The Wadden Sea salt marshes have high conservation value as they harbor a wealth of plant and animal species, some of which are endemic to this area. These salt marshes are therefore protected under the EU Habitats Directive (EC Habitats Directive
1992). The experiment was conducted in one of these salt marshes, the natural high productive (1120 ± 201 g dw m
−2; mean ± 1 se; measured in 2018) salt marsh in the barrier island of Schiermonnikoog (53° 30′ N, 6° 10′ E), the Netherlands (Bakker
1985). A small western part of the salt marsh had been grazed by cattle up to 1958 when grazing stopped. Cessation of grazing led to the local dominance of the tall late successional grass,
E. atherica, which led to a decline in plant diversity over the following 10 years (Bakker
1985). The conservation managers wanted to reverse this trend. Hence, a field experiment to guide management practices started in 1972 in this area.
Four blocks were established in 1972, encompassing different plant communities characterized by different dominant species: block (1)
Festuca rubra and
Armeria maritima; block (2)
E. atherica; block (3)
F. rubra and
Artemisia maritima; block (4)
F. rubra and
Limonium vulgare. Block 1 and 2 were situated in high marsh, block 3 and 4 in the low marsh. Exclosures (ca. 8 m × 42 m) within blocks, consisted of two electrical metal strands running 0.5 and 1 m above ground supported by wooden posts every 3.5 m. Note that small herbivores like hares, geese, and insects could enter the exclosures freely. Hares and geese can substantially slow down species decline over time particularly when their abundance is high (Chen et al.
2019). A previous study using this experiment shows that cattle grazing facilitates hare grazing, and possibly also goose grazing, because cattle graze down the tall grass
E. atherica, which in turn promotes
F. rubra favored by hares and geese (Kuijper et al.
2008). Each block contained eight different treatments, including (1) an undisturbed control (C, i.e. natural succession without grazing and mowing), (2) early season mowing (M (E)), (3) late season mowing (M (L)), (4) both early and late season mowing (M (EL)), (5) grazing by cattle (G), (6) grazing plus early season mowing (G + M (E)), (7) grazing plus late season mowing (G + M (L)), (8) grazing plus both early and late season mowing (G + M (EL)) (plot size ca. 18 m
2 for mowing treatments; experimental design in Fig. S1). Treatments within blocks were randomized in the field. We usually mowed in late June or early July for the early season mowing, and in late August or early September for the late season mowing. We cut the vegetation to 2 cm above ground using a brush cutter. Plant material (including litter) was raked and collected, and dry weight was determined. Cattle grazing was continuous from May to November annually. Stocking density decreased from 1.5 to 0.5 head ha
−1 from 1993 onwards, as the cattle-grazed area increased (Bakker et al.
1993; Bos et al.
2002; Fig. S1). One permanent plot (2 m × 2 m) for each treatment was established in 1972. We recorded species occurrence and abundance in the permanent plots before the start of mowing from 1972 to 2017. We used the data from 1972, 1974–1980, 1984–1989, 2003, 2015 and 2017, as all the eight treatments were measured in those years (thus 17 years of surveys). We estimated abundance (percent cover) using the decimal scale of Londo (
1976). As we estimated percent cover for each species independently, the total cover of living plants can sometimes exceed 100% for the multilayer canopies. Plant species occurrence and abundance were recorded by a skilled field assistant for most years. A list of plant species occurred during the 46-year experiment can be found in Table S1.