Questions: How do naturalistic grazing and mowing vs. passive succession affect plant community composition and species richness in a temperate grassland grazed by semi-feral cattle and horses?
Location: Mols Laboratory, Denmark
Methods: In the rewilding area of the Mols Laboratory, 22 experimental
blocks were established in 2017, of which nine placed in moist or dry grassland
vegetation were investigated here. Each block consisted of five 5 × 9 m
plots, representing four fenced treatments, i.e. summer-only grazing,
winter-only grazing, annual mowing and full exclosure, embedded in the
year-round grazing matrix. Quantitative plant community composition was
assessed using the point-intercept method in 25 × 25 cm quadrats,
supplemented with calibration models based on additional quadrats that
were harvested after recording and biomass sorted to species and
weighed. Uniqueness was assessed as the sum of inverse range sizes for
constituent species.
Results: We found a major difference in species richness between grazing
and annual mowing/full exclosure, but only minor differences between grazing
treatments. In contrast, forb:graminoid ratio and uniqueness was found
to be highest in year-round and winter-only grazing, whereas summer-only
grazing (with close-to-natural grazing pressure) had lower uniqueness. Annual mowing was found to promote graminoids over forbs. Full exclosure
plots had accumulation of litter and the lowest species richness.
Differences in community composition were blurred by initial
heterogeneity between plots within blocks and large differences between
moist and dry grasslands. Data analysis using the biomass estimates derived from the calibration models yielded only minor differences in the patterns described above, when compared to the results obtained using the raw number of intercepts.
Conclusions: Grazing under near-natural conditions is a goal in itself
in ecological restoration, but also an efficient management tool to promote
conservation of grassland plants and communities. Both plant species
alpha diversity and prevalence of less widespread species (unicity) were promoted,
as well as the prevalence of forbs, which may translate into enhanced
floral resources for anthophilous insects. Summer-only grazing at low density
of large herbivores was quite similar to winter-only and year-round grazing, but our treatment is much closer to natural grazing than agricultural summer grazing at high stocking density. While the application of calibration models can be useful for approximating absolute biomass estimates non-destructively, they may not always warrant the extra time cost.
Keywords: biomass estimation, disturbance regime, point-intercept
method, rewilding, uniqueness
Introduction
Grazing by large herbivorous mammals is a key process shaping vegetation structure and habitat conditions for plants and other organisms (\citealp{Svenning_2015}; \citealp{Chytr__2022}). In European conservation management, there is a strong tradition of aiming at mimicking traditional practices in agriculture and livestock husbandry, e.g. extensive haymaking and summer grazing (\citealp{Varga_2016}). In reality, however, actual conservation management is often strongly constrained by the opportunities compatible with modern high input-high output farm management and agri-environment support schemes (\citealp{Newton_2011}). Either way, conservation management practice is not always rooted in ecological theory and often fails to deliver the desired outcomes for biodiversity (\citealp{Maxwell_2020}). Attempts to apply first principles to grazing management can be comprised under the term "naturalistic grazing", which may be characterized as landscape-scale conservation management, under which grazing as a natural process is seen as an aim in itself, where human intervention therefore is reduced to a minimum and where herbivore density is not human-controlled, but left to be resource-regulated \citep{RN3815}. Although "naturalistic grazing" is considered open-ended with regard to effects on herbivore populations and vegetation, monitoring the effects is crucial to our understanding of how grazing as a natural process works and interacts with other natural conditions and processes.
In large contiguous landscapes, area use by large herbivores often shows substantial variation in diurnal, seasonal and between-year patterns. Animal activity tend to be concentrated in certain areas, while large parts may be much more extensively used, e.g. wet areas may be avoided during winter, but preferred in spring and summer \citep{Górecka-Bruzda2020}. Traditional livestock husbandry had and continues to have the raising of domestic animals as its core purpose. Therefore, summer-only grazing on pastures and winter feeding of stabled animals was the norm traditionally, in particular for cattle, while some horses have traditionally been left on pastures year round. The pattern of summer-only grazing is strongly reinforced in modern livestock husbandry, in which standard practice is to turn livestock out at very high density during a short period of intensive grazing at the peak of the growing season. One way of investigating the resulting impact on vegetation of the annual timing of herbivore activity is to compare areas open to animals during controlled periods of the year \citep{RN1084}.
While large herbivores shape vegetation structure and habitat conditions for herbaceous plants, species richness and composition of grassland communities have a relatively strong influence on richness of mega-diverse consumer taxa, e.g. arthropods and fungi \citep{RN16669}. Flower-visiting insects have attracted particular attention, partly because this functional groups is particularly threatened by intensified grassland use, and partly because some anthophilous insect taxa have shown dramatic declines in species richness and abundance (e.g. \citealt{Hallmann2021}; \citealt{Warren2021}). The ratio in vegetation of forbs to graminoids has therefore been particularly highlighted, as most forbs have flowers offering resources to anthophilous insects, while graminoids all have wind-pollinated flowers.
The response of vegetation structure to contrasted grazing regimes are likely to differ in quantitative plant community composition, with the activities of large herbivores promoting the abundance of certain species, while limiting others. We therefore applied the point-intercept method to quantitatively recording vegetation structure (\citealt{RN4360}; \citealt{RN19634}; \citealt{RN18842}). Non-destructivity is a virtue of the method, which was desired in the current setup of long-term monitoring plots, also surveyed for other groups of organisms. However, because of differences in plant architecture, the intercept-based abundance does not translate directly to biomass-based abundance. We therefore made calibration models per species and/or functional groups, based on an additional set of quadrats, first subjected to point-intercept recording, next to total harvest and dry-mass estimation per species.
Alpha diversity of plant communities is of core interest to evaluations of vegetation under contrasting grazing regimes, although results may depend on the actual quadrat size applied. From the perspective of gamma diversity in the region or country, however, community unicity - the regional rarity of constituent species - is of higher relevance. One way to evaluate the contribution of individual communities to regional gamma diversity is the ‘Sum of inverse range-sizes’ (\citealp{RN15834}; \citealp{RN16594}), in which constituent species are given decreasing weight with increasing regional occupancy. Also from the perspective of biodiversity conservation, community unicity may be more relevant than alpha diversity, e.g. even locally species-poor communities may be of high regional conservation value, if they tend to consist of relatively rare species.
Our overarching aim was to assess differences in grassland vegetation structure, community richness and uniqueness as a snapshot after four years of naturalistic grazing, as compared to contrasted grazing and mowing regimes. Specifically, we aimed at investigating:
1) Does plot-scale plant species richness vary between year-round grazing, seasonal grazing (all at naturalistic herbivore density), mowing and passive succession?
2) Does forb to graminoid ratio vary between year-round grazing, seasonal grazing (all at naturalistic herbivore density), mowing and passive succession?
3) Does plant community unicity (index value reflecting uniqueness) vary between year-round grazing, seasonal grazing (all at naturalistic herbivore density), mowing and passive succession?
Materials and methods
Study site
The Mols Laboratory is an ecological research station, owned by the Natural History Museum Aarhus. It is located in the hilly, glacially shaped landscape of Mols, east-northeast of Aarhus. The area is known as a biodiversity hotspot for Denmark and is very heterogeneous by Danish standards, with wide variation in abiotic conditions and vegetation structure. Roughly half of the area is covered by open habitats, the other half by scrub and forests, with all types in a mosaic with gradients between open and canopy-covered habitats. The most frequent open habitat type, as categorized under the European Habitats Directive, was 6230 - Species-rich Nardus grasslands. Despite all blocks being assigned to this type of grassland, quite large variation in the species composition and topography between different parts of the area is evident, foremost between hilly glacial gravelly till and sandy marine foreland shaped by the higher sea-level of the Littorina transgression (Atlantic; 6800 - 3900 BCE). This contrast is presumably linked to hydrology, with the marine foreland being somewhat impacted by exfiltration of groundwater from the hills.