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Pretoria, South Africa

Everson C.S.,SAEON | Everson T.,University of KwaZulu - Natal
African Journal of Range and Forage Science | Year: 2016

Natural grasslands deliver essential ecosystem services through plant production, which enhances water supply, nutrient cycling, soil retention and greenhouse gas mitigation. Although the condition of montane grasslands for provision of ecosystem services is maintained by regular annual or biennial burning, controversy exists over the impact of different frequencies and seasons of burning on grassland productivity. The objective of this study was to determine the long-term effects of different burning regimes on primary production and quality of the montane grasslands of the KwaZulu-Natal Drakensberg. There were no significant differences in the mean standing live mass between 30 years of annual winter and biennial spring burning. However, in unburnt areas productivity was 20% lower (118.2 g m−2) than in regularly burnt grassland (144.7–154.5 g m−2). Crude protein did not vary between the annual winter and biennial spring treatments (95–113 kg ha−1), but was significantly lower in unburned areas (45 kg ha−1). However, an infrequent fire in a protected area caused a temporary spike in crude protein (16%) compared with regular burning (5–10%), which can benefit wildlife. We conclude that montane grasslands can be burnt annually or biennially in the dormant season to promote long-term productivity. © 2016 NISC (Pty) Ltd Source

Reduced grazing pressure is expected to promote recovery of degraded semi-arid savanna. Transformation of degraded livestock ranches to a wildlife reserve in 1982 created a system with about one-third of the grazing pressure, which was maintained until 2011. Herbaceous composition and condition were monitored on four occasions between 1982 and 2011. Baseline measurements were taken following eight years of above-average rainfall conditions. Palatable perennial grasses and sward condition deteriorated markedly in response to severe, sustained drought during the early 1990s. Perennial grasses did not respond to a single record high rainfall season (2000) but recovered following a sequence of four above-average years (2008–2011). This monitoring study illustrated that recovery of a degraded semi-arid savanna may require decades of reduced grazing pressure for some degree of recovery in the absence of intervention. An appropriate time frame for management of these systems is in the order of a century. © 2015 NISC (Pty) Ltd. Source

O'Connor T.G.,SAEON | O'Connor T.G.,Dohne Agricultural Development Institute | de Ridder C.H.,Dohne Agricultural Development Institute | Hobson F.O.,Dohne Agricultural Development Institute
African Journal of Range and Forage Science | Year: 2010

Seed ecology may play a role in the widespread increase of Acacia karroo in savanna and grassland. Accordingly, fecundity, predation, mortality by fire, dispersal by livestock, and seed longevity were studied. Seed production per tree (P of pod production, pods per tree) was positively related to tree height, negatively related to tree density, and was greater for sites receiving run-on. Predispersal predation by bruchids, cerambycids, and wasps depleted seed production by 18%. The ant Messor capensis effectively eliminated surface seed except during the early dry season following seed shed. Rodent predation of seed was minimal. Fires >100 °C killed most exposed seed but a thin layer of soil increased survival and maintained germinability. A greater proportion of ingested seeds passed through cattle (57%) and sheep (50%) than through goats (15%). 50% of seeds were recovered within 48 h. During passage, soft seeds were digested, some hard seeds were softened, but most were egested as hard seeds; germinability was not increased. Cattle were confirmed as dispersal agents. Using experimental seed banks, it was shown that buried or surface-exposed seeds did not persist for longer than a year, but 24% of surface-shaded seeds persisted for two years. Most seeds germinated but most buried seeds died before emergence. Seed persistence in deep shade should facilitate invasion into grassland. Seed banks of A. karroo did not exceed 18 seeds m -2 because of the above-described effects. Seed availability (fecundity, dispersal by cattle, seed longevity) is concluded to contribute to the invasiveness of A. karroo, but use of fire may offer partial control of seed availability. © NISC (Pty) Ltd. Source

Canopy tree survival and compositional change of the Greefswald forest on the Limpopo River, South Africa, were monitored between 1990 and 2007 in response to a severe drought (cessation of flow in 1991/2), water abstraction commencing in 1991, a mega-flood in 2000 and increasing impact of elephants since 2000. Aerial photographs confirmed that forest area had not decreased during the 35-years prior to the study. In total, 25% of 428 canopy trees tagged in 1990 had died by 2005. Tree density was reduced from 22.8 to 16.3 trees per hectare. Forest was thus transformed to woodland. Mortality was attributed mainly to drought stress (47%), drought in combination with creeper infestation (30%) and the flood (21%). Of the nine main canopy species, mortality was highest for Acacia xanthophloea (56%) and Faidherbia albida (37%) mainly because of drought-related stress, and Ficus sycamorus (25%) mainly because of the flood. Water extraction increased drought-related mortality in the area of extraction by 45%. Creepers rendered microphyllous but not broad-leafed species more vulnerable to drought-induced mortality. Elephants were responsible for a further 3% mortality between 2005 and 2007. Composition has shifted towards 'drought-tolerant' species not selected by elephants, namely Philenoptera violacea, Xanthocercis zambesiaca and Schotia brachypetala. Initial concern about water abstraction was eclipsed by a rapid, unpredictable concatenation of a series of rare events that transformed forest to woodland in less than 15-years. © 2009 The Author. Journal compilation © 2009 Ecological Society of Australia. Source

O'Connor T.G.,SAEON | Kuyler P.,University of the Free State | Kirkman K.P.,University of KwaZulu - Natal | Corcoran B.,World Wildlife Fund
African Journal of Range and Forage Science | Year: 2010

The relative benefit of different grazing management practices for maintaining grassland biodiversity integrity was assessed. Practices considered were grazing system, stocking rate, animal type, grazing season, fire-grazing interaction, plus burning regime. Sparse empirical data indicated that stocking rate, cattle-to-sheep ratio, and increased fire intervals can affect plant diversity. Certain species behave as increaser or decreaser species in response to stocking rate or cattle-to-sheep ratio. Plant diversity appears robust in the face of variation in fire regime except for preclusion of fire. Multicriteria analysis was used to assess the relative impact of grazing systems (continuous, conventional, or high-intensity grazing by sheep, cattle, or sheep plus cattle) on grassland biodiversity integrity. Selected indicators covered landscape composition, structure and functioning. High-intensity systems and continuous sheep grazing have the most, and continuous grazing with beef the least, negative impact, depending on stocking rate. The relation between veld condition and plant diversity needs to be tested further before veld condition is used as a management tool for maintaining biodiversity. Proposed research priorities include inventory of management systems used, rapid expansion of the empirical data base, an improved understanding of patterns of plant diversity across the biome, and evaluation of the pyro-grazing diversity paradigm. © NISC (Pty) Ltd. Source

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