Sward heterogeneity, not grazing intensity per se, determines plant diversity and functional characteristics in a cattle-grazed grasslan
Selective grazing by large herbivores can cause or enhance heterogeneity at several scales. Feedbacks between grazing intensity and plant quality may lead to so-called patch-grazing dynamics, where some patches are grazed repeatedly in one season, while others are not grazed at all. The resulting mosaic of short and tall patches may be relatively stable over time. As a result, a long-term differentiation between short and tall patches in terms of botanical composition and nutrient cycling is to be expected. In an eleven-year-old cattle grazing experiment with three grazing intensities we tested the hypothesis that patch type, and not grazing intensity per se, exerted a greater influence on plant alpha diversity and functional characteristics.
Plant species number and abundance were recorded, and topsoil samples were taken, in short and tall patches in nine paddocks of three different grazing intensities (long-term average yearly stocking rates: 670, 370, and 250 kg ha-1). Community weighted means (CWM) of the three plant strategy compounds according to Grime were calculated for each paddock and patch type. Bray-Curtis dissimilarity was calculated between short and tall patches within each paddock and between grazing intensities within patch type.
Short patches were characterized by a higher point diversity, higher proportions of legumes and forbs, lower proportions of grasses, higher CWM for stress tolerance and ruderality, and lower CWM for competitiveness, compared to tall patches (p<0.001). Plant available soil phosphorus and potassium concentrations were lower under short than under tall patches (p<0.01). Grazing intensity did not significantly affect these parameters. Bray-Curtis dissimilarity between short and tall patches within each paddock was independent of grazing intensity and higher (0.54) than average distance within the same patch type between grazing intensities (0.44). Long-term average proportion of short patches increased with grazing intensity (22, 29, 52%, for the three grazing intensities, respectively), while that of tall patches decreased (46, 40, 15%). We explain the observed differences between short and tall patches through higher defoliation frequency in short patches and nutrient redistribution from tall to short patches, making short patches nutrient- and tall patches light-limited. Our results show that effects of grazing intensity on plant communities may be indirect and mediated through differences in the relative proportion of patch types. In this system, explicit consideration of the heterogeneous patch structure is necessary to understand grazing effects on the plant community.