1. Introduction
Over the past decade, global nut production has grown steadily, reaching over 5.3 million tonnes in the 2020/2021 season. Almond (
Prunus dulcis Mill. =
Prunus amygdalus Batsch) is the most cultivated nut crop, representing 31% of the global share, followed by walnut and pistachio at 19%. In 2018 the global almond market revenue was enormous, with a production of 2.4 million tons, a year-on-year increase of 3.8%. The State of California is the world’s leading producer of almonds, representing 79.2% of the world production. Spain and Australia follow with around 7% each [
1,
2,
3]. In Europe, the last 50 years have been crucial for the almond crop, changing production in terms of volume and producing countries. In the 20th century, the main almond producing countries were European countries, led by Italy and Spain. However, after the 1980s, European production declined significantly, while other countries increased their production significantly. Almond orchards continue to be converted to other crops in Italy, and the worst moment for Italian almond production was in 2013, which was 255,916 tons less than in 1961, −77.9% in 52 years, and 32,171 tons in 2000, −30.7% in 13 years. Since 2013, the situation has started to change, and production has increased by 9.7% over the past 5 years [
4,
5]. The traditional almond cropping system has a low density of old trees, making efficiency of inputs difficult [
3,
4,
6,
7,
8]. The latest innovations in cultivation systems have been achieved by applying more than 20 years of experience in super high-density (SHD) olive groves and adapting them to almond trees [
3,
7,
9,
10,
11,
12]. The first SHD almond orchard was planted near Lleida (Spain) in 2010, and soon all almond growing countries started to establish SHD orchards [
4,
13,
14]. In 2022, the global SHD almond orchard area has reached 14,622 ha, mainly planted in Spain and Portugal, followed by Italy and the USA (Iglesias et al. 2023, personal communications). This new SHD planting system, including plant material and orchard management, is also known as the “Sustainable Efficiency System” (SES) because it optimizes the use of natural resources such as soil and water and agronomic inputs such as fertilizers and chemical treatments associated with open orchards [
3,
7,
15,
16].
The choice of row orientation plays a key role in these new planting systems, improving sun exposure, affecting quantity and direction of the radiation on the canopies [
17,
18,
19], influencing each species differently [
20]. Vineyards were the first hedgerow model to be studied, from vegetative and reproductive points of view [
21,
22,
23,
24,
25,
26]. Some research supplied important evidence on the best row orientation, observing better results for a north-south orientation [
21,
27,
28,
29].
After vineyards, deep research was carried out on SHD olive orchards, observing a wide range of parameters [
17,
18,
19,
20,
30,
31,
32]. Results have demonstrated that there are no differences among hedgerow orientations [
17,
20,
33], although recent studies have negatively judged east-west orientation as worsening the status of the orchard over long periods [
34,
35].
Few publications have been produced on SHD almond orchards until now, giving insights on biometric characteristics [
36], evaluating and improving row spacing and light interception [
15,
16] and comparing different cultivars [
35,
37,
38]. Row orientation is also essential for successful production: indeed, 10% of a correct design in SHD almond orchard depends on it. Until now, only one research paper has been published on this topic, comparing different row orientations on cv. Guara Tuono [
19]. Therefore, further insights are needed on other cultivars to better understand this important cultivation technique. Cv. Lauranne Avijor is now the most planted cultivar in SHD orchards, so deeper research is needed on it [
39,
40].
Based on previous studies, the influence of row orientation on this cultivar grown in SHD planting system is hypothesised, influencing a whole range of parameters and leading to relevant differences between east-west (EW) and north-south (NS) orientations.
The aim of this two-year field research was to gain deeper knowledge regarding the interaction of row orientation, almond tree age and available PAR on canopy growth, yield and harvesting efficiencies in this very new planting system.
4. Discussion
Due to the scarcity in the bibliography regarding the effects of row orientation in almond SHD orchard, we also compared our results with studies performed on olive SHD orchards, vineyards and other deciduous species.
