Analysis of the soil conditions in the natural habitats of a climate-resistant species Nerium oleander L.

Climate change, whose effects have become increasingly pronounced in recent years, is causing environmental challenges, such as rising temperatures, droughts, and irregular precipitation. These adversities threaten the availability of natural resources, making it more difficult to meet the needs of living organisms. By 2030, global water demand is projected to increase by 50% (National Intelligence Council, 2012), which will directly impact humans, plants, and animals. As a result, a significant portion of existing water resources will be allocated to food production, leaving less for landscape irrigation.

To address this issue, sustainable landscaping practices must prioritize plant species with low water requirements and high drought resistance.

Today, many countries have already begun integrating genotypes that occur naturally in their flora and, at the same time, have low consumption of natural resources (Karaguzel, 2007; Benschop et al., 2010; Alam et al., 2013). It is considered very important that Türkiye has a very rich flora in this respect, with 10460 species, 2066 subspecies, 888 varieties, and 287 hybrid taxa (Ozhatay et al., 2022).

One of the species in this rich flora is Nerium oleander L., the genus Nerium, a member of the Apocynaceae family, is monotypic, and has only one species. This widely distributed plant thrives in Mediterranean regions, parts of Africa, Asia, South America, and the Southern United States. It is also cultivated as an ornamental plant in parks and gardens. The species is considered native to the Mediterranean, Indian subcontinent, and Western China. The presence of Nerium around the Mediterranean Basin has been documented since the Miocene (Ergun, 1992; Mateu-Andrés et al., 2015).

Although it generally grows in humid and low places, it can reach an altitude of 2500 m in some areas, such as the Atlas Mountains (Senses, 2009). Nerium grows primarily along ephemeral streams, ravines, and other upland seasonal streams where flooding in spring and autumn is common; typically, these streams remain dry for several months during the summer (Mateu-Andrés et al., 2015). It grows naturally in Türkiye, especially in coastal areas where temperatures are high, such as the Mediterranean, Aegean, and Marmara regions (Ergun, 1992; Dik et al., 2013; Simsek, 2013).

According to the information in the Flora of Turkey (Davis, 1978), it is naturally distributed in Manisa, Çanakkale, Balıkesir, Muğla, Denizli, Aydın, Antalya, İçel, Adana, Hatay-İskenderun, and Adıyaman (as subsp. kurdicum Rech. fil., fide Rech. in litt.).

N. oleander is an evergreen, densely branched shrub or small tree that can grow up to 2–6 m tall. It is a fastdeveloping species with a hard and durable stem. While the bark of the body can be bright green, dark brown, and gray, its surface is mostly smooth. It is common to have more than one stem. When the twig of the plant is cut, sticky latex is released (Mamikoglu, 2007; Simsek, 2013; Celik, 2020).

The lanceolate leaves are 2–3 cm wide and 10–15 cm long, usually in groups of three. The leaf tips are acute, narrowing at the base, and terminating in a short stalk (Davis, 1978). The leaf blade is 6–30 cm × 1–3 cm in size. The midrib protrudes on the lower face; the side veins are almost perpendicular to the midrib and parallel, and both sides are naked and coriaceous. Its upper surface is dark, and its lower surface is light green (Ergun, 1992).

The flowers bloom between June and November and are located at the ends of the shoots. It is known that there are more than 400 Nerium cultivars, whose flower colors are mostly in shades of pink or white, with red, purple, yellow, and orange flower colors, rarely fragrant (usually those with layered flowers) (Davis, 1978; Simsek, 2013; Anonymous 1, 2022).

This species is highly adaptable to various soil types, including well-drained and heavy clay soils. It thrives in full sun or partial shade and exhibits remarkable tolerance to heat, drought, and salinity. It is well-suited for coastal areas, enduring salt-laden winds. While it can survive temperatures as low as –5°C and briefly withstand -10°C, leaf damage may occur (Anonymous 1, 2022; Anonymous 2, 2022; Anonymous 3, 2022).

