CHANGES IN PLANT COMMUNITIES OF A DRAINAGE DITCH OVER THE LAST 30 YEARS

Hanna Gołdyn
Institute for Agricultural and Forest Environment, Polish Academy of Sciences, Poznań, Poland

ABSTRACT

The goal of this study was to analyse changes in plant communities of the Wyskoć Ditch (near Poznań in western Poland) over the last 30 years. Vegetation diversity increased, but the red-listed association Sagittario-Sparganietum emersi (dominant in the 1970s) was replaced by plant associations characteristic of more fertile waters. In 1979, in the same section of the watercourse, only 3 plant associations were found, compared to 12 in 2007. In contrast, the area covered by vegetation decreased nearly twofold. These transformations are due to environmental stress, linked with changes in chemical conditions caused mostly by intensive farming, but also with changes in physical conditions, resulting primarily from direct human interference (ditch restoration works) as well as the growth of aquatic plants and trees and shrubs on the banks. Shading of long sections of the watercourse by trees and shrubs limits the growth of plants in water and has led to a substantial reduction of the area covered by them. Results of this study indicate that under moderate human pressure, some red-listed aquatic vegetation may disappear from watercourses, but their vegetation diversity may increase anyway and the ecosystems still greatly enhance the biodiversity of agricultural landscape.

Key words: changes, diversity, human impact, physical conditions, plant communities.

INTRODUCTION

In rural areas, often the only habitats supporting wetland vegetation are regulated watercourses, including drainage ditches. Such plant communities are threatened with extinction because of the increasing human pressure. Their conservation and protection are extremely important for preservation of biodiversity in the landscape dominated by arable fields.

Distribution of plant communities in running waters depends on physical and chemical conditions, which to a large extent are related to the given region and landscape.

The most important factors include, e.g., water flow rate [1,2,7,15,18,24,33], access of light and shading [26,33], type of substrate, and of riparian vegetation [7,13,14,21], as well as concentrations of nutrients in water and in the substrate [2,25,27,29]. These factors can be modified in the course of plant succession [1,15], but also directly by human impact and management of the watercourse [5,21,26], or indirectly by human activity in the catchment [13,20,29].

The goal of this study was to analyse changes in plant communities of the Wyskoć Ditch over the last 30 years. This ditch collects water from numerous minor drainage ditches from farmland in the General Chłapowski Landscape Park 40 km south from Poznań city in western Poland.

Research on transformations of aquatic and wetland vegetation over a long timespan is relatively rarely reported in the literature because usually the current state cannot be compared with any historical data. Fortunately, the vegetation of the Wyskoć Ditch was documented in the 1970s by the author [9]. Also data on the distribution of vegetation and the area covered by individual types of plant communities are available. This enabled an assessment of changes in plant communities. An attempt was made to determine which of them were due to human activity within the catchment area and its direct disturbance of the habitat by ditch restoration works.

The studies were supported by the Polish Committee for Scientific Research (grant no 2 P04G 05729, "Natural and anthropogenic transformation of water and marsh vegetation in General Chłapowski Landscape Park").

STUDY AREA, MATERIAL AND METHODS

Plant species and communities as well as their distribution were studied in the Wyskoć Ditch and along its banks. This watercourse is located within the General Chłapowski Landscape Park, where cultivated fields constitute 82% of the area (Fig. 1). The ditch flows out of Lake Zbęchy, collects water from fields and meadows, and carries it to the extensive system of channels of the Obra River. The Wyskoć Ditch and the smaller watercourses that are its tributaries jointly constitute a drainage system. The study comprised the 1025-metre-long section of the ditch, which was surrounded by meadows and cultivated fields.

Generally in both investigated periods the water depth in this section of watercourse was similar – the highest in spring (about 100 cm) and lowest in summer (5–40 cm in 1976–1979 and 10–50 cm in 2007). Mean width of the ditch was 4.5 m. In summer the water flowed mainly in the current in middle part of width of the ditch, whereas stagnated near the riversides. However, the areas with stagnating water were significantly larger in 2007 than in 1976–1979.

Vegetation of the Wyskoć Ditch was first studied from 1976 to 1979 [9], so investigations were repeated in the same fragment of the ditch in 2007. This allowed analysing the transformation of vegetation during the last 30 years.

Every time, the plant communities were defined, and the areas occupied by their patches were measured. The vegetation of both periods was valorised by estimation of the number of communities and the area occupied by each of them. The communities differed in genesis, response to human impact, threat category, and frequency in the Wielkopolska region. Names of plant communities, as well as the diagnosis of their origin, their response to human pressure, threat category, and frequency follow mainly Brzeg and Wojterska [6].

