Flower Strips as an Ecological Tool to Strengthen the Environmental Balance of Fields: Case Study of a National Park Zone in Western Poland

Flower Strips as an Ecological Tool to Strengthen the Environmental Balance of Fields: Case Study of a National Park Zone in Western Poland

1. Introduction

Agriculture has an impact on every aspect of human life and the future of humanity depends on progress in sustainable agriculture [1], which is defined as an integrated system of production practices that will meet human food needs without depleting the Earth’s resources or polluting its environment but instead enhance environmental quality and sustain the economic viability of farm operations [2]. The need to minimize losses in field biodiversity is undisputed. For this to happen, programs have been prepared to support different ecological activities and encourage farmers to sow flower strips or to set up biodiversity gardens on arable land. One of the main goals of the Green Deal is to move to a more sustainable agricultural system [3]. Diversified landscapes may have the greatest potential to protect biodiversity and maintain pest control functions [4,5,6,7,8]. Flower strips not only increase the ecological value of agricultural landscapes. A promising alternative is also the use of the resulting biomass as a co-substrate for biomethanization, which contributes to the production of climate-friendly energy [9].
Flower strips can be defined as deliberately created strips of profusely flowering plants which attract beneficial arthropods and become a habitat for various groups of organisms living in agrocenosis. The main purpose of flower strips is to enrich the biodiversity of farmland fauna with pollinators and beneficial species that reduce the density of pest populations [10,11,12,13,14,15,16,17,18]. In intensively cultivated fields, there are very few flowering plants that can be used by arthropods that provide ecosystem services [19]. It is obvious that fields cultivated in organic systems or established flower strips provide an area with a more diverse species composition of flowering plants, and thus are more attractive to insects than conventional fields [20].
The establishment of flower strips on arable land may be supported through economic subsidies. Currently, in Poland, this is possible as part of a rural development intervention called “Multiannual flower strips” (second pillar), which is being implemented as part of the CAP Strategic Plan for 2023–2027. Biodiversity may be strongly influenced by local habitat factors; therefore, it is very important to appropriately adjust the botanical composition of the flower strip to a specific area, taking into account the climatic conditions [7]. Adverse conditions for the development of flower zone vegetation may result in the appearance a large percentage of opportunistic species, and even lead to their complete drying out [21]. The possibility of overwintering of sown plant species and the species variability of the long-term strip as a result of the emergence of plants from the soil seed bank are also important [22]. Species mixtures can be composed of annuals, perennials, or a combination of plants with different life cycles [4,17,18,19,20,21,22,23,24,25,26].
An unquestionable advantage of flower strips is that the number of bees increases significantly, both in flower strips composed of annual and perennial plants [14,17,27]. In a study by Balzan et al. [14], the inclusion of strips with Calendula officinalis in tomato fields was associated not only with an increased abundance of bees, but also of parasitoids and other groups of arthropods, with the highest abundance of bees, of course, recorded during the full bloom of plants. In fields with stripes of C. officinalis, a reduction in total pest damage caused by Lepidoptera was also noted. Insects inhabiting perennial plants are more diverse and specialized [28], and older strips are additionally a place of living and wintering [29].
The diversity of plants in the strip should attract and support a large variety of natural enemies of pests [30]. Arthropods provide basic ecosystem functions such as natural pest and weed regulation and pollination. Research by Triquet et al. [31] demonstrates that ground carabid communities are shaped by habitat type, and strips of catch crops can support their occurrence and contribute to increasing their diversity from the first year of establishment. The activity of Poecilus cupreus and Pterostichus melanarius was higher near the strip, and the distance from the strip also affected the size of the insects’ bodies. Anjum-Zubair et al. [32] showed that the species richness of carabid beetles was significantly higher at the edges of fields than in the centers of fields but the total numbers of arthropods were higher in the centers. However, according to McCullough et al. [33], in the short term, maintaining existing semi-natural habitats in the landscape may be more beneficial for plant production than creating flower strips adjacent to crops. According to Beyer et al. [34], the feeding area for pollinators shapes both the surrounding landscape and crops, and they are subject to seasonal fluctuations and annual cycles.
The flowering period of plants is not the only indicator of the attractiveness of plants to arthropods. There are clear preferences of individual groups of arthropods for specific plant species [35]. This is related to the unique set of morphological features of flowers and anatomical features of insects [36] and to the behavior of specific insect species [37]. Results by Carrié et al. [38] suggest that Knautia arvensis and Achillea millefolium seem to be more universal in their attractiveness than others. K. arvensis showed particularly high attractiveness to bees, aphids, and Lepidoptera, and A. millefolium to predatory beetles, hoverflies, all Hemiptera, and Lepidoptera. The authors suggest that Leucanthemum vulgare is also highly attractive to beneficial insects, e.g., Syrphidae and Lepidoptera. Studies by Franzén and Nilsson [39] showed that K. arvensis flowers are attractive to solitary bees and butterflies. In research by Hussain et al. [40], syrphid abundance and species richness were significantly higher in flower strips than in grasslands.
Harbo et al. [41] indicate that flower strips not only increase biodiversity, but also reduce greenhouse gas emissions through increased carbon sequestration. The authors found that in the initial 20-year period after establishment, flower strips absorbed on average 0.48 ± 0.36 Mg C ha−1 year−1, concluding that the conversion of 1% of the total area of arable land in Gerseveral into flower strips might lead to a reduction in emissions of 0.24 Tg CO2 year−1, which equals 0.4% of the current greenhouse gas emissions in this country. Therefore, this function of flower strips is also extremely important in the overall view of sustainable agriculture.

