3.2. Climatological, Topographical, and Soil Characteristics of Current Olive Cultivation Areas
The current olive cultivation areas in the study area were evaluated in terms of 15 selected parameters, and the ecological conditions effective in olive cultivation on a local scale were determined and then compared with similar studies in the literature.
Considering the study area in terms of elevation, which has a key role in crop distribution in agriculture, it can be seen that 9.68% of the current olive fields are located in the elevation range of 130–510 m, and 82.26% of them are located in the elevation range of 510–890 m (
Table 3). It is reported in the literature that there are different opinions about the elevations where olive trees can grow and that the geographical and climatic characteristics of the regions also effect the suitability of different elevations [
51]. Although it is stated that olive plants can be grown optimally in areas up to 400 m above sea level where the Mediterranean climate is observed [
12], it was found by Kesici [
72] that olive fields can be located at elevations of 950–1000 m in Kilis (Turkey), and it was found by Ustaoğlu and Uzun [
49] that olive cultivation can be carried out at up to 500 m elevation in the Mediterranean region. Both Bilgilioğlu [
5] in Mersin, Turkey, and Moriondo et al. [
37] in the Mediterranean Basin found that olives might be grown successfully at altitudes of up to 800 m. It was stated by Ustaoğlu and Uzun [
49] that the slope of the land is important for olive production, as in all agricultural crops, and a slope of up to 20% is acceptable. However, it was determined that 75% of the olive fields in our country are located in sloping areas [
6,
51] and the average slope of the olive fields in the Aegean region is around 28% [
55]. In this context, when the current olive fields in the study area were examined, it was observed that 86.40% of the olive fields are located in areas where the slope is above 20% (
Table 3). The very rough nature of the study area and the average slope of 62% made this result inevitable.
On the other hand, it is a known fact that insolation in olive fields is effective and important. In this context, when the aspect of the current olive fields was examined, it was determined that 56.86% of them are flat and located on east, west, and south aspects (
Table 3). Tombesi et al. [
73] also emphasized that the south, west, and east aspects are the best options for crop quality and volume.
The climate characteristics of the current olive fields in the study area are presented in
Table 4. Accordingly, it was observed that 62.27% of the current olive fields are located in the 300–600 mm precipitation zone, and 30.55% receive precipitation below 300 mm. Koca [
2] and Çelik and Cin [
12] stated that the olive tree can be grown without irrigation in places where the annual precipitation is at least 600 mm; however, irrigation should be carried out especially in summer in order to increase yield and quality. Ozturk et al. [
6], on the other hand, stated that 85% of the olives cultivated in the world are not irrigated and therefore olives can be considered as a drought-resistant species. In addition to this, Guo et al. [
31] and Gucci and Fereres [
74] stated that the annual precipitation should be over 600 mm, while Shadeed et al. [
13] stated that olive cultivation can be carried out with precipitation between 200–300 mm. However, considering the fact that humidity and high precipitation reduce the oil rate in olives, while the inadequacy in humidity and precipitation reduces the yield [
49], we can say that the precipitation values in the study area are at a good level for olive farming.
It is also stated in the literature that olive plants can be grown in regions with an average annual temperature of 15–20 °C and can tolerate temperatures up to 35–40 °C at the highest [
49] and must be irrigated in order to withstand temperatures above 40 °C [
2]. In addition, it is emphasized that the lowest temperature for olive cultivation should not fall below −7 °C [
31,
47,
52,
75], and these values define the geographical areas where olive cultivation can be carried out. In this context, when the study area is examined, it is seen that 94.01% of the current olive fields have average annual minimum temperatures between 6–10 °C, 78.76% have average annual temperatures between 11–15.69 °C, and 95.89% have average annual maximum temperatures between 17–21.86 °C. Furthermore, the lowest temperature in the study area was measured at −6 °C in December, while the highest temperatures were measured between 30.4–37.9 °C in August. There are very few days when the temperature drops below zero or rises above 30 °C in the study area [
76]. These figures show that the climate characteristics of the study area are compatible with the climate demand of the olive tree.
Considering the current olive fields in terms of solar radiation, another climate factor, it was observed that 88.48% of the olive fields are located in regions with a solar radiation value of 1315.26–1560.71 kWh/m
2 (
Table 4). As Guo et al. [
77] point out, substantial amounts of sunshine are favorable for the production of olive fruit.