Additionally, mean trophic rank (MTR) scores of characteristic species of plant communities present in the ditch were calculated on the basis of a method developed by Szoszkiewicz et al. [30] for biological monitoring of inland rivers. MTR scores are means of species trophic rank (STR) scores, each indicating the mean trophic state in which the given species occurs. The scores vary from 1 in strongly eutrophic to 10 in oligotrophic habitats.

RESULTS

Changes of the chemical variables of water quality and the physical conditions in the watercourse
Analyses of the chemical composition of water of the Wyskoć Ditch, carried out in 1973–1991, showed a gradual increase in its trophic state [4]. The decrease in water quality is mostly due to intensive farming (particularly since the 1970s) and to development of the food processing industry in the 1990s. A great threat to water quality in the period between this and the previous study, was posed by growing nitrate concentrations in groundwater of arable fields, which in the 1970s was on average 12 mg dm^-3 N-NO₃ ^-, and reached 20–40 mg dm^-3 N-NO₃^- in the 1980s, but the maximum values were then close to 100 mg dm^-3 N-NO₃^-. Large areal nitrogen loads from farmland are the reason why some parts of the catchment of the Wyskoć Ditch are classified as areas that are particularly vulnerable to pollution. A list of such areas was complied because of implementation of EU regulations in Poland, especially the Council Directive 91/676/EEC concerning the protection of waters against pollution caused by nitrates.

Research conducted by Bartoszewicz [4] revealed a gradual deterioration of water quality evaluated on the basis of concentrations of N-NO₃^-, N-NH₄⁺, P-PO₄^- and K in 1973–1991. The strongest increase was observed in the concentration of phosphates an ammonium nitrogen. In 1973–1991, the mean concentration of N-NH₄⁺ was 1.7 mg dm^-3, while that of P-PO₄^- was 0.248 mg dm^-3 . An analysis conducted in August 2007 showed that water quality deteriorated even further. The values were then much higher: 2.52 mg dm^-3 and 0.72 mg dm^-3, respectively (unpubl. data). On the basis of published data [35] it can be supposed that water quality declined most strongly in 1986–1990. This could be associated with the construction of a weir at the outlet of the Wyskoć Ditch from Lake Zbęchy, in the 1980s. In that period, waters of both the lake and watercourse contained large amounts of nutrients, especially of ammonium ions (2.78 mg dm^-3  N-NH₄⁺  in that lake and as much as 3. 28 mg dm^-3 N-NH₄⁺  in the ditch). At the same time, algal blooms appeared in the lake. As reported by Gołdyn [12] from Lake Strykowskie, even a small rise in lake water table causes an increase in concentrations of nitrogen and phosphorus, phytoplankton abundance, a decrease in water transparency, and consequently a decline of submerged vegetation. This is consistent with observations in Lake Zbęchy, where submerged vegetation nearly completely disappeared [10].

Drainage and restoration works carried out in the Wyskoć Ditch and its vicinity (drainage of the meadows) were reflected also in chemical water quality variables, but additionally changed the physical factors, such as water flow rate, depth, and type of substrate.

In 1976–1979 about half of the length of the investigated ditch section (over 600 m) was shaded by trees and shrubs growing on the banks, while the other half was not shaded. Between 1979 and 2007, the length of the shaded section increased. Shrub vegetation spread out abundantly on the banks, and the trees that grew there already 30 years ago, achieved a considerable height. In the meantime, dredging deepened the ditch bed and the banks became steeper.

Changes in plant communities
During the first season of investigation only patches of 3 plant associations were found in the studied section of the ditch (Table 1).

Two of them, Sagittario-Sparganietum and Nupharo-Nymphaeetum albae, are natural communities that decline if exposed to human pressure (i.e. are perdochoric), red-listed in Wielkopolska, while Elodeetum canadensis is an alien-dominated community (i.e. xenospontaneous). In all unshaded parts of the watercourse, patches of Elodeetum canadensis were found on silty sites, with vegetation coverage of 20–100%, and Sagittario-Sparganietum, covering sandy or sandy-silty parts of the bed, characterized by lower vegetation coverage (15–70%). The largest area was occupied by Sagittario-Sparganietum (Table 2).

