Species, functional groups and community structure in seed banks of the arid Nama Karoo: Grazing impacts and implications for rangeland restoration

Abstract

Please cite this article in press as: Dreber, N., et al., Species, functional groups and community structure in seed banks of the arid Nama Karoo: Grazing impacts and implications for rangeland restoration. Agric. Ecosyst. Environ. (2011) The regeneration potential of grazing-affected Nama Karoo vegetation was evaluated by comparing soil seed banks of different microsites across a fence-line contrast in arid Namibia. Seed banks under low and high grazing pressure reflected the condition of the standing vegetation in terms of composition, community structure and species abundance distributions. However, a close concordance between vegetation and seed bank was restricted to the herbaceous and grassy vegetation within the inter-shrub matrix. The divergence of seed bank communities across the fence-line was low at community level but high at the level of species abundances. Continuous severe grazing increased the abundance of small-seeded, prostrate forbs with round seeds and favored unpalatable, annual grasses over palatable, perennial grasses. Microsites provided a source of small-scale variation in seed bank community composition and were dissimilar between the rangelands.Results indicated an advanced divergence in the vegetation at the degraded site with seed banks of species common under sustainable grazing being drastically reduced. Their low abundance, even in safe sites, suggests that long-distance dispersal is one of the main limiting factors for natural re-establishment after disturbance. The inertia in recovery of Namibian degraded rangelands through seed limitation can be overcome only by active species introduction.

Temporary grazing exclusion promotes rapid recovery of species richness and productivity in a long‐term overgrazed Campos grassland

“…Potential negative effects of high grazing intensities have been repeatedly described (Dreber et al ; Cingolani et al ). Species richness in individual quadrats was similar in the two evaluated grazing intensities, but at the plot scale we recorded new species more frequently in the moderate grazing paddocks.…”

Forb ecology research in dry African savannas: Knowledge, gaps, and future perspectives

“… Superscripts refer to selected studies only. 1 Buitenwerf et al (), 2 O'Connor (), 3 Gilo and Kelkay (), 4 Jacobs and Naiman () , 5 Odadi et al (), 6 Young et al (), 7 Zerbo et al (), 8 Dreber and Esler (), 9 O'Connor, , 10 Shackleton (), 11 Walker and Knoop (), 12 Belsky et al (), 13 Augustine (); 14 Ludwig, De Kroon, Berendse, & Prins, (), 15 van der Waal et al (), 16 Linstädter et al (), 17 Muvengwi et al (), 18 Masunga et al () , 19 Smit (), 20 Dreber et al (), 21 Rutherford et al (), 22 Hejcmanová et al (), 23 Odadi et al (), 24 Tessema et al (), 25 Angassa and Oba (), 26 Keya (), 27 Metzger, Coughenour, Reich, and Boone (), 28 Hanke et al (), 29 Tessema et al (), 30 Wesuls et al (), 31 Riginos et al (), 32 Riginos and Grace (), 33 Koerner et al (), 34 Belsky et al (), 35 Linstädter et al (), 36 Britz and Ward (), 37 Burkepile et al (), 38 Eby et al (), 39 Mlambo et al (), 40 Chikorowondo et al (), 41 Clegg and O'Connor (), 42 Traill (). …”

“…Plant–herbivore interactions in dry savanna systems are consistently shown to increase the risk of vegetation transitions into alternative states. With respect to the herbaceous layer, these changes become commonly manifested in transformations of plant communities toward the dominance of grazing‐resistant or grazing‐tolerant forbs and grasses, both in the standing vegetation and in the soil seed bank (Dreber, Oldeland, & Van Rooyen, ; Kassahun, Snyman, & Smit, ; Tessema, De Boer, & Prins, ). Although such a herbaceous layer may still provide a nutritious graze or browse for large and small stock (Donaldson & Kelk, 1970; section 3.4), the potential favoring of single species can have lasting effects on overall carrying capacities and livestock production (Wagner et al, ).…”

“…Although such a herbaceous layer may still provide a nutritious graze or browse for large and small stock (Donaldson & Kelk, 1970; section 3.4), the potential favoring of single species can have lasting effects on overall carrying capacities and livestock production (Wagner et al, ). Livestock grazing regimes affect the composition and structure of herbaceous communities and forb assemblages in multiple ways depending on the intensity (Dreber et al, ; Hanke et al, ; Linstädter et al, ; Rutherford et al, ), duration (Odadi, Fargione, & Rubenstein, ; Tessema et al, ), and seasonality (Angassa & Oba, ; Keya, ) of grazing impacts. Similarly, the type of herbivory and grazer mix may select for certain forb species (Odadi et al, ; compare section 3.4).…”

Nitrogen enrichment and grazing accelerate vegetation restoration in degraded grassland patches

“…In grazed plots, plant seeds are carried into degraded patches by livestock from nearby natural grasslands, thus increasing the species similarity and biodiversity. Additionally, the feeding preferences of grazing animals both directly and indirectly alter interspecific plant competition, further influencing both species composition and community structure (Dreber et al, 2011;Noymeir et al, 1989;Wan et al, 2011). It should be noted that grazing intensity is a crucial factor regulating the restoration of degraded grassland patches.…”