| Restorative mowing on semi-natural grasslands: community-level changes and species-level responses | ||
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The damage caused by mowing, in particular, affects equally all species with similar architecture and the same phenological state. However, some species are able to replace the lost biomass more quickly and thus suffer relatively less than the others. In Finnish managed grasslands, the losers are typically tall competitors for light, often perennial forest species with a wide ecological amplitude. Tall perennials and tree saplings invade meadow vegetation during irregular or temporary breaks in management. Even if these competitors and/or stress tolerators could compensate for lost tissue by, for instance, postponing their reproductive effort to the following year, the difference compared to species able to tolerate regular tissue losses and to respond to damage within the ongoing growth period is sufficient to maintain the dominance of true grassland species (cf. Owen & Wiegert 1981, Tolvanen et al. 1992, 1994, Wegener & Odasz 1997). Thus, species with a good compensatory capacity indirectly gain from being injured, provided that other species within the community are simultaneously also damaged. When management discontinues, this slight advantage in favour of meadow species disappears and typical meadow species are replaced by ones representing later seral stages.
The situation in regularly mown grasslands is not completely analogous to grazed ones. Mowing affects all species beyond certain height, and only a few species with low-creeping shoots or rosettes pressed tightly against the ground avoid damage (Losvik 1988). Moreover, some plants may escape damage by flowering and setting seed before or after mowing has taken place (Lennartsson et al. 1997, Crawley 1997). However, grazing and mowing also have many parallel effects on grassland vegetation. The greatest differences appear in their effects on the patchiness of vegetation and the properties of soil. The effects of grazing and different types of mowing on certain community characteristics are shown as summarized from a number of references in Table 1.
Table 1. Effects of grazing and early- and late mowing on grassland vegetation
Parameter | Grazing | Mowing and raking | References |
|---|---|---|---|
Stature | Unevenly degrading | Early: evenly and strongly degrading Late: evenly degrading | Tamm 1956, Andersen et al. 1990, Bakker & de Vries 1992 |
Shoot density | Stimulating | Stimulating | Valentine et al. 1997 |
Biomass production | Highly stimulating | Early: stimulating Late: stimulating (up to certain threshold) | Prins et al. 1980, McNaughton 1984, 1986a, Westoby 1989, Hik & Jefferies 1990, Cargill & Jefferies 1984, |
Species diversity | Increasing | Early: increasing or decreasing Late: increasing or stabilizing | During & Willems 1984, van Andel.& van der Bergh 1987, Losvik 1991, Belsky 1992, Eriksson et al. 1995, Zobel et al. 1996, Güsewell et al. 1998, Linusson et al. 1998, Stampfli & Zeiter 1999, Smith et al. 2000 |
Colonization of new species | Stimulating | Moderately stimulating | Hertz 1934, Willems 1983, Milchunas et al. 1992, Gibson et al. 1993, Diaz et al. 1994, Stampfli & Zeiter 1999 |
Litter layer | Scatteredly decreasing with increasing light and radiation | Early: decreasing with evenly increasing light and radiation Late: moderately decreasing with evenly increasing light and radiation | Facelli & Facelli 1993, Carson & Peterson 1990, Humphrey & Patterson 2000 |
Patchiness | Highly stimulating | Moderately stimulating | Rodriquez et al. 1995, Malkamäki & Hæggström 1997 |
Soil porosity | Highly compacting | Compacting or indifferent | Malkamäki & Hæggström 1997 |
Soil nutrient level | Stabilizing or increasing | Early: decreasing Late: slowly decreasing or stabilizing | McNaughton 1979, Spatz 1980, van Duuren et al. 1981, Bakker 1987, Pegtel 1987, van Andel & van der Bergh 1987, Parr & Way 1988, Oomes 1990, Mathews et al. 1994, Pettit et al. 1995 |
Tamm (1956) already noticed that some species were common in regularly mown areas, but rare or absent in grazed ones. Furthermore, certain species seemed to favour grazed pastures and occurred scarcely also in mown meadows. According to Tamm (1956), the vegetation sward is shorter in mown meadows and more clearly dominated by herbaceous species, although grasses are also common. Mown and grazed grasslands differ physiognomically. Depending on the intensity of grazing, the vegetation in grazed areas is typically patchy. In mown meadows, the vegetation sward is more uniform (Table 1), even though the species composition between the two may not differ greatly.A similar situation seems to prevail between the mown and abandoned meadows studied here. Particularly grass-dominated meadows seem to remain quite unchanged for a long time.