Small portions of the bottom were covered by patches of Nupharo-Nymphaeetum albae. Each association was composed of 2–7 plant species. In total, 11 plant species were recorded. Patches of vegetation, with varying coverage, occupied 37% of the length of the studied section of the ditch. Their total area reached 1700 m². The mean vegetation coverage in the whole studied section of the ditch was 15% (Table 3). The MTR score of characteristic species of these associations indicated a relatively high trophic state, as it amounted to 4.3. The plant associations that are red-listed in the Wielkopolska (Sagittario-Sparganietum and Nupharo-Nymphaeetum albae) covered a total area of 1100 m² (65% of the area covered by vegetation).

In 2007, in this section of the ditch, 12 species of plants were recorded (Table 1). During the last 30 years, the perdochoric association Sagittario-Sparganietum disappeared, Nupharo-Nymphaeetum albae was rare there already in 1976–1979 (Table 2). In contrast, patches of the xenospontaneous association Elodeetum canadensis still thrive in the ditch, and cover the largest total area. Most of the new associations are tolerant to human pressure (i.e. they are auxochoric), while the response of Lemno-Hydrocharitetum morsus-ranae to human disturbance has not been determined yet. Apart from Nupharo-Nymphaeetum albae, whose patches were found in the ditch also in the first study period, four other associations red-listed in Wielkopolska appeared. In 2007, their phytocoenoses covered in total 21% of the vegetated part of the ditch. In individual patches, 1–8 plant species were recorded, and their total number in the studied section of the ditch reached 28 (Table 3).

Vegetation covered a much smaller part of the bed than in the 1970s (25%), but its patches were more compact. For this reason, the mean vegetation coverage in the whole studied section of the ditch increased to 22%. The MTR score of characteristic species for these associations decreased to 3.7, which attests to a higher trophic state than in 1976–1979. Characteristic species of the association Sagittario-Sparganietum (Sparganium emersum and Sagittaria sagittifolia) are assigned higher STR scores than characteristic species of the associations currently found in the ditch (Table 4).

DISCUSSION

Aquatic plants and their communities are indicators of the trophic state of rivers [8,27,28,30]. Bioindicative properties of various species enable an assessment of the environmental condition of rivers. To reach this aim the distribution and the share of these species in plant cover have to be recognized. Water quality assessment by means of evaluation of the environmental condition of the aquatic ecosystem is in accordance to the European Water Framework Directive.

Research on variation of plant communities in British and Irish rivers [27] proved that also associations of vegetation  are bioindicators, because their types are strongly correlated with trophic state. These investigations also provided data for distinguishing the communities, which differed in their trophic status.

The detected transformation of plant communities in the Wyskoć Ditch during the last 30 years  indicated water quality deterioration and an increase in trophic state. Most of the communities found there at present are characteristic for rivers which are rich in nutrients. However, the transformations of vegetation are not only a response to an increase in concentrations of nutrients in water, but are also due to a complex influence of chemical and physical conditions that changed in the period between this and the previous study. The changes in living conditions of aquatic plant communities, with respect to both chemical and physical factors, were caused to a large extent by human activity in the watercourse itself or its catchment.

The most important transformations of the vegetation of the Wyskoć Ditch include: (1) an increase in vegetation diversity; (2) disappearance of patches of Sagittario-Sparganietum and appearance of 10 new plant associations; and (3) a reduction of the total area covered by vegetation. The increase in vegetation diversity, as well as the increase in species richness in the watercourse, result from human activity. Such a response of vegetation  to moderate human interference on a local scale has been reported in the literature for a long time [32].

Nevertheless, the disappearance of the association Sagittario-Sparganietum has lead to a decrease in the total area covered by red-listed plant communities in Wielkopolska. It is generally quite common in Europe [5,19,21,23] and frequent in Polish rivers, but in the Wielkopolska region it is relatively rare [6]. It requires sandy or light muddy substrates, and slow-flowing, slightly eutrophic water [16,22]. It is characterized by a high potential for regeneration on disturbed sites, and its growth is stimulated by mowing [3,21,23], but it is negatively affected by high concentrations of ammonium nitrogen in the substrate [25]. However, the most important is that Sparganium emersum is very susceptible to shading, and even a small increase in the amount of shade leads to a loss of this species in rivers [17].

Physical properties of a site and their modifications are important factors that affect aquatic vegetation, and can even mask the influence of major point sources of pollution on aquatic plant communities. The dominant physical factors include shading by riparian vegetation [7,26]. Shading by trees growing on the south side of the watercourse controls mostly submerged aquatic plants.

