Changes in the grassland communities at these two sites have been relatively slow over the course of the experiments. This is in part due to the absence of any substantial invasion by new species, and the survival of almost all the original species in all treatments. The exceptions to this amongst the common species and species groups were few; A. odoratum was not detected in the Kirkton exclosures in 1995, dwarf shrubs were absent in the 3-cm plots at the same date, and at Cleish P. erecta was eliminated from plots grazed at 3 cm and 4·5 cm.
Despite the small changes in the species complement at both sites and the differences in species between sites, a number of patterns emerged from the data. However, a number of changes occurred across the treatments at each site, and such changes must be borne in mind when determining the actual response of the vegetation to the treatments. At Cleish there was a general decline in the importance of A. capillaris in all treatments accompanied by a general increase in G. saxatile. At Kirkton, there was an increase in a number of species and a decline in A. capillaris and A. odoratum, although the rate of decline differed between treatments.
This site initially had a more agriculturally productive sward than that at Kirkton (cf. Figs 1b and 4b). The maintenance of a short sward height, 3 cm or 4·5 cm, resulted in an increase in dominance of F. rubra, L. multiflora, T. repens and total moss cover. Both the shorter sward treatments also resulted in a decline in cover of Poa spp. and D. flexuosa. Overall, these two treatments resulted in a more open, moss-rich sward, with a higher contribution of forb species. Less intense grazing, i.e. maintaining the sward at 6 cm, allowed the increase of D. flexuosa and F. ovina, but appeared to cause a reduction in the cover of F. rubra.
An absence of grazing again favoured taller growing species as well as more grazing-sensitive species such as D. flexuosa and Poa spp. The removal of grazing resulted in a decline in the cover of lower growing species and those species tolerant of high grazing intensities, including F. rubra, L. multiflora and total moss cover. Deschampsia flexuosa showed consistent increases in cover during long-term exclosure studies at five sites in the northern Pennines (Rawes 1981), while P. pratensis increased in exclosures set up on Rhum (Ball 1974).
This site had an initially higher content of moorland species than the vegetation at Cleish. The maintenance of a short sward, 3 cm or 4·5 cm, resulted in an increase in dominance of N. stricta and a decrease in representation in the sward of M. caerulea and dwarf shrubs. Thus the maintenance of a short sward resulted in the expansion of rejected species (N. stricta) and the reduction of species intolerant of grazing, especially summer grazing (e.g. M. caerulea) (Grant et al. 1996a, b).
The imposition of the 6-cm treatment resulted in relatively few changes in species composition. Even this intensity of grazing was sufficient to reduce the cover of M. caerulea[cf. the 33% utilization rate shown by Grant et al. (1996a) to cause a reduction in cover], but it was not sufficient to allow the increase of the less preferred species such as N. stricta. The total removal of sheep grazing allowed for the expansion of the grazing-intolerant M. caerulea and the dwarf shrub species, and a reduction in species associated with a short turf and heavy grazing, such as A. capillaris, A. odoratum, C. pilulifera and N. stricta.
The behaviour of the communities at these two sites was, at least in part, as expected. Vegetation left ungrazed became dominated by taller growing, often grazing-intolerant, species, although the species that became dominant varied considerably in how they ranked with Agrostis–Festuca mixes in terms of preference by sheep; D. flexuosa was similar, M. caerulea was less preferred (Hunter 1962). This increase in dominance by D. flexuosa and M. caerulea was the same as that described by Hill, Evans & Bell (1992) for exclosures in Agrostis–Festuca grasslands in North Wales, as well as in other situations (Watt 1976; Rawes 1981). The taller growing species exclude the shorter ones such as F. ovina, F. rubra, L. multiflora and N. stricta. Vegetation kept short by grazing was increasingly dominated by shorter growing species and species resistant to grazing or not preferred by sheep (Hunter 1962). However, the trends were complicated to some extent by site-wide changes in species composition; the overall decrease in A. capillaris seen at Cleish, and the increase in a number of mosses in all treatments at Kirkton. The general trends in species behaviour at the two sites are summarized in Table 5.