The most essential difference between mowing and grazing is seen in the frequency and intensity of disturbance (i.e. removal of plant biomass, Grime 1977, 1979). Mowing causes high-intensity disturbance with a low frequency. In contrast, the vegetation in a grazed meadow is exposed to low-intensity and high-frequency disturbance (During & Willems 1984). Unlike mowing, grazing is often selective (Arnthórsdottir 1994), and certain species are favoured and some avoided by grazers. Grazing favours rosette plants and prostrate growth forms, which leads to the development of dense field and bottom layers. Mowing, on the other hand, favours late-flowering, non-rosette plants and prostrate herbs. The height of the vegetation sward is lower in grazed pastures than in mown meadows (Table 1). Thus, light penetration is greater in pastures than in mown meadows, not to mention abandoned ones. The number of emerging seedlings is greater in grazed than in mown meadows (cf. Losvik 1988, Lennartsson 1997), although many seedlings die due to trampling at early stages. When grazing is replaced by mowing, the multi-layered vertical stratification usually decreases (Losvik 1988, but see Roxburgh et al. 1993). According to Losvik (1988), the numbers of short-lived perennials, rosette plants and creeping growth forms diminish because openings are no longer created by grazing and trampling. Finally, they are replaced by more erect species that are less affected by mowing than grazing.
Traditionally, semi-natural meadows were used for mowing and aftermath grazing (Ekstam et al. 1988). At present, mowing is more often replaced by grazing than vice versa. In most semi-natural grasslands, it is mowing rather than grazing that has ceased, and these areas are used - if at all - for incidental cattle grazing. Furthermore, they are often only slightly grazed (cf. Vainio & Kekäläinen 1997, Schaminée & Meertens 1992). Unfortunately, there is no data available from the Kiiminki meadow (III) immediately after the management ceased, but only seven years after the abandonment. Therefore, the question of whether mowing alone is enough to replace grazing in this particular case remains open. The seven years before restorative mowing was started may have been enough to cause profound changes in species abundance. Particularly the species that have only a short-term seed bank may have disappeared (O´Connor 1991).
Mowing and grazing also have different impacts on the soil nutrient status (Table 1). Mowing leads to a continuous nutrient loss and even depletion in poor soils (Tamm 1956), whereas in rich soils it may take decades before the soil nutrient status is markedly affected (Berendse et al. 1992). If the total nutrient pool in soil is only reduced by 5 % annually, the change does not have any far-reaching effects on the long-term nutrient status, nor is it reflected in the species composition (Pegtel 1987). In contrast to mowing, grazing causes part of the nutrients to be recycled back into soil (Mathews et al. 1994). However, intense grazing may erode the humus layer, especially in dry stands (Tamm 1956). Soil fertility may affect both the productivity and the diversity of species in grasslands. Generally, it is assumed that the productivity of grassland communities correlates negatively with species diversity (Silvertown 1980, Bakker 1987). This relationship is not, however, unambiguous. According to Bakker and de Vries (1992), even when the standing crop is equal in both mown and grazed areas, the species diversity is greater in grazed ones. During and Willems (1984) point out that the differences between grazing and mowing become clearer towards higher productivity. Mowing at different times during the growing season results in different amounts of yield, but it also has various effects on the reproduction of different species. Moreover, according Oomes (1992), one cannot make very far-reaching conclusions about structurally different vegetation types merely by considering production. However, there is consensus among researchers that mowing and grazing lead to an increase in the number of species in abandoned meadows (During & Willems 1984, Gibson et al. 1987). Mowing and particularly grazing create gaps in the vegetation, which is a prerequisite for a great number of species (Pegtel 1987, Lennartsson 1997). According to Bakker (1987), particularly grazing changes the competitive relations in grasslands, which leads to an increased number of species. In contrast, in meadows with a long mowing history, competition for below-ground resources may be more important. According to Vinther (1983), a certain grazing intensity enables the development of a “floristic optimum“, i.e. the maximum number of species that can coexist in a given place. A decrease or increase in grazing intensity causes a decline in the number of species. Thus, it could be claimed that mowing, in habitats where it maintains a high level of diversity and is a “normal component“ of the environment, is not a “negative disturbance factor“ at all (Kull & Zobel 1991).