Optimal PAR interception is one of the features required in a SHD orchard and solar radiation has a key role in flower bud differentiation, as reported for olive, acting via photosynthesis in the availability of assimilate for bud induction and flowering [
17]. It is crucial to not allow shaded areas in the canopy, to avoid less efficiently involved areas in the photosynthesis process [
42]. On the other hand, net photosynthesis rises with PAR up until light saturation, after that extra photosynthetic photon flux density does not promote carboxylation, rendering around half of the available light excessive [
43]. The saturation point for almond cultivars should be fixed around 1300 µmol m
−2 s
−1 as the average [
43]. Moreover, for maximum productivity, all the foliage must be illuminated in a threshold range between 20% and 30% of incident radiation, especially for the critical phase of fruit growth and ripening [
44]. At this stage, both row orientations received equal quantities of PAR above the canopy (between 1880 and 1896 µmol m
−2 s
−1 as mean, for 2021 and 2022, respectively), thus allowing non-limiting light saturation external conditions, as expected. In all expositions, in both layers and for both years, the available PAR values were always over the threshold range of 20–30% of the incident radiation. This is due to the limited/controlled volume of the canopy compared with other training systems, such as open vase. Even if the trees worked at non-limiting light saturation internal conditions, N–S row orientation allowed the same PAR distribution between the E and W sides, with a mean of 37% and 55% for L and T layers, respectively (
Figure 6). On the contrary, the E–W row orientation lead to a strong asymmetry in available PAR values between the N and S sides. Indeed, while S exposition presented the maximum available PAR values (65% and 70% for the L and T layers, respectively), N exposition showed the minimum ones (29% and 52% for the L and T layers, respectively). This unevenness in PAR distribution inside the whole E–W oriented canopies could represent the driving engine of the effect on growth and yield parameters. The same results were found in almond, for cv. Guara Tuono [
19], and in olive [
33].
Thus, in these conditions, LAI variations represent the first consequence. LAI showed a general decline from layer L to layer T in all the expositions. These data are in line with previous studies [
15,
19]. LAI was higher in N–S row orientation in the bottom part of the canopy for both years; in the E–W row orientation, LAI was highest at the 4
th year in the top of the canopy, but it was the worst in the following year. The row orientation effect on LAI parameter decrease when latitude increases: if at 55° latitude N LAI is highest with N–S orientation during the summer months and with E–W orientations for the rest of the year, at 41° it is very similar [
45].
All canopy growth parameters observed in the N–S row orientation were significantly higher than E–W. TH showed between 5 and 14% higher values (
Figure 2); an even higher difference was found for TW (11–20%) (
Figure 3) and, consequently, for CV (27–36%). TCSA values increased by 28% and 15% for N–S and E–W orientations, respectively (
Figure 5). No comparison can be performed with other SHD almond orchards, but in a previous work [
19], it was hypothesized that, for the E–W orientation, the more available PAR on the S side could lead to over-excitation of chlorophyll, leading to the production of reactive oxygen species (ROS), increasing the risk of photo-inhibition (i.e., photo-damage) [
46]. Through direct and indirect impacts on cell structure, radiation can seriously harm almond trees, resulting in a drop in chlorophyll content and photosynthetic rate [
33,
47,
48,
49]. In olive tree, contrarily, no significant differences for tree height, tree width or trunk diameter of the canopy were found in different row orientations [
17]. Canopy size in January 2022 decreased by 10% for both row orientations (
Figure 3), due to hedging and topping operations, made in order to restrain the canopy at the optimal size required by SHD orchards [
19], and to adapt it to the harvester machine: indeed, canopy dimensions must not exceed 0.7–0.8 m in width and 2.75 m in height [
36].