Although the olive plant does not show much selectivity in terms of soil requirements, Koca [
2] stated that sandy loamy, loamy, loamy-sandy, clayey-loamy, and silty-loamy soils are suitable for olive trees with their permeable structures and sufficient water-holding capacities. In addition, Guo et al. [
31] stated that while neutral and calcareous soils are suitable for olive cultivation, sticky yellow, ginger stone, and sand yellow soils are not. It can be seen that 91.86% of the olive fields taken into consideration in the study are in the M (brown forest soils) soil group and 2.69% are in the A (alluvial) soil group (
Table 5). It was reported that brown forest soil (M) is used for perennial orchards such as olives, apples, pears, and vines, and has clayey-loamy and sandy-clayey-loamy textures [
78]. At the same time, Hantekin [
3], Varol [
47], and Tuğaç and Sefer [
4] stated that soils rich in nutrients, without salinity problems, and with good water holding capacity are suitable for olive cultivation. In this context, sub-capability classification (LUCS) is determined by the land capability classification’s limiting factors [
18,
79]. Approximately 53.63% of the current olive fields in the study area have slope and erosion damage, and soil inadequacy (es-se), and 38.23% have slope and erosion damage constraints. The city’s rough and highly sloping land structure has caused erosion sensitivity and insufficient soil depth problems and has caused agriculture to be carried out by forming terraces in practice. In fact, when the soil depths of the olive fields are examined, it can be seen that 38.23% of them are in the middle depth group, while 32.92% of them are in the shallow and very shallow groups. While the best depth for olive cultivation is considered to be between 1 m and 1.5 m, it is stated that the olive tree can provide a reasonable yield at half a meter depth under suitable conditions [
1]. While generally less sloping and alluvial areas are used for field agriculture in Turkey [
2], it has been seen that olive production can be carried out on barren, stony, rocky, pebbly, sandy, and calcareous soils where agriculture cannot be carried out efficiently [
12]. A similar situation is encountered in Artvin Province as well. Soils are classified into eight classes in the creation of LUCCs [
80] based on the limits they impose on the agricultural crops to be grown on them. While lands with class I land use capability are considered the best for cultivation, places with class VIII soil capability are considered inappropriate for cultivation [
5]. Class VI lands are those used for forests or pastures and require a moderate level of measures, while class VII lands are those with steep slopes or high erosion risk, as well as those with stony, shallow, or dry soils, or other undesirable soil types [
18]. Approximately 70.65% of the olive fields taken into consideration in this study are class VI and VII lands (
Table 5). In the study conducted by Tuğaç and Sefer [
4], it can be seen that class I-VII lands are considered suitable for olive cultivation.
Lastly, the land cover parameter is addressed in this title, where the characteristics of the current olive fields are examined. In this context, it can be seen that 29.16% of the olive fields are in the trees category, 56.13% are in the scrub/shrub category, and 12.22% are in the built area category (
Table 6). The surface area of the olive plots used for the training of the RF algorithm varies between 6.36 m
2 and 8708.17 m
2. In particular, the fact that olive plots with an area of fewer than 200 m
2 are located in the village settlements caused olive fields to be included in the built area category.
At this point, it can be said that the climate, soil, and topography characteristics of the olive fields in the selected study area of Artvin are compatible with the ecological demands of the olive tree.
3.3. Suitability Evaluation for Olive Cultivation with RF Algorithm
At this stage of the study, suitability analysis for olive farming was carried out using the RF algorithm with 10 parameters that were determined not to be multicollinear and to be effective in olive farming, and then a suitability map for olive farming was produced (
Figure 8). The results of the suitability evaluation were classified into five classes of natural breaks (currently not suitable (N1), permanently not suitable (N2), highly suitable (S1), moderately suitable (S2), and marginally suitable (S3)) according to FAO guidelines [
14,
18].
Because agricultural activities are not permitted in forest and pastureland under Turkish law, and water is being/will be held in the aforementioned dams, these regions were eliminated from the suitability map, and the final suitability map was created (
Figure 9).
Figure 10 shows the areal distribution graph of suitability evaluation groups.
According to the land suitability map, 17.59% (60,981.04 ha) of the study area is suitable for olive cultivation, but 12.40% (3153.86 ha) of this area cannot be used for olive farming since it has a forest and pasture nature, and 18.09% (3832.61 ha) of it is flooded/will be flooded due to dams. As a result, while 2.65% (53,994.57 ha) of the study area is suitable for olive cultivation, 97.35% (90,988.24 ha) is permanently and currently unsuitable (
Figure 10). Thus, on the scale of the study area, a total area of 53,994.57 hectares, 7666.02 hectares of which is highly suitable, was found to be suitable for olive cultivation. In the study, it can be seen that the ecological demands of olive trees (especially climate and topography) are effective in the low ratio of suitable areas for olive cultivation. In addition, the rapid increase in elevation in the study area, and therefore the decrease in temperature, as well as flooding of the valley floor due to large dams, limited the areas where olive trees can grow. At the same time, the fact that the dominant land cover in the city is forest and pasture with 86.24% became effective in this result. Bilgilioğlu [
5], in his study in Mersin, also found that 8% of the study area is suitable for olive cultivation and the remaining area is unsuitable.
It is also important to analyze the effects and relative importance levels of the parameters used in researching the suitability of the land for olive cultivation on a regional scale. In this context, the relative importance levels of the parameters used for olive cultivation with the RF algorithm are seen in
Figure 11. This figure was produced using the built-in variable importance (or feature importance) function (
varImp) of the RF algorithm. The variable importance function assigns a score that expresses how useful the input variables are in estimating a target variable, using techniques such as Gini importance and permutation variable importance [
81]. Relative importance scores can be used in a predictive model to better understand the data and the model and reduce the number of input features. In the RF model produced in this study, average temperature and annual total precipitation were the two most important and effective parameters, while LUCS and slope were the least important and effective parameters. Again, solar radiation, aspect, land cover, OSP, soil depth, and LUCC parameters were also effective in order of importance (
Figure 11).
Similarly, Ustaoğlu and Uzun [
49] also determined temperature and precipitation as the most effective parameters for olive cultivation. However, Bilgilioğlu [
5] found that land use capability, the average annual temperature, annual average precipitation, and elevation criteria are more effective than other criteria in determining the sites suitable for olive cultivation.