The manuscript is a case study conducted in Wielkopolska National Park, a valuable area located in the temperate conditions of western Poland. The aim of the study was to analyze the species composition of plants in the flower strip two years after sowing a mixture of seeds of perennial plants, and to determine the diversity of the collected arthropods depending on the flowering intensity of plants in the strip.

2. Materials and Methods

The study area covered a 6 m wide and 244 m long flower strip established in an area adjacent to a chemically protected maize crop cultivated in a monoculture system in Wielkopolska National Park (WNP) located in western Poland. This was an area where maize had been grown for several years before. The flower strip was established in the spring of 2021. A total of 37.74 kg/ha of seeds were sown, constituting a mixture composed mainly of perennial plant species. The flower strip was located along the longer side of the maize field. The flower strip was located on averagely good arable soils, in a very good rye complex, quality class IIIb (light clay sands), and a humus layer of about 25 cm.

Botanical field observations were carried out in situ from May to July 2022 (18 May, 14 June, 13 July). One plant species inventory was made in each month and consisted of 5 projections of frames. Inventories of plants were carried out using a metal frame with dimensions of 0.5 m × 0.5 m in a random manner every 50 m along the entire length of the strip. A sampling of five points along the entire transect was made to better average the data from the entire strip. Both species richness and the number of individual species flowering at a given time were inventoried. Due to the need to assess the attractiveness of plants for insects, the number of plants in full bloom [per 0.25 m2] was counted. Whole flowering plants were counted, while in the case of rhizome species and compact inflorescences, which can be considered as one functional pollination unit, flower shoots were counted.

After the plant inventory, the percentage of species in the total number of flowering plants, both from the sown mixture and from the soil seed bank, was determined. One of the factors of attractiveness to arthropods is the color of the inflorescences, so it should be taken into account when choosing a flower mixture for a strip. Therefore, the color of the flowers was also recorded each time. The flower strip was not sheared or grazed during the observation period. No fertilizers or pesticides were used.

In the June–July period (at the time of the most numerous blooming flowers), arthropods were caught using the sweep net method (quickly moving the net back and forth over the flower strip, 25 times directly over the plants). Specimens caught in 25 scoop strokes were assumed as one sample. Arthropods were divided into functional groups: beneficial and potentially harmful. Beneficial arthropods included predators (those catching and eating other organisms, such as insects or mites) and parasitoids (insects that parasitize other insects, their immature stages developing on or inside the host, ultimately killing it) which may affect the number of pests in crops adjacent to the flower strip and pollinators. The numerous arthropod populations causing damage to maize crops (but also cereals and grasses) to which the flower strip was adjoined (as mentioned at the beginning of this chapter) were classified in the manuscript as potentially harmful arthropods (feeding on the main crop). Both groups of arthropods (beneficial and harmful) determined the diversity of the observed arthropods in the studied agrocenosis. The order Hemiptera was additionally described due to the potential role of the caught specimens. Representatives of this order occurred both in the group of harmful and beneficial arthropods and this division was made on the basis of their food preferences and behavior. The percentage share of individual taxonomic groups in relation to all captured arthropods was calculated and their differentiation was presented, depending on the date of observation. The dry weight of individual arthropods was also determined, which is given in grams and applies to all arthropods collected in a given month.

