All previous studies addressing the effect of wetland mowing on the fauna were restricted to one or a few years (e.g. Graveland 1999; Schmidt et al. 2005). Although such studies are helpful in assessing the negative effects of mowing mediated by the reduction of favourable surfaces within that short time-frame, they are insufficient for revealing the long-term effects on vegetation structure and communities, and their consequences on the fauna. In addition, our long-term population trends showed large within-site oscillations over the years (up to threefold in some species) despite little long-term evolution. Therefore, results from studies conducted over short time periods could be confounded by only optimal habitats being occupied during years of low density, while less optimal habitats could also be chosen when density is higher for reasons unrelated to management (e.g. climate, mortality during migration, food availability in wintering grounds). To fill this gap, we measured the effect of mowing on habitat suitability for the five most abundant breeding birds in wetland reserves in Switzerland over a period of 30 years.
We found that mowing efficiently reduced shrub development by cutting growing saplings. It is important to note, however, that cutting stems does not totally suppress invasion by shrubs. Alnus, Frangula and Salix, which are the most common woody species that settle in wet meadows, are able to regrow when cut. Furthermore, mowing could even favour the establishment of dispersing seeds by decreasing competition for light with herbaceous species, especially at the borders of forest where propagules are numerous. Mowing therefore restrains shrubs growth, but does not decrease the number of individual woody plants. Thus, shrub cover could quickly increase if mowing is abandoned.
Another possible drawback of mowing is the homogenization of plant communities at the landscape level. Mowing can sometimes increase local diversity, for instance, through the opening of vegetation that leaves some space for pioneer species, or through the creation of ruts by the mowing machine. However, applying the same constraint to a large area can also buffer other, more localised, perturbations, thus decreasing large-scale species and landscape diversity (Deák et al. 2015).
In the Recommendations for management section, we discuss additional management measures that could be implemented to overcome these limitations.
Long-term population trends
Our long-term trends analysis showed no consistent change among species or location. This result confirms that rotational mowing can be used without long-term harm to breeding birds. This result is likely dependant on mowing plots being kept small enough (not more than a few hectares) so as to leave enough favourable habitat each year. Mowing larger surfaces at once would likely cause more casualties in birds and even more so in taxa with more limited dispersal abilities (Schmidt et al. 2005). In addition, freshly cut plots adjacent to ones more suitable for breeding can serve as foraging grounds for birds breeding in the surroundings, thus potentially increasing density there. The absence of overall detectable demographic change when mowing started suggests that birds redistributed in suitable nesting grounds.
Despite no consistent population decrease, two out of the four species present in Cheyres decreased slightly at this location. The two species with decreasing populations (water rail and reed warbler) mostly occupy the wettest portion of the landscape. These results could therefore be explained by a succession towards drier communities despite management, as shown by Güsewell and Le Nédic (2004).
In order to control for factors independent of management, we assessed population trends in the rest of Switzerland. The reed bunting and the water rail were decreasing at national level, while neither of them showed a consistent decrease in the Grande Cariçaie. This suggests that management in the Grande Cariçaie reserves helped to maintain the populations stable despite national decline. The reed warbler was stable in Switzerland and did not consistently increase or decrease in the Grande Cariçaie, indicating that management there is sufficient to sustain stable populations of that species. The only species that was strongly increasing in Switzerland and stable in the Grande Cariçaie was Savi’s warbler. This species started breeding in Switzerland in 1956 and quickly expanded thereafter. It first colonized the Grande Cariçaie, and started expanding to other parts of Switzerland later on (Aebischer and Antoniazza 1995). For this reason, the observed strong increase likely results from ongoing range expansion in Switzerland, while the Grande Cariçaie population has likely reached its carrying capacity already.
Overall, these results show that rotational mowing has no long-term detrimental effects on the demography of the most abundant species at our location.
Optimal mowing regime
In addition to assessing the long-term effects of mowing on breeding birds, our study was aimed at determining the optimal mowing regime for birds. We showed that plots of age 1 are almost systematically avoided, which is consistent with previous results (Graveland 1999; Poulin and Lefebvre 2002). This is most likely explained by the short growing vegetation offering very few breeding opportunities, as most of the nests in age 1 plots were located in restricted patches that could not be mown due to their proximity with water or an obstacle.
Plots of age 2 were also significantly less favourable than older ones for all species except the water rail. This is consistent with the common expectation that more complex structures are more beneficial because they offer more opportunities for breeding, and it shows that vegetation structure takes more than one full growing season to reach an optimal level for birds. The water rail lives in the wettest part of the marshes (Brambilla and Rubolini 2004), where reed is most abundant. Vegetation structure there is different from upstream meadows in that vegetation is more dense, so that reed size and structure could become favourable sooner than in the rest of the marshes. In addition, this species feeds mostly on aquatic invertebrates and thus depends less on high reed stem density than the other species.
