Bioindication of Environmental Conditions Using Solar Park Vegetation

: Solar parks are currently considered a new source of carbon-free energy. At the same time, they encroach on our landscape and create new conditions for ﬂora and fauna. Vegetation responds to changes in living conditions. The aim of this study was to evaluate the changes in soil conditions induced by solar parks using vegetation bioindicators. A vegetation assessment was carried out in a solar park located in the cadastral territory of the municipality of Un í n (Czech Republic). Among the soil conditions observed, this study focused on soil moisture, pH, nitrogen availability, phosphorus availability


Introduction
Today's world is powered by conventional fossil fuels, such as coal, gasoline, diesel, and natural gas. Fossil fuels are convenient to use, however, they also have their consequences, including the release of harmful NO X , SO X , and CO X gases [1]. Recently, the increase in demand for energy, concerns about the state of the natural environment, and the shortage of fossil fuel resources-as well as the Ukraine-Russia conflict-have resulted in increased interest in alternative sources of clean and renewable energy [2,3]. In addition, the outbreak of military conflict has impacted supply chains and the availability of fossil fuels, which are particularly needed in the energy sector worldwide [4]. One way to solve this energy crisis is to increase the use of renewable energy as a power source. In the renewable energy sector, solar energy is one of the best alternative energy sources because it has less harmful effects on the surrounding environment. In terms of sustainability and quality, solar energy has the potential to meet current energy needs [1,5].
Solar energy is one of the most rapidly growing carbonless energy technologies. In 2016, the global increment of network capacity was made up of one-third of solar energy [6]. The capacity of solar energy is increasing and is assumed to increase by up to 50% between The solar park is on a land property of 4966 m 2 , which used to be farmland before the PV plant construction. In previous years, wheat, rapeseed, and barley were grown here. The photovoltaic power plant was built there in the second half of 2009, its installed capacity being 0.627 MW.

Methodology of Vegetation Assessment
Vegetation was assessed using the method of phytocoenological relevés, the size of which was 20 m 2 (2 × 10 m). In the solar part, the relevés were recorded at different locations-between the PV panels and under the PV panels. At each location, three permanent plots were laid out, on which the relevés were recorded. The relevés were recorded on the same plots in 2016, 2017, and 2018, at all times in spring, summer, and autumn. All present vascular plant taxa (mainly species) were recorded, and their cover was estimated using a modified Braun-Blanquet nine-grade scale [46]. The taxonomic nomenclature of plants followed that of Kaplan et al. [47].
Vegetation between the PV panels is controlled by mowing and mulching. The frequency of management measures is adjusted so that the height of the vegetation does not reach the height of the panels and which are not shaded. Vegetation under the PV panels is controlled by mowing with the removal of biomass and the application of total herbicides.
In the study area, the cover values of different plant species were determined using multivariate analysis of ecological data. This analytical method was chosen based on the length of the gradients detected through Detrended Correspondence Analysis (DCA). The The solar park is on a land property of 4966 m 2 , which used to be farmland before the PV plant construction. In previous years, wheat, rapeseed, and barley were grown here. The photovoltaic power plant was built there in the second half of 2009, its installed capacity being 0.627 MW.

