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
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
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
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
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.
The study was conducted in the forested catchment of Zalve stream (Figure 1), located in the central part of Latvia (hemi-boreal vegetation zone) and actively managed by JSC (Latvia’s State Forests). The catchment is dominated by conifer stands, with an admixture of deciduous trees, mainly on mesotrophic mineral soils. The dominating tree species are
The location of the research area.
In 2016, vegetation monitoring plots were established along four forest ditch edges with a different management history. Plant species were inventoried along two recently reconstructed forest ditches (reconstruction year 2015) and two unmanaged forest ditches (left unattended for at least 20 years). At each site, a vegetation survey was performed along a 1-km-long section of the ditch, with ten 3 m × 10 m plots positioned with the long side perpendicular to the ditch on the left and right sides of the ditch, 100 m plots were located immediately adjacent to the ditch (Matisone
Ellenberg indicator values (nitrogen, moisture, light, soil reaction) were used to describe the environmental conditions of each studied plot (Simmel
Generalized linear mixed-effects models were used to assess the differences between survey years, the effects of the object type (managed or unmanaged) and of Ellenberg variables on species richness. The residuals were adjusted according to the Poisson distribution (log link function). The predictors were checked for co-linearity using the variance inflation factor; variables showing values >5 were discarded.
The plant species composition was analyzed using the detrended correspondence analysis (DCA) based on species cover data. The relationship between two canonical axes and species community characteristics (total species richness, richness of insect-pollinated flowers, species diversity index (H’)) and Ellenberg variables were calculated with Pearson’s correlation analysis. Canonical axes were rescaled and rare species were downweighed. Successional vectors were drawn to assess changes in plant species communities between the studied ditches according to the survey year. Data analysis was conducted in
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
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 χ
Strength (
Richness of species | Insect-pollinated plant species | ||||||||
---|---|---|---|---|---|---|---|---|---|
Effects | Variable | Chi-Square (χ2) | p-value | Mean | Group | Chi-Square (χ2) | p-value | Mean | Group |
Year | 2016 | 16.77 | <0.001 | 38 ± 7.5 | a | 14.35 | <0.001 | 22 ± 6.2 | a |
2020 | 43 ± 8.2 | b | 25 ± 5.3 | b | |||||
Object | Reconstructed (A) | 31.77 | <0.001 | 47 ± 7.7 | a | 41.07 | <0.001 | 27 ± 5.1 | a |
Reconstructed (B) | 43 ± 6.9 | ab | 27 ± 5.4 | a | |||||
Old (C) | 34 ± 6.8 | c | 19 ± 5 | b | |||||
Old (D) | 39 ± 6.2 | b | 22 ± 4.1 | b | |||||
Light | 3.81 | 0.05 | 0.17 | 0.68 | |||||
Nitrogen | 4.8 | 0.03 | 1.09 | 0.3 |
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
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
DCA ordination of studied plots in survey years 2016 and 2020. The first two canonical axes are shown. (A) shows plot ordination according to management intensity. The vectors show correlation between calculated Ellenberg indicator values (light, soil reaction, moisture, nitrogen) and between species communities metrics. Abbreviations: Spe_rich – species richness, Ins_pol_rich – insect-pollinated species richness. (B) shows species ordination with successional changes on studied plots. The successional changes in the species composition among the observations in 2016 and 2020 are indicated with black vectors.
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.