Table 5. Overall trends in the cover of species that showed significant differences during the experiment at Cleish (C) and Kirkton (K) summarized by grazing treatment. ‘+’, an increase over the experiment, ‘.’, little change; ‘–’, a decrease. No entry means the species was not present
| ||3 cm||4·5 cm||6 cm||Exclosure|
|Conopodium majus||+|| ||+|| ||+|| ||+|| |
|Deschampsia flexuosa||–|| ||.|| ||+|| ||+|| |
|Festuca rubra||+|| ||+|| ||.|| ||–|| |
|Poa spp.||–|| ||–|| ||.|| ||+|| |
|Trifolium repens||+|| ||.|| ||.|| ||–|| |
|Anthoxanthum odoratum||.||–||.||–||.||–|| ||–|
|Carex pilulifera|| ||.|| ||+|| ||+|| ||–|
|Dwarf shrubs|| ||–|| ||.|| ||.|| ||+|
|Molinia caerulea|| ||–|| ||–|| ||.|| ||+|
|Nardus stricta|| ||+|| ||+|| ||.|| ||–|
However, the differences observed between the two sites are illuminating. The initial vegetation at Cleish had no M. caerulea and very little N. stricta; at Kirkton the former had an initial cover of 19%, the latter 29%. Thus the potential for the conversion of the vegetation to one that was relatively unproductive for grazing differed between the two sites. Both N. stricta and M. caerulea are low in nutrients and show lower digestibility values than mixtures of A. capillaris and F. ovina (Armstrong, Common & Smith, 1986). The treatments involving the maintenance of a short sward at Kirkton produced a community with an increasing cover of N. stricta. This was not possible at Cleish, and the resulting community was one with a high proportion of cover made up by mosses. Similarly, in the absence of grazing, there was an increase in the cover of M. caerulea at Kirkton. In its absence at Cleish, the exclosures became dominated by D. flexuosa and Poa spp.
It should be noted that at Kirkton N. stricta was present outside the experimental area, but it appeared not to be increasing despite the relatively short sward produced by sheep grazing. It is possible that N. stricta increased inside the plots because it was not subjected to winter grazing as the open hill was. This is an issue that needs investigation.
It would appear from comparison of movement in ordination space and analysis of the response of individual species, that to maintain the initial vegetation composition the most appropriate management at Cleish was to graze the sward at 4·5 cm (Tables 1 and 5). At Kirkton it appeared that grazing to 6 cm was most appropriate (Tables 3 and 5). There was sufficient grazing pressure in the summer to prevent the spread of M. caerulea, but it was not sufficient to encourage the spread of N. stricta at the expense of more preferred grasses, and changes in the cover of A. capillaris were minimal. The results from these two sites demonstrate that sustainable management of upland grazing, specifically British upland Agrostis–Festuca grassland, needs to take into account the presence of species capable of achieving dominance under different conditions, as well as the overall productivity of the vegetation and its constituents, a similar conclusion to that derived by Hill, Evans & Bell (1992).
In this grassland type, as in all others, the key to maintaining both agronomic and ecological sustainability is to manage the sward to maintain a substantial cover of palatable species, and to prevent the dominance of species that are unproductive or that alter the structure of the community. Changes in composition or structure, such as allowing the expansion of non-preferred species or allowing litter build-up, can make pastures more susceptible to change (Kemp, Dowling & Michalk 1996). Understanding the ecology of the species present is critical to predicting the effects of management on systems. This is true whether those systems are mediterranean-type rangelands threatened with the replacement of productive perennial grasses by annual grasses (Kemp, Dowling & Michalk 1996), or tussock grasslands with a balance to be maintained between productive intertussock grasses and tussock grasses (Allan, O’Connor & White 1992), or arid systems where dwarf shrubs expand at the expense of perennial grasses at high grazing intensities (Bisigato & Bertiller 1997). As in all systems, stocking rate (as set by sward height in this case) is the most important variable in grazing management. If this is not near the optimal level, then, regardless of other management practices, the community will undergo change (Walker 1997).
The overall small change in community composition over the 6 and 7 years of treatment at Kirkton and Cleish, respectively, has other implications. The relatively small number of species lost from these communities implies that moderating the severity of treatments should result in the reversal of trends in species composition. Vegetation change is considerably slower when species have to regenerate from seed or even re-invade the site (Mountford, Lakhani & Holland 1996). Thus these communities appear to have a certain resilience that allows the effects of a few years of inappropriate management to be rectified. It is not known how long this period lasts and how it is affected by the changes in sward potential for livestock production as species change in dominance. However, the increase in dominance of D. flexuosa (a species preferred by sheep, particularly in the spring), or the moss-dominated vegetation at Cleish in the more extreme treatments, may be more easily replaced by the desired A. capillaris and F. ovina than the less preferred M. caerulea and N. stricta, which are present at Kirkton, particularly as these latter two species are both less preferred species and can build up large quantities of unpalatable litter that hinder foraging.