Methodology of Vegetation Assessment
Vegetation was assessed using the method of phytocoenological relevés, the size of which was 20 m 2 (2 × 10 m). In the solar part, the relevés were recorded at different locations-between the PV panels and under the PV panels. At each location, three permanent plots were laid out, on which the relevés were recorded. The relevés were recorded on the same plots in 2016, 2017, and 2018, at all times in spring, summer, and autumn. All present vascular plant taxa (mainly species) were recorded, and their cover was estimated using a modified Braun-Blanquet nine-grade scale [46]. The taxonomic nomenclature of plants followed that of Kaplan et al. [47].
Vegetation between the PV panels is controlled by mowing and mulching. The frequency of management measures is adjusted so that the height of the vegetation does not reach the height of the panels and which are not shaded. Vegetation under the PV panels is controlled by mowing with the removal of biomass and the application of total herbicides.
In the study area, the cover values of different plant species were determined using multivariate analysis of ecological data. This analytical method was chosen based on the length of the gradients detected through Detrended Correspondence Analysis (DCA). The purpose of this study was to understand the relationships between different plant species and their environments.
Canonical Correspondence Analysis (CCA) was performed as part of the multivariate analysis. CCA helps identify the environmental variables most strongly associated with the distribution of plant species. Using both DCA and CCA, it was possible to gain a more comprehensive understanding of the plant community in the study area.
To establish statistical significance, a Monte Carlo test was conducted using 999 calculated permutations. This test is a non-parametric statistical method that can Environments 2023, 10, 86 4 of 16 be used to analyze complex datasets, such as those obtained through multivariate analysis. It involves creating multiple random permutations of the data and calculating a test statistic for each. By comparing the actual test statistics to the distribution of test statistics obtained from the permutations, it was possible to determine whether the observed pattern was statistically significant.
Finally, the data were processed using the SW Canoco 4.0. This software is specifically designed to analyze multivariate ecological data and is widely used in ecological research. Using this software, it was possible to conduct a comprehensive and rigorous analysis of plant communities in the study area [48].
Based on the database compiled by Tyler et al. [49] criteria were chosen, characterizing 5 selected soil conditions. Plant species were used for the bioindication of local soil conditions.
The first criterion was soil moisture content, which is expressed as average and realized moisture content together with water conditions in the niche of plant species. The criterion is expressed by the bioindication scale of seven degrees: (i) Mo1 = very dry; (ii) Mo2 = dry; (iii) Mo3 = mildly dry; (iv) Mo4 = average moisture content, common water regime; (v) Mo5 = moderately moist, not occurring on extremely wet or dry niches; (vi) Mo6 = moist, not occurring on extremely dry niches; (vii) Mo7 = wet soils; (viii) Mo8 = wet and poorly aerated soils; (ix) Mo9 = temporarily flooded; (x) Mo10-Mo12 = permanently flooded.
The next criterion is availability of N, which is the mean realized limit of the availability of this nutrient to plants from the soil. The scale of nine degrees is as follows: (i) N1 = nitrogen difficult to access; (ii) N2 = nitrogen difficult to access up to poorly available; (iii) N3 = nitrogen poorly available; (iv) N4 = nitrogen available; (v) N5 = soils moderately rich in nitrogen; (vi) N6 = soils moderately rich to rich in nitrogen; (vii) N7 = soils rich in nitrogen; (viii) N8 = soils very rich in natural nitrogen; (ix) N9 = soils very rich in nitrogen, usually artificially enriched with N.
Another criterion is the availability of phosphorus (P), which is expressed as the mean realized limit of the availability of phosphorus to plants from the soil. The scale of five degrees is as follows: (i) P1 = plant species avoiding soils with high availability of phosphorus; (ii) P2 = plant species disadvantaged by high availability of phosphorus; (iii) P3 = plant species thriving on average availability of phosphorus; (iv) P4 = plant species preferring high availability of phosphorus; (v) P5 = plant species occurring only in soils with high availability of phosphorus.
The next criterion is soil salinity which is expressed by tolerance to the gradient of salinity, and is characterized by a scale of five degrees: (i) S1 = plant species intolerant to salt, also avoiding conditions with a weak salt solution; (ii) S2 = plant species moderately tolerant to salts but preferring conditions without salts; (iii) S3 = plant species tolerant to mild salinization but their occurrence is not restricted to such habitats; (iv) S4 = plant species are more competitive at moderate to high soil salinity; (v) S5 = plant species are competitive at high soil salinity only. A block diagram explaining the process of this study is presented in Appendix A Figure A1.

