Vegetation response to forest ditch reconstruction: Promoting a potential habitat for insect-pollinated plant species?

During previous centuries, the human impact on nature has been steadily increasing. The result is a high degree of intervention in previously undisturbed ecosystems, bearing with it significant alterations of natural processes (Jackson & Jackson, 2000; Marsh, 2003; Goudie, 2018). It is often impossible to revert these ecosystems back to their near-natural state, and it is increasingly important to safeguard their remaining natural features and to ensure that they support the ecosystem processes in the best possible way (Jackson et al., 2009). For that, a profound understanding of the functioning of these novel ecosystems is of crucial importance.

In countries with a developed forest industry, linear forest infrastructure objects, such as forest roads and ditches constructed primarily to increase forest productivity and improve accessibility, are among the most widespread examples of human-altered ecosystems (Avon et al., 2013; Lõhmus et al., 2015), and drainage networks are common linear elements in forest landscapes all over Europe (Paavilainen & Päivänen, 1995; Rydin & Jeglum, 2006; Päivänen & Hånell, 2012). These anthropogenically created and maintained features may cause significant changes in forest ecosystem functions, for example, by disrupting population connectivity, acting as a barrier to dispersal, altering ecological community composition and decreasing community diversity at landscape level, and acting as pathways for the spread of invasive alien species (Smart et al., 2006; Flory & Clay, 2009).

At the same time, due to more varied micro-site conditions, ditches and ditch edges support higher species diversity, providing habitats for plants with different ecological requirements (Zielińska, 2007; Zielińska et al., 2017; Karim & Mallik, 2008). They are of a special importance for hygrophilous bryophytes and may help to preserve rare species in a managed forest landscape (Staniaszek-Kik et al., 2016). Zielińska et al. (2017) highlight the importance of forest ditch edges in increasing the habitat diversification for vascular plants and bryophytes at meso- and microscales. In landscapes heavily influenced by human activity ditch networks may enhance the functional connectivity of populations (Favre-Bac et al., 2016).

To address the decline of pollinator populations, measures resulting in the establishment of flowering plant communities with the primary aim of improving the nutrition basis for larvae and adult insects have been implemented in Europe and North America (Wratten et al., 2012). Ditch slopes and verges may positively contribute to this objective, as they provide habitats for a variety of flowering vascular plants (Rasran & Vogt, 2018), in this regard being more suitable for pollinator communities (especially bees, flies and butterflies) than closed forests and woodlands, moreover, linear forest structures are facilitating the dispersal of pollinators across the landscape (Hanula et al., 2016; Phillips et al., 2020). Thus, they may serve as ecological corridors and stepping-stones for pollinator communities: a significant aspect in the light of the increasing importance of pollination as essential ecosystem service with a declining tendency due to the intensification of agriculture practices, expansion of parasites, more frequent biological invasions, habitat loss and fragmentation (Liss et al., 2013; Porto et al., 2020). At the same time, linear infrastructure elements may promote the dispersal of invasive alien species, potentially acting as competitors for pollination services (Hanula et al., 2016). Knowledge about the specific effects that the creation and management of linear forest infrastructure elements have on vascular species’ composition and its role in providing habitats and as food base for pollinating insects is still scarce.

It is hypothesized that ditch reconstruction could increase the plant species diversity on forest ditch edges, in turn affecting the abundance of flowering plant species. The aim of our study was to monitor changes in plant species composition on drainage forest ditches and ditch edges, with emphasis on quantifying the amount of insect-pollinated plants, potentially beneficial to pollinator communities. This paper reflects the results of the analysis of flora along managed forest drainage ditches 3–5 years after reconstruction works, when the plant communities have re-established.

In total, 254 plant taxa were recorded along the studied ditches: 34 bryophytes and 220 vascular plants – some individuals could be identified only down to genera. The species community represented the local pool of common species. The most common vascular plant species were Athyrium filix-femina (L.) Roth, Calamagrostis canescens (Weber) Roth, Cirsium oleraceum (L.) Scop., Dryopteris carthusiana (Vill.) H.P. Fuchs, Lysimachia vulgaris L., Rubus idaeus L. and Urtica dioica L. and the most common bryophytes were Brachythecium rutabulum (Hedw.) Schimp., Plagiomnium ellipticum (Brid.) T.J. Kop., Calliergonella cuspidata (Hedw.) Loeske (Appendix 1). For the second survey in 2020, 44 new taxa had emerged and 43 taxa had disappeared (Appendix 1). In the studied plots one protected orchid species Platanthera bifolia (L.) Rich. (Cabinet Regulation No. 396, 2000) was found once in the studied time period, but this species is rather common along forest edges. The majority of the species were forbs and graminoids. More than half of all identified species were insect-pollinated flowering plant species (135 species) and made up around 60% of all recorded vascular plant species (Appendix 1).