Extreme heat and drought caused by climate change, the effects of which we have felt more intensely in recent years, will adversely affect the existence of water resources and will also directly or indirectly affect the landscape and ornamental plants used in landscape design works and will lead to the emergence of partial changes. Given the increasing impact of climate change, extreme heat and drought conditions will continue to threaten water resources and landscape vegetation. This necessitates a shift toward drought-resistant species, such as N. oleander, which requires minimal water while thriving in challenging environments.

Therefore, this study aims to analyse soil characteristics in its natural habitats to assess potential limitations or preferences for its use in landscaping.

This study’s material consisted of soil samples collected from different locations where the N. oleander L. species naturally occurs in the flora of Türkiye.

The natural distribution locations of the species were identified based on from Flora of Turkey (Davis, 1978), ISTO (Istanbul University—Cerrahpasa Faculty of Forestry Herbarium), Istanbul University Faculty of Science Herbarium, and Istanbul University Faculty of Pharmacy Herbarium.

For this purpose, the provinces of Manisa, Çanakkale, Balıkesir, Muğla, Denizli, Aydın, Antalya, İçel, Adana, Hatay, and Adıyaman were selected as sampling locations. Although İzmir province is not recorded in Flora of Türkiye (1978), it was included in this study as part of the species’ natural distribution. In total, 22 soil samples (Table 1) were collected from natural distribution areas across 12 provinces.

In July and August, 2023, soil samples were collected from a depth of 0–30 cm in the species’ natural habitat, following standard procedures (Jackson, 1958). The samples were placed in polyethylene bags and labelled separately. The physical and chemical analyses of soil samples taken from different locations were conducted in three replicates at S.S. Nilufer Agricultural Development Cooperative Analysis Laboratory located in Ulutek Technopark.

For the classification of selected physical and chemical properties of soil samples, the following sources were used: Dewis and Freitas (1990) for exchangeable calcium (Ca) and magnesium (Mg); TSE (1990) for saturation content; Ulgen and Yurtsever (1995), Richards (1954), Grieve et al. (2011), and Ulgen and Yurtsever (1995) for electrical conductivity (EC) content; Eyupoglu (1999) for pH content; Hizalan and Unal (1966) for lime content; Anonymous (1985) for organic matter content; Olsen et al. (1954) for available phosphorus (P) content; and Pizer (1967) for exchangeable potassium (K) content.

Additionally, the elevation map of the sampling locations where the samples were classified into five categories, while the aspect map was divided into seven categories, both derived from the digital elevation model in ArcMap 10.2.2 (Esri Inc., Redlands, California, USA.) programme. On the contrary, maps of selected physical and chemical properties of soil samples from different locations were created using ArcMap 10.2.2 programme.

Climatic data of different locations

Meteorological data of the provinces where the distribution areas evaluated in this study were located and where soil samples were taken from the website of the General Directorate of Meteorology (MGM) are given in Figures 1 and 2.

Figure 1.

Long-term (1929–2024) average temperature values of the provinces where the natural distribution areas of Nerium oleander L. are located (MGM, 2024).

Figure 2.

Long-term (1929–2024) average rainfall values of the provinces where the natural distribution areas of Nerium oleander L. are located (MGM, 2024).

This study was carried out to reveal the similarities or differences between the soil conditions of N. oleander species, which has a natural distribution in different localities in the flora of Türkiye, and it was determined that the species has a natural distribution within the borders of 12 provinces in Türkiye. Within the scope of the study, soil samples taken from 22 different localities where the species has a natural distribution were analysed in terms of physical and chemical properties.

It has been observed that N. oleander L., one of the maquis formation plants, which is the characteristic plant formation of the Mediterranean climate, distributes naturally in coastal areas, dry waterways, and valleys and that the altitudes of the places where soil samples were taken vary between 0 m and 724 m and that they do not distribute naturally above this altitude limit. It has been observed that the species has a natural distribution in localities with the Mediterranean climate type, and its distribution areas are restricted due to the decrease in temperatures and climate type changes at altitudes of ≥750 m. As a matter of fact, researchers, such as Davis (1978) and Aksoy and Ozturk (1997), reported that the distribution height of the species is at most 800 m. In all natural distribution areas examined for the species, it was determined that the average temperature is 17.3°C, the average highest temperature is 22.9°C, the average lowest temperature is 12.12°C, the lowest temperature is –11.37°C, the highest temperature is 43.75°C, and the average annual precipitation amount is 783.28 mm.