Results
There were altogether 77 taxa of plants with a dominant position of permanent species found on the assessed solar park site. Lower coverage of all plant categories was recorded under the PV panels with a conspicuous predominance of annual plant species. The steady species composition of solar park vegetation showed only slight differences between the competitive at high soil salinity only. A block diagram explaining the process of this study is presented in Appendix A Figure A1.

Results
There were altogether 77 taxa of plants with a dominant position of permanent species found on the assessed solar park site. Lower coverage of all plant categories was recorded under the PV panels with a conspicuous predominance of annual plant species. The steady species composition of solar park vegetation showed only slight differences between the evaluated years. The coverage of the plant categories during the monitored period and at the investigated sites is shown in Figure 2. The correlation between the identified plant species and the monitored sites was assessed using CCA analysis. Figure 3 graphically shows the results of the analysis. Table  1 shows the division of plant species into groups. Group 1 of plant species (Under) showed a predominant occurrence and higher coverage in the area under the PV panels and associated species, indicating dry sites. Group 2 of plant species (Under) occurred predominantly and showed higher coverage under the PV panels, associating species with diverse indications. Group 3 (indifferent) associated plant species with no significant site preference were recorded at both sites (Between and Under). Group 4 of the plant species (Between) occurred more frequently between the PV panels. The correlation between the identified plant species and the monitored sites was assessed using CCA analysis. Figure 3 graphically shows the results of the analysis. Table 1 shows the division of plant species into groups. Group 1 of plant species (Under) showed a predominant occurrence and higher coverage in the area under the PV panels and associated species, indicating dry sites. Group 2 of plant species (Under) occurred predominantly and showed higher coverage under the PV panels, associating species with diverse indications. Group 3 (indifferent) associated plant species with no significant site preference were recorded at both sites (Between and Under). Group 4 of the plant species (Between) occurred more frequently between the PV panels.
The first assessed criterion was moisture. Results of the bioindication of conditions are shown in Figure 4. The site under the PV panels exhibits a significantly higher share of drought-resistant plant taxa.
The second assessed criterion was soil pH. Results of the bioindication of conditions are shown in Figure 5. The site under PV panels exhibits a higher share of plant species, indicating acidic soil pH but also neutral soil pH.
The third assessed criterion was the availability of N. Results of the bioindication of conditions are shown in Figure 6. In this criterion too, the site under the PV panels exhibits a higher share of plant species indicating low availability of N but also a higher share of plant species indicating a high surplus of N.
The fourth assessed criterion was the availability of P. Results of the bioindication of conditions are shown in Figure 7. In this criterion, once again, the site under the PV panels exhibits a higher share of plant taxa indicating low availability of P but also a higher share of plant species indicating high enough phosphorus.

Groups of Taxa Taxon
Group 1 prefers sites Under PV and on dries sites  The first assessed criterion was moisture. Results of the bioindication of conditions are shown in Figure 4. The site under the PV panels exhibits a significantly higher share of drought-resistant plant taxa. The second assessed criterion was soil pH. Results of the bioindication of conditions are shown in Figure 5. The site under PV panels exhibits a higher share of plant species, indicating acidic soil pH but also neutral soil pH.  The first assessed criterion was moisture. Results of the bioindication of conditions are shown in Figure 4. The site under the PV panels exhibits a significantly higher share of drought-resistant plant taxa. The second assessed criterion was soil pH. Results of the bioindication of conditions are shown in Figure 5. The site under PV panels exhibits a higher share of plant species, indicating acidic soil pH but also neutral soil pH.   The third assessed criterion was the availability of N. Results of the bioindication of conditions are shown in Figure 6. In this criterion too, the site under the PV panels exhibits a higher share of plant species indicating low availability of N but also a higher share of plant species indicating a high surplus of N. The fourth assessed criterion was the availability of P. Results of the bioindication of conditions are shown in Figure 7. In this criterion, once again, the site under the PV panels exhibits a higher share of plant taxa indicating low availability of P but also a higher share of plant species indicating high enough phosphorus.

