1. Introduction
Plant communities owe their existence to relations between plants and their surrounding environment. A number of environmental factors (e.g., soil nutrients and moisture content) impose a profound effect on the unique characteristics of plants as well as plant and vegetation communities [
1,
2,
3,
4,
5]. The introduced relations between plant communities and environmental factors underpin a very important debate in ecology [
6]. Arid and semi-arid areas are mostly characterized by their scarce vegetation cover which makes its ecosystems fragile and vulnerable to external or internal components such as human activities [
7]. The distribution, pattern, and abundance of plant species in arid and semi-arid areas have most often been related to three groups of factors including physical environmental, soil chemistry, and human factors [
8]. In order to plan better conservative measures and perform effective well-informed management, one should understand the differences between species of distinct ecological categories within a studied location [
9]. The existence of relationships between the environment, humans, and vegetation dictates the progress or regress of a given species within a given area.
The relations between human activities and plant communities have been studied by many researchers. Lavergne et al. (2005) [
10] analyzed the implications of environmental and human factors on the presence of rare species, exclusion, and preservation of the species in the Mediterranean region. Comparing the floristic lists of 1886 and 2001 showed that in 2001, rare species tended to migrate to upper elevations. Guo et al. (2007) [
11] evaluated that both undisturbed and disturbed communities showed the same changeable bell-shaped trend with an elevation increase. Suzart de Albuquerque et al. (2011) [
12] scrutinized human effects on the richness of endemic and exotic plants. Their results showed a strong relationship between the endemic and exotic species. Zhang (2009) [
13] also evaluated the influences of grazing intensity, soil, and topography on the status and diversity and found that grazing intensity was an important factor influencing vegetation composition and structure. Soil and topographic variables were also important to vegetation composition, although most soil variables were sensitive to grazing intensity. Similarly, El-khouly (2004) [
14] found that overgrazing was the most important activity influencing vegetation diversity and floristic composition. Salinity was also found to be a significant factor that limits the plant species diversity in the location of Siwa Oasis, Egypt. A study by Pueyo et al. (2006) [
15] demonstrated that grazing gives rise to the biological diversity in plant species. Mligo (2006) [
16] also pointed out that the lightest grazing intensity areas were the main cause of plants reaching a peak in diversity.
Considering the high importance of plants and soils in the lives of humans, and in order to have effective management of a natural system, such as rangeland ecosystem, identifying the relationship between plant and soil has a great importance [
17,
18,
19,
20]. Soule and Piper (1992) [
21] and Weil (2004) [
22] discussed that relationships between soil properties and the dominance of certain plant species in a natural environment may assist in tailoring suitable species for particular soil types within a managed rangeland. In this regard, through field visits and the measuring of various environmental factors in Kamyaran rangelands in Iran, Gholinejad et al. (2012) [
23] surveyed environmental determinants including the main soils, and physical and chemical parameters on plant communities’ distribution as one of the most significant structures in semi-arid rangelands. Their findings showed that clay, nitrogen, sand, altitude, and slope have the highest relationships with principal components and are the most important factors affecting the distribution of plant communities in semi-arid rangelands. Accordingly, the first hypothesis of the current study is that there is a significant difference between soils’ physical properties in different rangeland sites. Zare Chahouki et al. (2008) [
24] evaluated the relationship between vegetation distribution patterns and soil characteristics in the Poshtkouh rangelands of Yazd Province, Iran. Their results revealed that the vegetation distribution pattern was mainly correlated to soil parameters such assalinity, soluble potassium, texture, gypsum, and lime. Based on the previous studies, the second hypothesis of this study is that there is a significant difference between species diversity in different rangeland sites. In relation to the effects of soil parameters on the diversity of plant species, Munhoz et al. (2008) [
25] also worked in the species–environment relationship in the herb-subshrub layer of a moist Savanna site in central Brazil. They found a significant relationship between the soil texture and soil moisture parameters with the plant distribution based on the canonical-correlation analysis. Timsina et al. (2011) [
26] found a significant difference between invaded transitional and non-invaded plots in species composition and soil properties in rangelands communities in Nepal. Zhang (2009) [
13] also evaluated the influences of soil variables and topographical features on the plant variety and reported that topographical characteristics could alter plant variety while soil variables could permanently alter vegetation varieties and structure. There is sufficient evidence that land use/land cover change is a major driving factor for the balance of SOM (soil organic matter) [
27,
28,
29]. In particular, changes among land use types such as cropland, forestland, and pastureland will result in clear changes in SOM reserves [
27]. Houghton (2003) [
30] showed that global land use changes since 1850 had caused 156 Pg of soil carbon release into the atmosphere.