According to the limit values reported by TSE (1990), Ulgen and Yurtsever (1995), it was observed that 22.73% of the soils in the areas where the plant was naturally distributed were clayey-loamy, 72.72% loamy, and 4.55% sandy textures. Moreover, according to Eyupoglu (1999), it was observed that 59.09% of the soils were slightly alkaline, 31.81% were neutral, and 9.10% were slightly acidic.

N. oleander is a species that can adapt to many soil conditions, such as poor, sandy, etc. (Uslu et al., 2018) and can withstand different soil reactions (acid-alkaline-neutral). The species easily adapt to pH levels between 5.0 and 8.3 (Gilman et al., 1993). In fact, the species has been detected in Rio Tinto, (Huelva Province, Andalusia, Spain), one of the world’s largest hyper acidic environments, in vegetation tolerant to water, sediments, and soils with pH <3 and high S, Fe, Cu, and other metal concentrations (Rufo et al., 2011). It was observed that soil pH levels varied between 5.92 and 8.05 in the natural distribution areas examined

The average exchangeable Ca content of soil samples taken from different locations was 18.51 cmol Ca · kg⁻¹ (Table 2), and according to the limit values reported by Dewis and Freitas (1990), it has been determined that 54.55% of the soils had medium levels of exchangeable Ca and 45.45% had high levels of exchangeable Ca.

The average exchangeable Mg content was found to be 4.21 cmol Mg · kg⁻¹ (Table 2). According to the limit values reported by Dewis and Freitas (1990), it was determined that 9.09% of the soils had very low, 45.45% low, 27.28% had medium, 9.09% high, and 9.09% very high levels of exchangeable Mg.

On the contrary, according to the limit values reported by Richards (1954); Grieve et al. (2011); and Ulgen and Yurtsever (1995), it has been determined that 86.36% of the soils in which the plants distributed were found to be nonsaline, 9.10% were found to be very slightly saline and 4.54% were found to be very saline.

The results showed that the species are resistant to soils with a wide range of salinity, including very saline, slightly saline, and nonsaline, as well as saline conditions and salty winds by the sea. Researchers, such as Gilman et al. (1993); Lenzi et al. (2009), and Seridou et al. (2023), reported that the species is resistant to salt in their studies.

On the contrary, the lime content in the soils of natural distributing areas (Hizalan and Unal, 1966) was found to be high at only 9.10% and very high at 18.18%. It was determined that there was an increase in the content of lime in the soils in which the plants distribute due to the increase in altitude.

According to the limit values reported by Anonymous (1985), it was observed that the amount of organic matter in 86.35% of the examined soils was very low and at low levels. This suggests that the species can easily adapt to soil conditions where the content of organic matter is low or very low. As a matter of fact, researchers, such as Trigueros et al. (2012), and Uslu et al. (2018), stated in their studies that the species could easily develop in poor marginal lands and nutrient-deficient soils.

It was determined that only 4.55% of the examined soils were high in terms of available P (Olsen et al., 1954) and 36.36% were very high in terms of exchangeable K (Pizer, 1967).

On the contrary, it was found that there was a 1% statistically positive correlation between the EC content of the soils located in different aspects (Figure 8) and the potassium and magnesium content of the soils. The main reason for this relationship can be explained as the increase in the EC content of the soils due to the presence and increase in the concentration of Mg salts in the soil.

A positive correlation was obtained between the organic matter content and saturation levels of soils at a 1% statistical level. The main reason for this correlation can be shown as the increase in the saturation capacity of soils in parallel with the increase in their organic matter content.

N. Oleander, which is one of the native plants of the Mediterranean region and is of great ornamental interest, can adapt to adverse environmental conditions (Bañon et al., 2006), can also withstand long-term droughts because it has some xerophytic adaptations (Lenzi et al., 2009; Sinha and Biswas, 2016), and has been found naturally in areas with quite different soil conditions. The results showed that N. oleander can easily adapt to areas with different soil conditions. This suggests that the species may be preferred to be used in landscaping areas with different soil properties and also as a potted plant in the coming years, which will experience rapid change due to climate change.