|Schmiedel, U; Röwer, IU; Luther-Mosebach, J; Dengler, J; Oldeland, J; Gröngröft, A: Effect of grazing on vegetation and soil of the heuweltjieveld in the Succulent Karoo, South Africa, Acta Oecologica, 77, 27-36 (2016), online: 2017-01-03, doi:10.1016/j.actao.2016.08.012 [Link]|
We asked how historical and recent grazing intensity affect vegetation and soil characteristics in the patchy landscape of the heuweltjieveld in the semi-arid biodiversity hotspot Succulent Karoo. The study was carried out on the communal farmland of the Soebatsfontein community, 80 km south-west of Springbok, in Namaqualand (South Africa). Heuweltjies are roughly circular earth mounds that are regularly distributed in this landscape. We sampled plant species and life-form composition, diversity measures, habitat and soil variables in 100 m² plots, placed in three visually distinguishable heuweltjie zones (centre, fringe, and matrix) and distributed across grazing camps with different recent and historic grazing intensities. Differences between heuweltjie zones were assessed with ANOVAs and multiple linear regressions. The effect of past and recent grazing intensity on soil and plant variables was analyzed by Generalized Linear Models for each heuweltjie zone separately. The three heuweltjie zones constituted clearly distinguishable units in terms of vegetation and soil characteristics. Soil pH and cover of annual plants increased from matrix to centres, while total vegetation cover, species richness and perennial plant cover decreased in the same direction. Historic (pre-2000) grazing patterns had the strongest effects on heuweltjie fringes, showing the strongest soil and vegetation related signs of overutilization with increased stocking density. Heuweltjie centres showed signs of overutilization irrespective of the stocking density. The much shorter exposure to recent grazing pattern (post-2000), which was nearly inverse to the historic grazing pattern, showed increase of vegetation cover (centres) and species richness (matrix) with recent grazing intensity. We interpret these effects as still visible responses of the lower grazing intensity in these camps during the historic period. No recovery under low-intensity recent grazing was observed at any of the zones. We conclude that irrespective of their conducive growing conditions, once transformed to a disturbed state, heuweltjie centres recover slowly, whereas the less impacted soil and vegetation of heuweltjie fringes are more responsive than centres and matrix. Heuweltjie fringes are thus more suitable as indicators for overutilization.