Based on the role of plants that bring balance into ecosystems and their products which humans directly as well as indirectly consume, there is an urgent need to understand the relationship between the plants, the surrounding environment as well as human activities to reach stability and sustainability. Given the role of human on ecosystems, the third hypothesis of the current study is that human activities have a significant impact on species diversity. Reasonable management and utilization of rangelands requires recognition of the vegetation per se and the relationship that it has with the environment and humans. It is obvious that soil, climate, and biological characteristics of the region have a direct influence on the diverse plant community [
31]. Assessing the mentioned factors is inevitable for finding the reasons of plant distribution and variety and also site capability. Vegetation and diversity conservation, recognition of plant communities, environmental factors, and human effects are fundamental principles for the preservation of natural ecosystems. With respect to the importance of species diversity in managing rangelands, this paper aims to identify and assess the types and potential influences of some influential human and environmental factors regarding plant distribution and diversity in two types of rangelands (degraded and good condition) and abandoned rain-fed regions in the Fereydan district, Isfahan province, Iran.
3. Results
Table 2 shows the physicochemical properties of soils in different land management situations in this study. The results of the ten experimented physicochemical properties of the soil indicated that only the amount of organic matter has a significant difference between the studied rangeland sites. The rangeland in good conditions had the highest amount of organic matter compared to the other two studied rangelands. When water logging occurs in the soil, it will result in the accumulation of organic matter. The higher densities of the dominant species can also cause more organic matter in the soil.
Table 3 presents the results for investigating the effects of human activities on plant communities showing the distance from a few man-made facilities in the study site. As the table shows, the good condition rangeland sites are located more than 1 km away from human infrastructure (such as road, watering place etc.) and rural sites whereas abandoned rain-fed and poor condition rangelands are located in the vicinity of the human infrastructures. Accordingly, these two categories of rangelands have less than 200 and 500 m distance from the roads and water holes, respectively.
In order to identify the most important environmental factors influencing the vegetation diversity, the PCA (principle component analysis) was applied which supports the importance of the variables devised in each factor [
41]. This method is concerned with establishing which linear components exist within the data and how a particular variable might contribute to that component [
42]. In accordance with eigenvalues of the PCA axis, which are useful for selecting the axis, the most important axis was selected. According to the results, the first and second axis justify the most significant changes of the eigenvalues, respectively 0.402 and 0.234. Therefore, the most important factors correspond to the first and then second axis. Thus, according to the eigenvalues, the most effective factors evaluated in the study are located in the first and second axis including CaCO
3, watering place, village distance, and roads.
Figure 2 and
Figure 3 show the result of the PCA method for distribution of plant species in the study site and the effective environmental and anthropogenic disturbance in the coordinate axis. Accordingly, SP (saturation percentage) percentage and OM (organic matter) axis are in the same direction and the low angle is a sign of high correlation between these two factors. Shannon and Hill diversity indices in good condition rangelands in the second quarter of the coordinate axis show high plant diversity. Poor condition rangeland is located in the third quarter of the coordinate axis and soil factors including silt, pH, and gravel content are effective in the distribution of poor quality plant species while the abandoned rain-fed site is located in the first and fourth quarters of the coordinate axis. Floristic list in the designated site is shown in the
Table 4 with the percentage of the relative frequency. Major vegetation of abandoned rain-fed areas has low or non-palatability and the vegetation of abandoned rain-fed includes invasive species. Main species is
Eryngium billardieri and accompanying species are
Boissiera squarrosa and
Euphorbia descipiens. In the good condition rangelands, the dominant species is
Bromus tomentellus and other accompanying species are:
Astragalus verus,
Ferula ovina, and
Eremo songarica. Annual species and non-palatable ones such as
Bromus tectorum and
Astragalus verus are the main plant cover of poor condition rangelands.
Furthermore, the indices of plants richness and diversity were analyzed. In doing so, the Hill N1 and Shannon–Weiner were applied as appropriate indices to approximate species diversity.
