Figure 3. Conceptual model of the Resource-mediated Successional Grazing Cycle of Calluna and Deschampsia.
as patch in a mosaic with a shifting configuration. The composition (patch proportions) may remain constant by opposite, compensatory transitions as long as the herbivore load and the nutrient pool of the whole mosaic (area) remain constant.
The hypothesised RSGC might be valid across a broader range of spatial scales, plants and herbivores in nutrient-limited environments. The studied sub-plots and plots constituted mosaics at community scale (25m2, Bokdam & Gleichman 2000). Patterns of
Calluna-dominated and Deschampsia-dominated areas of larger size (> 1 ha) in the study
area reflected soil fertility patterns due to differential land use and management history (Bokdam & Gleichman 2000). Grazing-induced transitions between Molinia caerulea and Erica and related mosaics occurred in the study area (Bokdam & Gleichman, 2000 Fig 1 e-f; Bokdam personal observations). The RSGC-model may generate qualitative predictions of effects of different herbivores, herbivores management strategies (e.g. free-ranging vs. herding, summer grazing vs. winter grazing) and additional vegetation management. These variables will affect the cycling rate and related mosaics, according to the resulting habitat use, diet selection, treading pressure and nutrient redistribution patterns. The model suggests that heathland restoration and maintenance require different grazing regimes. Restoration of Calluna in Deschampsia grass-heaths will be maximised by large grazers with preferential grass diets, displaying large-scale spatial segregation of foraging and resting habitats and a high hoof pressure. Heavy browsing or trampling in this stage would impair the establishment and lateral expansion of Calluna. Moderate browsing may favour the clonal growth of Calluna. Restoration success may therefore increase with body size: heather beetle < small rodents < rabbits < sheep < red deer < horses < cattle. Maintenance of Calluna will be maximised by moderate browsing (by rabbits, sheep or red deer) if the soil nutrient pool remains at a low level. Browsing will delay ageing and favour vegetative regeneration and Calluna re-establishment in
Deschampsia-free gaps, but uneven defoliation and nutrient redistribution will inevitably
lead to small-scaled mosaics with local grass invasion. Cattle and other large grazers might accelerate this process, leading to heather-grass mosaics, while large-scaled homogeneous, even-aged Calluna requires machines and fire.
The RSGC-model, linking herbivory and soil fertility to vegetation succession and biodiversity may become a useful tool for conservation managers. Implementation requires further testing of its validity range and quantification (Berendse 1985; Van Oene
Acknowledgements
Thanks are due to the Vereniging Natuurmonumenten for their permission to carry out the research project in the ‘Wolfhezerheide’ and to Willem Aandeweg for his technical support. Arend Vermaat investigated the plots in 1989 and 1991. Maurits Gleichman provided technical assistance and critical remarks. The comments of Michiel WallisDeVries, Frank Berendse, Herbert Diemont and an anonymous referee of the Journal of Vegetation Science improved the manuscript. Joy Burrough advised on the English.
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