Discussion
Solar parks and the presence of PV panels cause changes and fragmentation of the living conditions of plants. Different conditions are reflected in the different species composition of the vegetation. The sites between the PV panels were dominated by perennial species, both grasses (Festuca rubra, Lolium perenne, and Dactylis glomerata) as well as by some permanent dicotyledonous species (Trifolium repens). The species composition of vegetation indicates that the sites between the PV panels feature conditions similar to those of meadows and pastures. The dominance of grasses in the vegetation was also confirmed by Armstrong et al. [28], Uldrijan et al. [50], and Vaverková et al. [51].
The site under the PV panels differs in the species composition of vegetation and was less favorable for biomass formation. Perennial species represented there in greater numbers are for example Calamagrostis epigejos, Cirsium arvense, and Urtica dioica. Conditions of this site are also favorable for some annual plant species such as Senecio vulgaris, Capsella bursa-pastoris, and Bromus sterilis. As these species are mostly nitrophilous, we can assume that the drift of nutrients, namely N, occurs in these places, and better availability of water can be expected as well. PV panels provide shade and limit solar radiation but also change the distribution of water precipitation. Irregular and lacking precipitation shows in the different compositions of vegetation on sites under the PV panels. Drier places are then dominated by plant species tolerant to low water supply, for example, Artemisia vulgaris, Sedum sexangulare, and Polygonum aviculare. It follows that PV panels change site conditions to which the vegetation adapts [50,52] and there are more research studies showing that some plant species benefit from the shade provided by them [53][54][55][56].
The bioindication of vegetation in solar parks shows that the living conditions for plants are very diverse. Environmental heterogeneity is the basis for landscape fragmentation and hence, for enhanced biodiversity. Increased botanical biodiversity following different microclimates in solar plants (shade, rainfall diversion) can result in a higher number of invertebrates and a greater diversity of avian species and invertebrate animals, including pollinators [21]. Solar parks have a considerable potential to enhance the biological diversity of pollinators because of the availability of food sources and places for reproduction [57]. Solar parks also have specific impacts which are shown in the landscape as a longterm disturbance of life potential, and therefore, their operation should be considered with responsibility [58,59]. The growing capacities of solar parks will increase land confiscation, which will lead to ecosystem disturbance. Therefore, local environments must be considered. Solar parks offer land areas suitable for the creation of various biotopes and minimize the impact of marginal agricultural effects (e.g., pesticide spraying) [57].
The massive transformation of agricultural practices has led to the conversion of a structurally diverse and complex landscape into a homogeneous and intensively used environment with a substantial loss of biodiversity-rich sites such as hedgerows, field margins, and other uncultivated areas [60,61]. The intensification of agriculture has had a major impact on the overall loss of biological diversity in the landscape [62][63][64]. Biodiversity loss has a myriad of consequences for the function and stability of landscapes, including a decline in ecosystem services [65][66][67]. Reductions in ecosystem services have greatly affected the suitability and availability of food for wildlife and have led to further declines in biodiversity [68][69][70]. The construction of the solar park and the diverse management of vegetation together creates diverse conditions for the solar park. High diversity creates favorable living conditions for various plant types. Therefore, a prerequisite is created to increase the biodiversity of vegetation with wider ecosystem services.
Vegetation in the landscape fulfills a number of ecosystem functions, such as reducing soil erosion, improving soil health, phytomedical functions, food sources for animals, and aesthetic and cultural functions [71][72][73][74]. Higher species diversity of vegetation is a prerequisite for the successful fulfillment of these functions [75,76]. The vegetation of solar parks has a high potential to fulfill all these functions in the landscape. Solar parks can become islands of biodiversity stabilizing ecosystems in the landscape.
The fundamental problem of the current agricultural landscape is the homogenization that has taken place in recent decades and is still taking place today [77,78]. The result of the homogenization of the landscape was primarily the merging of plots of land and the expansion of arable land. This has led to the loss of habitats, linear greenery, meadows, and field margins, and reduced landscape fragmentation [79]. The conditions of the solar park, where alternating PV-panel strips shade the area and divert rainfall, fragment the site conditions. According to Steffan-Dewenter [80], Paesel et al. [81], and Fattorini [82], vegetation composition changes based on microclimate, with different vegetation compositions in shaded areas and permanently sunlit areas.
Another important factor is vegetation management, which significantly affects ecosystem functions and biodiversity [83][84][85]. Vegetation management applied to solar park lands can take different forms. Its main objective is to limit the height of vegetation and prevent the shading of PV panels caused by vegetation. Vegetation management also affects the water regime [86][87][88]. Herbicide use often results in completely bare soil that has limited infiltration, increases stormwater runoff, and can increase soil erosion [89,90]. The frequency of vegetation mowing is a key indicator of plant species diversity [91]. Studies conducted by Chollet et al. [92] reported that lawns with low mowing frequency are characterized by higher plant biodiversity than green areas with high mowing frequency. Moreover, the results obtained by Watson et al. [93] suggested that frequent mowing may facilitate weed invasion. On the one hand, vegetation management of solar parks must ensure the functionality of PV panels and limit the negative impacts of vegetation; on the other hand, high-intensity vegetation control will limit diversity and leads to the spread of unwanted invasive plant species.
The bioindication of soil conditions in the solar park shows that conditions under the PV panels significantly differ. Uldrijan et al. [94] warn about the change in living conditions under the PV panels, too. It is to be expected that life's extremes will be increasing due to limited precipitation as well as due to shading. The question is what the cumulative action of these changes will be and how it will affect the ecosystem.
The increasing extremes in soil conditions are not good news for local native plant species. Changes in cumulative action may result in the withdrawal of some plant species. In a vacant place, species with more plastic life requirements may occur as well as species of invasive characteristics. Plant species of invasive character already occurring in the solar park are, for example, Artemisia vulgaris, Digitaria sanguinalis, Bromus hordeaceus, B. sterilis, B. tectorum, Calamagrostis epigejos, Cirsium arvense, Impatiens parviflora, and Epilobium adenocaulon.
Extreme conditions under the PV panels are utilized by more tolerant plant species. The anthropogenic conditions of solar parks, however, create space for the involvement of plant species with anthropogenic life strategy [95]. The vegetation of solar parks is a result of the co-evolution of human civilization and synanthropic vegetation.