The results of the Simpson, Shannon, Hill N1, and Hill N2 are presented in
Table 5 and
Table 6 illustrating that the maximum and minimum values belong to the Hill N1 and Simpson diversity indices, respectively. The estimated Shannon and Hill N1 indices show significant difference between the good rangeland site and abandoned rain-fed or poor condition rangeland based on all species (
p ≤ 0.05). Also, the calculated diversity indices based on perennial species coverage show significant difference between the good condition rangeland site and abandoned rain-fed or poor condition rangeland (
p ≤ 0.05). In the multivariate regression analysis, the Shannon–Weiner and Hill N1 species diversity index is applied as dependent, and soil and anthropogenic activities used as independent variables.
4. Discussion
According to the results of this study, among all the tested physicochemical properties of soil, the amount of organic matter indicated a significant difference between the three types of rangeland sites (i.e.,1.4, 1, and 1.3 percent for good rangeland, degraded rangeland, and abandoned dry land farming, respectively). Accordingly, the first hypothesis is rejected as only organic matter indicated a significant difference between the rangeland sites. Similarly, using the same methodology, Zare et al. (2011) [
43] found that salinity, soil texture, effective soil depth, potassium, organic matter, available nitrogen, lime, and soil moisture criteria were the main soil parameters responsible for diversity in vegetation pattern in Shahriyar rangelands, Iran. If water logging occurs in the soil, it will cause the accumulation of organic matter [
44]. The higher densities of dominant species can also be the cause for more organic matter in the soil. While the proper utilization of the natural ecosystems is leading to the preservation of biological diversity, exclusive conservation will not bring about biological diversity at its most appropriate condition [
45]. In this study, organic matter is projected to be the most influential soil component in the plant species to establish their diversity (i.e., 1.4, 1, and 1.3 percent for good rangeland, degraded rangeland, and abandoned dry land farming, respectively). Hersak (2004) [
46] also suggested that soil organic matter and nitrogen content should be considered as the most important factors to define plant succession series over the sand dunes of the northern parts of Croatia.
Examining different rangelands in the Fereydan district showed the existence of a significant difference of species diversity between good condition rangelands compared with the other sites. Accordingly, the second hypothesis of the study is accepted as plant diversity is different between rangeland sites. This study showed that among soil chemical and physical characteristics, only soil organic matter has a direct relationship with the vegetation cover. According to the fact that in the studied rangelands, geological and physiographic features show slight differences, it could be concluded that species diversity has no effect on the edaphic factors and any change made in the soil organic matter is the overall outcome of the interactions of soil and vegetation which itself is under the influence of the utilization level.
With regard to the effects of human-related factors, this study showed that the rangeland areas are in good condition when they are located at a far distance from rural areas and constructed facilities (such as road, watering place) by human in the study site whereas abandoned rain-fed and degraded rangelands are located much closer to these three human infrastructures. Accordingly, the third hypothesis of the study is also accepted as human activities have influential impacts on species diversity. Perhaps, in farther distances, grazing intensity decreases, thus species diversity increases. One of the factors that is effective in plant communities is grazing due to livestock density around the watering places. Findings of Zhao (2007) [
47] and Todd and Hoffman (2009) [
48] suggest that grazing has significant influence on vegetation to the extent that the number of the palatable species decreases and coverage of ephemerals increases considerably. Similarly, decreased local species diversity is confirmed as a widespread impact of human activity [
49,
50]. De Bello (2006) [
51] observed the highest number of rare species in abandoned areas, while abandoned areas correspond to the lowest species diversity in this study. Mitchell et al. (2002) [
32] reported that the reduction in species diversity is preceded by destructive human operations and this could result in a loss of primary production. Also, communities that lost the species dominant at high diversity had higher pathogen loads, presumably because relaxed competition allowed greater in-creases in host abundances. In total, their results support the hypothesis that decreased species richness will increase foliar pathogen load if this increases the host abundance and, therefore, disease transmission. Bornman et al. (2008) [
52] provided the evidence that distance to water acts as one of the most critical variables determining the distribution of plant types in Southern Africa that is consistent with our findings. According to the previous study, there are environmental variables that had the greatest influence on the distribution of the dominant salt marsh species. These environmental variables included soil moisture, distance from the estuary, elevation above mean sea level, and depth to the water table. The most important ecological driver for salt marsh vegetation, especially along the arid west coast of southern Africa, is moisture. Woldewahid et al. (2007) [
53] also reported that access to water and land-use conditions are the main human factors affecting the distribution of plant types along the coasts of the Red Sea, Sudan that is also in line with the current study findings.