A gradual yield increase in terms of hulled almonds in the N–S orientation (from 1890 to 2930 kg ha
−1) and a sharp decrease in the E–W orientation (from 2680 to 2190 kg ha
−1) were observed, moving from the 4th to the 5th year. According to Trentacoste et al. [
17] in an olive orchard, rows with N–S orientation are the most common and are promoted because the two expositions intercept the same quantity of solar irradiation, in both sunny and cloudy conditions. For this reason, they produced more. Moreover, according to Tous et al. [
49], in an SHD olive orchard with E–W rows orientation, a 40% lower average production was observed compared with N–S. In almond SHD orchards, at the 4th year from planting (2021), an opposite trend was observed, with a higher yield for N and S expositions (E–W orientation) (
Table 3). These data appeared anomalous and in disagreement with the ones found for cv. Guara Tuono [
19]. As for biometric parameters, light distribution could have influenced more N–S bud induction. This could be related to the biological phase of the orchard; rows with E–W orientation seemed to undergo a strong exit from the unproductive phase, with a greater production of fruits. The situation changed in 2022 (5th year from planting), with significantly higher values of FY in the N–S orientation (
Table 3). Therefore, in the N–S row orientation, a gradual rise was observed, while, in the E–W a sharp decrease was detected. These results are in line with the one in the bibliography, in which N–S orientation trees produced 15% higher FY than E–W [
19]. However, FY values are lower than reported [
38], because of the younger age of the orchard. Moreover, fruit number seemed to follow production trend, as for previous research [
19]. FN was higher in the E–W orientation in 2021 and in N–S orientation in 2022 (
Table 4).
Average almond weight values were significantly higher in both years and in E and W expositions (N–S orientation) (
Table 5). These data were strictly related to FY and FN and, for this, lower FN values in 2021 offset low FY values. These results are not in line with a previous study on cv. Guara, in which AW showed no differences among expositions [
19]. The polar gauge of fruits was significantly higher in the N–S orientation for 2022 and, in general, was not significant in previous year (
Table 5). These results are in contrast with a previous study, in which higher values were observed in the E–W orientation [
19]. According to our study, shelling percentage was influenced by row orientation in 2021, with the highest mean value in E–W orientation, while in 2022 no significant differences were found among the four expositions (
Table 5). These results are in contrast with a previous study, in which N and S expositions (E–W orientation) showed the higher SP. All these different behaviours could be related to the varietal and age effect [
19]. However, cv. Lauranne SP mean values ranged around 30–37%, 38%, in line with another study on SHD almond orchards [
38] and with other cropping systems [
50,
51,
52]. As with SP, hull tight nut percentage seemed not to be influenced by row orientation in 2022, but in 2021 this parameter is significantly higher in the E–W orientation, with an average percentage of 2.5% (
Table 5). Double kernel data strongly direct production to certain types of market, excluding it from others. In two years, an average percentage of double kernels close to 2.5% was observed. Hull tight nuts and double kernel parameters are strongly influenced by genotype and the external factors of the cultivated area [
50,
53,
54,
55,
56]. Furthermore, double kernel percentage mean values always ranged in the varietal standard interval (0–10%) [
50,
51,
56], with the same trends of SP and HT.
Yield efficiencies were also higher in N–S orientation in the 5th year, while no differences were detected in the previous year (
Table 6). These results are strictly dependent on canopy growth, as in previous research, in which, during the second year, a severe pruning was made to reduce the cv. Lauranne canopy [
38]. Greater sizes of the canopy in 2021 led to no differences, while after pruning, the higher yield of the N–S oriented almond trees gave significantly higher efficiency values in the subsequent year (
Table 6).
A mechanical harvesting efficiency higher than 95% was found over the years and between the orientations. The goal is to have values as close as possible to 100%, with values higher than 90% as acceptable [
57,
58,
59,
60,
61,
62]. No statistical differences were found for this parameter between row orientations in both years. Differences were found in 2021, in which fallen fruits were significantly higher than pending fruits, for both row orientations, due to some meteoric events, such as a hailstorm that hit the area (
Table 6,
Figure 1). Meteoric events can cause a strong reduction in production [
63] and are a limiting factor for all harvest systems, except when almonds are picked up from the ground. In 2022, however, an opposite trend was found for the delayed harvesting time due to the forecast of an adverse meteoric event, causing the non–detachment of some fruits (12%).