Conclusions
• Vegetation in solar parks provides valuable information regarding the impact of PV panels on the environment; • Different living conditions, bioindicated by plants, suggest increasing extreme soil conditions due to PV panels; • PV panels change the distribution of water precipitation, affecting the soil water regime, availability of nitrogen and phosphorus, soil pH, and salinity; • Vegetation responds to these changes by withdrawing sensitive plant species and increasing tolerant species; • It remains unclear to what extent these changes will be permanent and whether they could affect the subsequent use of the land; • Heterogeneous conditions in solar parks may lead to increased biodiversity of vegetation, subsequently reflected in the increased biodiversity of animals; • Changes in the environment also open up opportunities for invasive plant species, therefore monitoring, bioindication, and subsequent management of vegetation in solar parks is crucial; • Solar parks represent a new living space for plant species, some of which find conditions favorable for growth and reproduction, whereas others have to adapt to new conditions; • Solar parks can be considered the next stage in the co-evolution of vegetation and human civilization.
Consideration: The future impact of invasive species on solar parks will depend on a variety of factors, including the specific species present in the area, management practices in place, and climatic conditions. As climate change continues to alter ecosystems and create new opportunities for invasive species to establish themselves, solar parks and other facilities need to be vigilant in monitoring and controlling the spread of invasive species to minimize their impact. This may include implementing early detection and rapid response strategies, using integrated pest management techniques, and working with local communities to raise awareness of the importance of preventing the introduction and spread of invasive species.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author and shared after his consideration.