In the classification based on the genesis and response to human impact [6] of plant communities found in Wielkopolska, the association Sagittario-Sparganietum was assigned to a group of natural communities that decline under the influence of human disturbance (i.e. perdochoric communities). This group includes also Nupharo-Nymphaeetum albae, whose patches are now smaller than 30 years ago. The new communities are mostly well-developed, or even expanding under the stress caused by human activity (i.e. auxochoric communities). Elodeetum canadensis, which dominated in 2007, is termed xenospontaneous, because its characteristic species is alien. Elodea canadensis, currently widespread in Europe, was first recorded in the early 19^th century, on the British Isles [31]. It started to be reported from Poland in the mid 19^th century [34]. This species is characterized by a great tolerance and a wide ecological scale. It is found in various types of freshwater ecosystems, ranging from mesotrophic to eutrophic. It can accumulate large amounts of phosphorus and is adapted to its seasonal variation. It is very expansive, because it is more effective in assimilation of nutrients, phosphorus in particular, than native species [31].

The association Elodeetum canadensis is common in Poland and generally in Europe. The dominant species, very much like Sparganium emersum, has a high potential for regeneration, and mowing stimulates its growth [3,16,19,21,22,23]. Thanks to its expansiveness and wide ecological scale, patches of this association are still, like 30 years ago, found in the Wyskoć Ditch, and cover large parts of the bottom available to plants.

Besides the shading and eutrophication of waters, the transformations of vegetation in the Wyskoć Ditch reflect changes in water flow rate. Water management measures undertaken in the 1980s consisted in dredging of the ditch and removing of the plants that formed dense patches within its bed. These actions caused a lowering of both the bed and water table within the ditch. As a consequence, drainage of surrounding areas was observed, which caused intensive leaching of nutrients. Overdrying and changes in depth of the ditch bed caused a decrease in water flow rate and an increased sedimentation of organic matter drifting from the catchment area and produced by autochthonous water plants. Most of the plant associations recorded in 2007 are typical for eutrophic waters with muddy substrates.

The decrease in water flow rate was additionally affected by the greater vegetation coverage, which also could hamper the growth of Sagittario-Sparganietum. Such an influence of submerged vegetation on water velocity is reported by Madsen et al. [15]. The growth of aquatic plants changes local conditions of flow, which facilitates sedimentation or erosion, or has a favourable influence on other species instead of those that occupied this site previously [1].

Similar changes were observed at the same time in the vegetation of another fragment of the ditch, which was surrounded by peat meadows [11], and in Lake Zbęchy [10]. Drainage of meadows caused accelerated peat mineralization and intensive leaching of nutrients. A consequence was the increase in trophic state of water of the ditch and of the lake.

The decrease of total area occupied by vegetation by nearly a half is due to a complex of the factors mentioned above, associated with shading, changes in chemical composition of water, water flow rate, and type of substrate. For example, shading of larger fragments of the watercourse than 30 years ago, was most important for patches of Sagittario-Sparganietum, very sensitive to shading. However, they died out in the whole ditch, also in its unshaded fragments, which indicates that also other factors were important for this process.

Characteristic species of this association are assigned higher STR scores than characteristic species of the associations currently found in the ditch. The decrease in the MTR score of characteristic species of associations shows that the studied watercourse is increasingly eutrophic.

CONCLUSIONS

1. The increase in floristic and vegetation diversity, and the decrease in the MTR score of characteristic species of plant associations indicate that the Wyskoć Ditch was progressively eutrophicated in the last 30 years as a result of human activity in its catchment, dominated by farmland.

2. Parallel to changes in vegetation, also chemical indicators of water quality confirmed its slight deterioration.

3. The increase in trophic state is additionally evidenced by changes in dominance of plant associations: the disappearance of a plant association that is red-listed in the Wielkopolska region and declines under the influence of human activity, and the appearance of associations tolerant to it, associated with eutrophic waters. However, two vulnerable associations still exist in the watercourse, although their patches are small.

4. The observed transformations in vegetation diversity are partly associated with changes in physical conditions, resulting from ditch restoration works.

5. The nearly twofold decrease in the area covered by vegetation was probably mostly due to the greater shading of water by trees and shrubs growing on the banks of the ditch.

6. Results indicate that under moderate human pressure, some red-listed aquatic vegetation disappear from watercourses, but the whole diversity of vegetation increases anyway and the ecosystems still greatly enhance the biodiversity of agricultural landscape.

REFERENCES

Accepted for print: 26.10.2009

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