Variable environmental factors are important for the wintering of plants. To determine these, we used meteorological data from the Adam Mickiewicz University Ecological Station in Jeziory, which is located in close vicinity of the flower strip. Data from March to December 2021 and from January to July 2022 were analyzed. The average monthly air temperature in both seasons was similar. The warmest months were June and July. After the flower strip was sown, in the winter months, the lowest temperatures at the ground level were recorded in December and in January 2022. The lowest air temperatures were recorded in December 2021, at −13.7 °C, and in January, at −9.8 °C (Figure 1). The flower strip is located in the warmest region of Poland. Precipitation measurements was also carried out (using a Hellmann rain gauge). Monthly averages from the growing seasons are presented (Figure 2).

The results were statistically processed using the Statistica 12.0 program (StatSoft Polska, Kraków, Poland). Prior to the analysis, the Shapiro–Wilk test for normal distribution was performed. For normally distributed data, the results were statistically processed by one-way analysis of variance, and the means were compared using Fisher’s least significant difference (LSD) test at a significance level of α = 0.05.

3. Results

Generally, eight flowering plant species were recorded, most of them from the sown mixture. The first observation was performed in May. It was a period with a strong dominance of T. repens with white inflorescences, which accounted for 87.50% of all flowering plants at that time. Ch. leucanthemum with white-yellow inflorescences (6.67%), T. pratense with light purple inflorescences (5.00%) and L. corniculatus with yellow inflorescences (0.83%) also started to bloom (Table 1).
In June, T. repens still dominated, accounting for an average of 81.52% of all flowering plants in that month, with a small share of A. millefolium (7.26%), Ch. leucanthemum (6.60%), and T. pratense (2.97%). A single D. carota (0.99%) and E. vulgare (0.66%) were recorded. In July, a similar effect was noted. While T. repens (53.29%) still dominated, the blooming of Ch. leucanthemum ended and L. corniculatus (4.19%) began to bloom. The flower strip was very compact, and the share of flowering plants from the seed bank did not exceed 2.4%. The species diversity of flowering plants in the strip depended on the date of observation. Although the largest number of plants bloomed in June, the largest number of flowering species (on average 6.20) was observed a month later (Table 1 and Table 2).
The total number of arthropod specimens caught in July was more than twice as high as that caught in June, which was similarly reflected in the dry mass of insects: 0.360 g in July, 0.157 g in June. It should be emphasized that in July, a significantly higher number of flowering species was found than in June, which determines the greater diversity of inflorescences (Table 2).
In June, the majority (58.74%) were beneficial arthropods, with the predominance of species from the Hymenoptera order (51.91%), while T. repens dominated among flowering plants. In July, the opposite situation was observed—the majority (64.84%) were potentially harmful species, although this high percentage is mainly due to species belonging to the order Thysanoptera (85.98%). At the same time, a slow decline in flowering T. repens plants (53.29%) was recorded in July, and all other flowering species accounted for 46.71%, including D. carota (11.98%) and A. millefolium (17.37%). For two months, the strip was dominated by white flowering plant species (Table 1, Figure 3 and Figure 4).
In both dates of observation, a large diversity (belonging to different taxonomic orders) was observed for beneficial arthropods. The diversity of potentially harmful orders was lower, and representatives of only three orders were recorded in both dates (Figure 3 and Figure 4).
In June, arthropods were dominated by insect species from the order Hymenoptera, suborder Parasitica, which accounted for 29.15% of all specimens caught that month, while in July, specimens from the order Thysanoptera, belonging to the family Thripidae, dominated; they accounted for 55.75% of all arthropods caught in that month (Table 3).