The results showed that during the 4-year-period the richness of insect-pollinated plant species and the richness of total species had significantly increased, from 22 to 25 species and from 38 to 43 species per studied plot, respectively (Table 1). According to glmer models, the object type was the main predictor of the insect-pollinated plant species richness and total species richness as indicated by the highest χ² values (Table 1). The highest insect-pollinated flower species richness was associated with managed ditches, and it was significantly higher than along unmanaged ditches (Table 1). The highest total species richness was identified along both reconstructed ditches, and the differences were significant for the strongest contrasts (compared to unmanaged ditch C) (Table 1). This confirms previously drawn conclusions that ditches as anthropogenic structures impact the plant species biodiversity in a managed forest landscape by increasing floristic richness (Zielińska et al., 2013; Zielińska et al., 2017). They do not, however, support endangered plant species. The explained factors for higher species richness were associated with higher Ellenberg light and nitrogen values (Table 1) that generally increase after the forest infrastructure reconstruction (Godefroid & Koedam, 2004). Our results showed that Ellenberg light values were significantly higher on the studied managed ditches (Appendix 2), indicating that management was improving the light conditions, while Ellenberg nitrogen values showed significantly strongest contrasts only between both managed ditches (Appendix 2).

According to our results, especially reconstructed diches provide important habitats for insect-pollinated plant species and thus could be favorable for groups of pollinators. Our results did not show the Ellenberg nutrient value as a significant factor for higher insect-pollinated species richness. According to the literature, it is possible that nitrogen enrichment benefits the growth of grasses more than that of forbs (Bråthen et al., 2021). While light availability could be a limiting factor for flowering plant species, our results did not show Ellenberg light values as a significant factor for higher insect-pollinator species richness as well. We hypothesize that pollinator species richness could be affected also by other factors, such as microtopography, features of soil and forest type and management in the studied area (Zielińska et al., 2017), which were not analyzed in this study.

The DCA ordination of the studied plots indicated an influence of management on the species composition. The higher number of insect-pollinated plants were more strongly associated with plots on reconstructed ditches (located on the left part of the DCA ordination) (Figure 2). Richness of insect-pollinated flowers were related to the first gradient (r=0.672), which accounted for 35% of the total variation (eigenvalue=0.348). According to the results, the species composition of managed forest ditches was associated with higher Ellenberg light values (the Pearson’s correlation coefficient between Ellenberg light and the first axis was 0.672 and second axis – 0.478, respectively) (Figure 2). Our results indicate that light could be related to higher richness of flowering plants.

The studied old ditches were characterized by lower light availability (Figure 2). The second gradient that accounted for 17% of the total variation (eigenvalue=0.165) and was explained by moisture, was most likely related to local specific conditions (Figure 2). The results confirm earlier findings that ditches could provide higher variability of microsite conditions resulting in high species richness (Bergès et al., 2013; Zielińska et al., 2013; Staniaszek-Kik et al., 2016). Several previous studies, e.g. Smith et al. (2007) and Baltzinger et al. (2011), mention especially roadsides with ditches as one of the most species-rich habitats in managed forests. The preliminary results obtained after the first survey in 2016 demonstrated that the species composition and species richness significantly differed between managed and unmanaged ditches, with Ellenberg nitrogen values as the main predictor of the differences (Matisone et al., 2018). Four years later, the differences were still significant, but the importance of nitrogen had decreased. After the second survey, light was the main environmental factor impacting the species composition.

Figure 2.

The DCA ordination indicated also successional changes in species communities over a 4-year period (Figure 2). The changes in species composition were specific to the studied plot and most explicit according to the first gradient. According to the main gradient, successional changes on the unmanaged ditches were small as indicated by the relative length of successional vectors. The changes in species communities could be related to light conditions, showing that successional changes along reconstructed ditches were expressed as an increase in species richness, including insect-pollinated species richness and diversity. Successional changes on the unmanaged ditches were less pronounced, except for some individual plots where the increase in species richness was high (Figure 2). Such changes could be explained by management of forest stands along the studied old ditches promoting better light availability and resulting in higher species richness in the second assessment.

Our research confirms that intensive forest management (ditch reconstruction) promotes higher plant species richness along ditch edges in comparison with unmanaged forest ditches. Ditch reconstruction changes species composition, promoting more light and nitrogen demanding species, including flowering plants important for pollinators, thus providing nutrition sources for pollinator communities. Anthropogenically created or altered ecosystems should be evaluated in a complex way, considering both the ecosystem services and disservices they deliver.