Plots of age 3 and older held the highest bird density. Vegetation structure evolution seems to slow after 3 years, with older, dead, decaying stems being replaced by new ones. Interestingly, bird density was systematically lower in unmown controls than year 3 and older plots, although this trend was significant in none of the species. The most likely explanation is that habitat quality quickly improves after mowing, and then plateaus. Only after several years or decades will the habitat shift towards drier and woody communities, thus preventing human life-scaled studies to detect a significant effect.
The fact that bird density is highest in plots of age 3 and 4 does not mean that mowing every 3–4 years is optimal. Indeed, delaying a cut to prevent going through age 1, which harbours very low densities, can be profitable on the long term. Consistently, we observed that predicted average densities were highest in our “every 6 years” scenario, and average density would likely keep on increasing thereafter. Bird density might, however, not rise indefinitely and birds might distribute more evenly if more favourable habitats become available, as was the case before mowing started.
Mowing regimes reported from other studies are often much more frequent than what our results suggest would be optimal (e.g. Graveland 1999; Poulin and Lefebvre 2002; Vadász et al. 2008). One factor that could explain these differences could be higher ecosystem productivity (and therefore vegetation regrowth rate) at other locations, causing the habitat to reach an optimal state more quickly after a cut. However, productivity is often thought to follow a latitudinal gradient (Yu et al. 2013; Gillman et al. 2015), and the Grande Cariçaie reserves are located in central Europe, at an intermediate latitude. Therefore, differences in productivity alone are unlikely to explain why optimal mowing regime at the Grande Cariçaie would be much less frequent than at other locations. For this reason, we encourage managers and researchers to assess the effect of delayed mowing at locations where mowing frequency is higher to (i) verify whether decreasing mowing frequency could favour birds and other taxa and (ii) investigate what factors are responsible for site-specific differences in optimal mowing frequency.
Recommendations for management
Based on our results, we suggest that mowing should take place no more frequently than every 3 years so that optimal habitats be available every year. Ideally, mowing should even not occur more often than every 6 years to delay periods of poor quality that follow a cut. We stress, however, that the aim of managers must not be to “produce birds”, but to preserve natural environments from alteration by conflicting human activities.
Adapting management to local habitat type allows to specifically optimise management to different taxa. While humid, reed-dominated, reedbeds provide ideal nesting grounds for many bird species, drier, sedge-dominated meadows are characterized by a much higher richness in plants and invertebrates. They are also the most susceptible to being invaded by shrubs. Mowing could therefore be spaced in the most humid parts of the marshes, but maintained more frequent in drier areas.
In addition, complementary interventions could be initiated to mitigate the potential drawbacks of mowing. Grazing by cattle is one such alternative. Unlike mowing, which tends to create homogeneous habitats, grazing has a random effect on vegetation, with some stems being eaten and others remaining. Another major difference is that organic matter is not removed when grazed, while it is often exported when mown. Depending on the breed, cattle can be more or less choosy and therefore efficient at removing woody species. The impact of grazing on the soil is also more heterogeneous, which can, under some circumstances, favour the establishment of shrubs. To prevent this, grazing is often used in combination with burning, with promising results (Middleton et al. 2006; Little et al. 2015; Mérő et al. 2015; Mester et al. 2015). Preliminary tests of grazing are ongoing in the Grande Cariçaie reserves and will give insights onto how widely applicable this technique is.
A more specific intervention is the targeted tearing of shrubs. Tests were conducted in the Grande Cariçaie reserves using an excavator terminated by forceps that pulls out both above- and below-ground parts of the plant, thus durably removing shrubs. This method presents the advantage of selectively removing shrubs to decrease competition for light and prevent deviation of the habitat towards wooded communities, while keeping sedge and reed structure intact. Its major drawbacks lie in its costs. It is time-consuming and, unlike grazing and mowing, produces no resource that can be valorised to compensate for its costs. Formal testing is still lacking, but shrub tearing is surely a promising technique in situations where economic productivity is not the central concern.
Another intervention that specifically targets the most humid part of the marshes is the recreation of open water bodies by excavating soil over a few dozen centimetres. These water bodies will be colonised by vegetation over the course of a few decades, thus recreating the natural succession of vegetation stages. This technique presents the advantage of necessitating only a single intervention, while lasting for several decades. However, as it is quite destructive, it must be executed with parsimony, avoiding when possible places of specific biological value.