4. Discussion

Restoring biodiversity in intensive agriculture lands represents a real conservation challenge. Research on the effectiveness of flower strips is very necessary. Ways must be found to use agricultural land productively, while preserving arthropod habitats and species diversity [40]. Kujawa et al. [42] indicate that annual flower strips located in the landscape of northwestern Poland are quickly becoming local refuges for arthropods, which can help control pests in neighboring crops. Previous studies have shown not only their effectiveness in reducing pests, but also the dependence of this effectiveness on the distance of the strips from the agricultural field [42,43,44] and its relation to the increase in the number and activity of pollinators [45]. However, we need to remember that one-year flower strips plowed in the next year are poor wintering habitats, because arthropods wintering in the ground will not survive plowing [29]. Research by Kowalska et al. [22] showed that when selecting the appropriate species composition of mixtures intended for the flower strips, both environmental conditions determining the success of their cultivation and the preferences of beneficial arthropods should be taken into account. Not all species’ seeds are able to survive the winter, even if it is not severe, and growing plants may not develop properly due to unfavorable hydrological conditions, which may result in a much lower share of flowering plants. In the year of observation, a very dry early spring was recorded. No rainfall was recorded at all in the research area. It was only at the end of March that accumulated rainfall began to increase across the country. Total precipitation since the beginning of 2022 was almost 20% lower compared to the long-term norm; spring was very dry (60% of the norm) and summer was dry (87% of the norm). Perhaps this aspect contributed to obtaining a small number of flowering plants for T. pratense, which has high soil and water requirements, or L. corniculatus (Table 1), although this one is more durable and tolerant, also to drought [46].
In studies by Buhk et al. [17], the abundance of bees clearly increased in areas with perennial flower strips. Strips of perennial flowers, however, must consist of plant species that flower for even several years, which requires careful planning at the stage of selecting the flower mixture and subsequent management of the strip, because the ability of plants to flower decreases over time [47,48]. It is very important to assess their durability for a given area, in specific environmental conditions. Schmied et al. [49] also point out that perennial flower strips may lose their attractiveness over the years. They call it the flower strip dilemma (FSD): on the one hand, the number of insects increases with flower strip age; on the other hand, perennial flower strips often become dominated by grass and are less attractive to several species, e.g., due to the color of the inflorescences, which acts as a visual attractant [50]. Staab et al. [51] showed that when the share of grass biomass exceeded 90%, it became harmful, adversely affecting the species diversity of flowers in the strip. In our own observations, the share of grasses in the second year from the establishment of the strip was still marginal, not posing a dominant threat to the flower strip (although due to the sporadic occurrence and lack of these units in research repetitions, they were assessed visually only). Creating flower strips composed of plants of different ages can be a means of compensating for the loss of species richness and abundance during their aging.
Ullrich’s research [52] showed that in a two-year-old and older flower strip, the diversity of arthropods was the highest, and the arthropods were more specialized. From the second year on, habitat and management factors have a significant impact on the vegetation, and the vegetation in turn strongly influences arthropods species composition. Many arthropod taxa hibernate in perennial flower strips. Ganser’s et al. [29] research showed that the age of perennial flower strips positively affected the wintering of spiders. The number of wintering pollinating flies and Staphylinidae beetles did not change significantly with the age of the strip. In a study by Balzan et al. [27], the most important Araneae predators were Thomisidae, which accounted for 70.2% of this group. In our study, Araneae accounted for less than 3% of all arthropods caught in both observation periods (Table 3).
In the present research study, in June, the percentage of beneficial arthropod specimens in the total number of insects was higher than in July (Figure 3), with a predominance of Hymenoptera (Figure 4), which were mainly represented by Parasitica and, to a lesser extent, Aculeata (Table 3). Parasitic wasps are involved in limiting the number of almost all groups of insects and some arachnids. They occur in all types of ecosystems [53]. They can be external or internal parasitoids. Adults usually feed on nectar and pollen, sometimes on honeydew or hemolymph taken from the body of the larvae [54]. The quality of the diet of adult parasitic wasps can affect their lifespan and fertility [55]. In June, pollen-producing and nectar-producing T. repens definitely dominated in the strip. The deep corolla of these flowers can make nectar inaccessible to parasitic Hymenoptera due to their typically short tongues, as can the narrow diameter of the corolla which can block the entry of an insect’s head [56]. The color plays a role as a visual attractant for insects [50]. Krzyżanowski [57] reports that dark green varieties with a high content of chlorophyll a and b and a low level of yellow-orange pigments (flavonols, carotenes, and xanthophylls) are much less accepted by winged forms of grain aphids, and Sobral et al. [58] found that herbivorous insects preferred to colonize plants with yellow flowers. In a study by Csanády et al. [59], the most preferred colors for hymenopterans were yellow, white, blue, purple, and the least commonly selected color was red; however, these preferences may change depending on the growing season and location. This was confirmed by our own observations—most Hymenoptera/Parasitica were noted during the flowering of white T. repens. It is worth noting that another important behavioral factor for arthropods is the smell of the flowers, which can attract insects from long distances [50].