Furthermore, the results of PCA in this study showed that the most effective factors affecting the plant diversity index are CaCO3, watering place, village distance and roads (i.e., the most significant changes of the eigenvalues are respectively 0.402 and 0.234). Similarly, Qian (2008), by applying a cluster analysis and PCA in China, considered soil chemical and physical characteristics such as nutrients, moisture, salinity, and pH as the main determinants of ecosystem’s homogeneity and the spatial distribution of plant societies in that area.
There are different studies that investigate the relations between various edaphic, environmental and human activities and their impact on plant diversity and distribution, such as Zare et al. (2011) [
43] in Iran and Wang et al. (2012) [
7] in Qinghai-Tibet Plateau of China. Similarly, Gholinejad et al. (2012) [
23] showed that major factors influencing the plant communities’ distribution in rangelands of Kamyaran rangelands, Iran are physical soil properties and physiographic parameters amongst which soil texture is one of the most effective determinants in the distribution of plant communities in the studied region. With regard to the distribution of plant species in the study area and effective environmental and anthropogenic disturbance, SP percentage, and OM showed a significant difference between the study sites. Moreover, Shannon and Hill diversity indices in good condition rangeland show high plant diversity. Whittaker (1977) [
54] observed that the highest diversity of plants is seen in good condition rangelands. Distance from village, clay percentage and lime content are effective in high diversity and the distribution of plant species such as
Bromus tomentellus,
Festuca ovina, and other desirable species for livestock. In contrast, soil factors such as silt, pH, and gravel content are effective in the distribution of poor quality plant species in poor condition rangelands. Rutherford and Powrie (2010) [
55] studied to quantify and understand the impact of severe land degradation on plant diversity in Succulent Karoo in South Africa and found that although the total number of species declined due to heavy grazing, greater equality resulted in an increase in species diversity.
According to this study, the Hill N1 index is the best for recognition the difference between different rangelands. This is in the line with Peet’s (1974) [
56] results about the Hill index. However, Magurran (1988) [
57] determined that the Shannon diversity and Simpson diversity indices are the best for recognition of species diversity. The estimated Shannon and Hill N1 indices in this study showed significant differences between the good rangeland area and abandoned rain-fed or poor condition rangelands based on all species as well as on perennial species coverage. Panitsa et al. (2010) [
58], applied stepwise regressions to test the effects of similar factors on the species richness of native and especially of endemic plants, at local scales. They concluded that a strong correlation exists between intercept values arising from species–area relationships at the family level and total richness of these families in their studied area (South Aegean, Greece).
5. Conclusions
The study showed that distance to roads, villages, and watering points can affect rangeland sites. Poor condition rangelands and good condition rangelands have the closest and the farthest distances to the human made facilities, respectively. Good condition rangelands have the highest level of species diversity and richness among all the mentioned sites. According to the multivariate regression analysis, the most influential factor on the species diversity index is the distance to village parameter; which signifies the important role of humans in degrading rangelands and reducing species diversity. Accordingly, this study concludes that the best way to conserve natural ecosystems, especially rangeland ecosystems is rangelands management in terms of extending and reserving plant species diversity. This study addressed some dimensions of interactions between human and environmental factors and plant vegetation in native rangelands within semi-arid areas of Iran. It was forecasted that the result of this study could be utilized as a basis for anticipating of the probability of the plant species in rangeland within similar ecosystems and recommend the suitable guidance for management and development of rangelands for similar regions.
The current study had some limitations in selecting soil properties. This study just focused on soil physical properties such as sand and silt content, clay, saturated soil’s moisture content, and gravel percentage as well as chemical properties such as lime content, pH, electro conductivity, and organic matter content. So, we recommend that future studies focus on determining biological and physical properties of soil quality assessment after land sue change, such as soil respiration, carbon biomass, hydraulic conductivity, soil aggregation, and mean weight diameter. It is also recommended that future studies focus on determining the degree of protection of the species and identify vulnerable, endangered, and critically endangered species.