Individual study: Introduction of sheep grazing encourages development of a varied sward structure in a previously uniform Yorkshire fog Holcus lanatus-dominated grass pasture at Westerholt, Drenthe, The Netherlands
Heil G.W. & Diemont W.H. (1983) Raised nutrient levels change heathland into grassland. Vegetatio, 53, 113-120
Large areas of grassland of low agricultural value are being abandoned in the Netherlands. Such land may be acquired for nature conservation purposes, when a major objective is to reduce the residual effects of fertilizers in order to increase species diversity, e.g. by hay-making or by grazing. Grazing at relatively low stocking rates is often considered advantageous in achieving this aim, as it leads to variation in soil compaction with variable removal of vegetation due to selectivity of the grazing animals, which hay-making cannot achieve. The present study investigated i) vegetation succession originating from an initially uniform Yorkshire fog Holcus lanatus-dominated grass sward under hay-making and grazing, and ii) the stability of vegetation 'micro-patterns' (which subsequently arose from the grazing activity).
Study area: The study was undertaken on a 6 ha area of grassland (heathland ploughed and sown to grass in the 1950's) at Westerholt (53º01'N, 6ºll'E), Drenthe province, The Netherlands. Until 1972 the grassland was cut for hay and aftermath grazed by cattle, and had been fertilized up to 1971. Under this management regime it had become a very uniform, species-poor, Yorkshire fog Holcus lanatus-dominated grassland.
Sheep grazing: In October 1972, sheep were introduced to graze part of the grassland at an average stocking rate of 3 sheep/ha; 25 grazed during the period July-December and 40 between January-July. After 1980, for 2 months each winter, the area was left ungrazed. (No indications are given in the original paper as to the size of the grazed area, and the size under hay-making). The vegetation had the same uniform appearance on either side of the fence (that divided the site into grazed and haymaking sections) in 1972 prior to introduction of the sheep.
Vegetation monitoring and mapping: In 1972, before sheep grazing started, permanent 2 x 2 m quadrats, were established on the then uniform sward, and vegetation was recorded annually. Grazing intensity (based on litter cover) was recorded in the quadrat as: lightly grazed >70% litter cover, heavily grazed < 30% litter cover, or intermediate. This was determined in February, when differences between heavily and lightly grazed areas were at their maximum. Species abundance was quantified on a dry weight basis; crude protein % was measured in the tops of the live portions.
A mosaic of taller tufts interspersed with shorter vegetation appeared only in the grazed area. To monitor change over time, vegetation was mapped several times from October 1979 to autumn 1982, in a 10 x 10 mÂ² area: patches with a height <5 cm and little litter accumulation were indicated as heavily grazed, and patches > 10 cm tall with more litter accumulation as lightly grazed.
Preferential grazing by the sheep gave rise to a 'macro-pattern' of various plant communities absent under the hay-making regime. Finer-scale vegetation patterns ('micro-patterns') were also evident, with short, heavily grazed patches interspersed with taller, lightly grazed patches. The sequential mapping indicated that the micro-pattern was more or less stable after there initial establishment, over the study period. The protein percentage of green material was higher in the heavily grazed areas than in the lightly grazed patches, and the continued selective grazing in these patches appeared responsible for the observed vegetation structure stability.
Heavily and lightly grazed areas differed little in species composition, but the abundances of occurring species differed considerably. The more heavily grazed areas were characterized by equal amounts of monocots and dicots, had higher abundances of rosette species and, to a lesser extent, greater persistence of perennial rye-grass Lolium perenne. The lightly grazed patches were dominated by common bent Agrostis tenuis (Syn: capillaris) and had a large amount of plant litter, which perhaps accounted for the lack of mosses.
Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: