, plants) [40].

In addition to SMSs and crop plantations, regenerative agriculture incorporates other practices, such as integrating livestock and reducing synthetic agrichemicals that benefit soil health and crop production and quality. This system promotes more soil organic matter (soil organic matter, total soil carbon, and total soil nitrogen), total soil carbon (TSC), and total soil nitrogen (TSN) when compared to conventional almond orchards [123]. It also improves biodiversity, reduces pest and disease damage, fosters water infiltration, and boosts microbial biomass [123].

4.1.3. Adaptative Cultural Practices

The almond tree’s susceptibility to climate change has prompted farmers to alter their cultural practices and adopt a wide range of technical strategies to mitigate the effects. Alterations to crop management, such as modifying pruning dates and training systems, hint at the potential to minimize the impact of climate change on the sustainability and productivity of orchards. Studies have suggested that changes in plant density, spacing, pruning methods, and mulching application may enhance production per unit volume of water applied [49,106].

4.1.4. Protection against Pests and Diseases

A wide range of pests and diseases may affect the health of almond trees. The constant search for agroecological solutions that can inhibit their presence or reduce their impacts is a priority. Detailed planning and simple phytosanitary control practices can make a great difference, such as selecting less susceptible varieties, adapting cultural practices for better air humidity control, avoiding excessive soil nitrogen, and adapting pruning. All of them can be applied for the control of fungus and favours aeration. Control methodologies must be elaborated for specific needs, as each pest or disease requires specialized measures. The mating disruption (MD) technology through pheromone application can be used for the control of Grapholita molesta and Anarsia lineatella Zeller [124]. Kaolin and potassium salts of fatty acids thyme essential oil (PSTEO) may be an option to control Monosteira unicostate and Phyllonorycter corylifoliella to reduce the individual’s number and conserve the almond orchard characteristics [125,126,127]. Moreover, the monitoring tools (such as camera traps, light traps, and sticky traps) help farmers track pest populations.

In Capnodis tenebrionis L. cases, the application of insecticides can be an option. Mites can be controlled by reducing water stress and by applying acaricides. Insecticides must be applied to combat the spread of aphids. Zeuzera pyrina L. and Cossus cossus L. can be controlled with chemical products [62]. However, pest and disease control using phytopharmaceuticals (insecticides, acaricides, and pesticides) is increasingly restricted by government policies.

4.1.5. Protection against Extreme Weather

The damaging effects of extreme events are unquestionable, and their impacts are increasingly devastating. Therefore, understanding the mechanisms of these events and developing adaptation strategies is crucial for the sustainability of any culture. In the almond tree case, frost spring events have a higher impact on phenological stages, especially in flowering. According to Guillamón et al. [22], late frosts can disturb ~40% of the flowering of extra-early and early varieties (such as “Desmayo” and “Marcona”). Additionally, changes in developing flower buds, pollinators’ activity, kernel quality and structure, and productivity can be observed [35,128]. Short-term strategies through air movement heaters or fans, sprayer irrigation, soil moisture, and ground cover have been used to reduce the effects of frost events [50]. Later varieties (“Tardona” and “Penta”) have been developed to combat the impact of spring frosts. During the period of the highest likelihood of frost, the plant is dormant, so the phenological cycle and other parameters will be less affected. In heatwave cases, light-reflecting leaf coverings or crop growth regulators that provide heat protection have been suggested [50]. As an illustration, biostimulants are used to enhance crop tolerance to drought under water-scarcity scenarios [45].

4.2. Long-Term Strategies

Adapting to climate change over a longer duration, such as three or more seasons, is an option available to growers, industry stakeholders, and decision-makers. However, this approach may be more challenging and expansive and may also require longer-term planning, thus being generally less attractive than short-term solutions. The following sections provide some examples of long-term adaptation strategies.

4.2.1. Genetic Improvement and Varietal Selection

Nowadays, the economic and social importance of the almond crop is based on a sustained effort toward genetic selection and modification [129]. Traditional almond varieties that express specific characteristics (morphological and mechanical) are a tool to develop varieties better adapted to new production contexts, which contributes to increasing the species’ diversity [130]. Effectively, breeding commercially sustainable cultivars is frequently focused on crop optimization against climate change impacts and optimizing plant efficiency in terms of yield, kernel quality, and resistance to several major diseases and pests [33].

A factor that is being optimized is the ability of the almond varieties to be self-compatible (such as “Belona” and “Guara”), reducing the problem of inadequate pollination in almond plantations [32]. Early flowering and exposition to spring frost may severely affect the phenological cycle and the profitability of production [22]. In this way, in colder regions, late flowering is often necessary to avoid damage and improve agronomic performance. These varieties may also be a preferred option for the colonization of new areas [6,22,35]. Developing more heat- and drought-tolerant varieties could be pursued as an adaptation strategy [19]. For example, in Italy, where only a few local varieties are still cultivated, success in the new intensive almond orchards was strongly supported by the selection of blooming and self-compatible cultivars, which guarantee effective pollination and higher resistance to pests and diseases [41]. Even though almond breeding has been effective in obtaining late blooming cultivars well adapted to regions with high frost risks, these late varieties may conversely show adaptation problems in low chilling regions due to a reduction in the number of cold hours that can affect the development of floral buds and fruits [131].

4.2.2. Relocation

According to Di Lena et al. [54], climate change can disrupt the viability of crops in specific regions. To mitigate the effects of warmer temperatures, it may be necessary to expand production to alternative areas. As a consequence of anticipated climate change in Europe, some crops currently grown in southern Europe may be better suited to Central and Northern Europe or higher elevations in Southern Europe. Lorite et al. and Di Lena et al. [35,54] suggest that identifying alternative locations for almond production is an effective adaptation strategy that can boost the economy and agricultural development of new regions. Nevertheless, enhanced variants that can adapt to local growing conditions must also be available for Southern Europe, which also benefits from complex orographic and microclimatic mosaics.

4.2.3. Policy and Regulatory Measures

The development and implementation of long-term policies and regulations that support sustainable almond production and climate change adaptation must be a priority for the producing country/region. These measures must be targeted to each situation. Collaboration between, political systems, identities, associations, and producers is a priority for the measure’s success. For example, the Almond Board in California (ABC) and Australia (ABA) are the peak bodies for almond growers, processors, marketers, exporters, and scientific teams, which help the sector at the production level (such as improvement of cultivation practices) and the commercialization level (marketing service) [132,133]. In California, among other measures, ABC proposes to reduce the amount of water used for almond produce by 20%, adopt environmentally friendly pest management tools by 25%, and reduce dust during harvesting by 50% until 2025 [134]. ABA takes focus on improving irrigation systems and improving the well-being of pollinators, which are critical to almond production [133]. Moreover, scientific projects have been developed to evaluate the climate change impacts on almond production systems and to promote adaptation measures to contribute to their sustainability.

5. Conclusions

Climate change is affecting the growth of almond trees in several ways, such as phenological and physiological modifications. The drought and heat tolerance as well as the varietal diversity of the almond tree play a vital role in preserving the species, but they may not be enough to cope with the new challenges brought about by climate change. Predicting future climate scenarios through socioeconomic projections can help identify and implement suitable adaptation strategies, including short- and long-term measures, such as irrigation and protective compounds. The effectiveness of these measures is not yet fully understood, but they have the potential to benefit the agricultural sector and minimize the impacts of climate change on the environment and human activities. To ensure the sustainability of the almond value chains, farmers need to invest in some of these strategies, duly supported and encouraged by governmental policies.