Interesting results concerning annual and perennial flower strips located in areas adjacent to Poland (Gerseveral) were obtained by Bednar et al. [60]. They showed that strips of flowers with different life cycles are habitats for distinct communities of earthworms and soil microorganisms. Although soil type was the strongest predictor of bacterial and fungal community composition, the plant mixture influenced the microbiome complex in each soil type. It was shown that earthworm density and biomass remained unaffected or declined in annual flower strips but increased in perennial ones as compared to the field margins.
In this study, a higher number of plant species was not associated with more intensive flowering; quite the opposite. Most of the plants bloomed in June (Table 2). During whole observation period, T. repens dominated, which is very common in Poland, both in natural habitats as well as in cultivation [61]. T. repens has a white, yellowish, or pinkish corolla and is a very variable plant species with high adaptability to different habitats [62,63]. However, in this study, in the second year of establishment of the flower strip, T. repens dominated the flower strip too much, thus hindering the development of other species, including from the soil seed bank.
In July, plant species diversity in the flower strip was higher than in June, but the share of harmful arthropod specimens increased, represented mainly by Thysanoptera from the Thripidae family (Table 2 and Table 3, Figure 3 and Figure 4). In the study by Balzan et al. [27], members of the order Thysanoptera accounted for 13.2% of the total sample size, while the Hymenopteran groups Formicidae, Apiformes, and Parasitica accounted for 0.71, 1.8, and 18% of the total sample size, respectively. The behavior and reproduction of thrips can be affected by both pollen quantity and quality [64]. Research by Visschers et al. [65] showed that the preferences of adult thrips were correlated with the absolute content of trehalose and fructose and the total amount of pollen in Capsicum flowers. In addition, thrips had clear varietal preferences and even plant part preferences. Particularly noteworthy is E. vulgare, which was flowering in this period in the flower strip. Although its percentage share in the total number of flowering plants was small (Table 1), the size of whole plants and the number of individual flowers available to arthropods was very large and was very often visited by them. In addition, tall plants growing in the strips, such as E. vulgare, can reduce wind erosion (by slowing down the wind speed), retain rainwater and snowmelt in the place of their formation and limit rapid surface runoff. The plant strip acts as a biological filter, and increasing the diversity of the soil environment under plants is conducive to the growth of humus-forming processes, which is so important in sustainable agriculture [66], and improves the aesthetics of the landscape. Tall plants can also constitute a form of mechanical barrier stopping spores of pathogenic fungi carried by the wind and are often food for birds, which are farmers’ natural allies in biological protection. E. vulgare is a host plant for thrips, e.g., of the genus Megalurothrips [67]. Thrips can choose plants in the flower strip and accumulate on them instead of on cultivated plants. D. carota and A. millefolium began to bloom at the time when thrips were noted in higher numbers, so it should be verified whether, like E. vulgare, they are not plants that attract thrips.
Schütz et al. [68] report that annual flower strips usually have a smaller impact on diversity than perennial ones, favoring less stability in the provision of ecosystem services. However, they provide primarily summer-flowering plants, which may reduce their effectiveness in supporting insect diversity in the long term [69]. In the studies of Kowalska et al. [22], in a flower strip composed mainly of annual plants, there were definitely more flowering plants in spring (May) than in summer (July). In the experiment that is the subject of this manuscript, the most intense flowering month was June, with a marked dominance of T. repens (Table 1 and Table 2), and the end of July was associated with the end of flowering of plants in the strip. Taking the above into account, it seems most beneficial to create strips of perennial plants which will be supplemented with annual species, especially those flowering in spring. The combination of annual plants recommended by Kowalska et al. [22], i.e., Trifolium incarnatum and Phacelia tanacetifolia with perennials such as T. repens, A. millefolium, and E. vulgare, can ensure abundant flowering and thus flower resources for several beneficial organisms living in the flower strip from May to the end of July.
However, it should be remembered that the colonization of different groups of arthropods is limited both by local vegetation conditions and by the presence of, distance from, and association with source populations, and restoring arthropod diversity is a relatively slow process [40]. It is, however, noteworthy due to the enormous importance of sustainable agriculture in the modern world. Taking the above into account, flower strips can be an instrument supporting the biological conservation method, contributing to the control of pests in several agricultural crops [26,28,43]. They can increase not only the number of beneficial arthropods, but also the density of numerous invertebrates [16]. Flower strips can therefore support the ecological intensification of agriculture by providing ecosystem services, but a comprehensive assessment of their effectiveness is needed [28,42,70]. The presented study is preliminary; it is a typical case study conducted in a national park zone in western Poland, illustrating floristic diversity (durability of the flower strip without additional treatments, such as sowing seeds in the second year of use) and the diversity of arthropods directly in the flower strip. However, further studies are planned to cover arthropod diversification not only in the flower strip, but also in croplands directly adjacent to it.

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