Factors affecting alien grass invasion into West Coast Renosterveld fragments

[摘要] ENGLISH ABSTRACT:With only 1.76% conserved West Coast Renosterveld remaining it is crucial to prioritizethe conservation and management of this fragile vegetation type. Because of itsoccurrence on fertile soils, renosterveld has been cleared for cultivation and otheragricultural use and is thus fragmented to a critical point. These remaining fragmentsare subject to extensive edge effects and the exact parameters of a viable fragment muststill be determined. The extent of alien grass species invasion into remnantrenosterveld patches was determined using data collected using the line-interceptmethod. Sampling included three different management treatment areas adjacent to oldlands: 1) a recently burned area, 2) a grazed area and 3) an unburnedlungrazed areaproviding a control sample. Additionally all small-scale soil disturbances along thelines were recorded.Results showed that Briza spp., Bromus spp. and Avena fatua were the most significantinvading alien grass species. There was a significantly high variation between aliengrass cover in each treatment, with the grazed area containing the highest overal! levelof alien grass invasion. The control area, with the exclusion of fire and animal activity,showed minimum alien grass invasion. Edge effects were apparent in all treatments,but were lowest in the control area, where percentage alien grass cover significantlydecreased after a distance of fifteen meters into the remnant patch. A weak positivecorrelation between the occurrence of alien grass and indigenous grass was found, aswell as evidence of a weak association between alien grass cover and shrub cover. Thepossibility that small-scale soil disturbances could be an important contributing factorto alien grass invasion was supported by the positive correlation between disturbancesand percentage alien grass cover. A positive correlation was found between theoccurrence of indigenous grass and small-scale soil disturbances, while shrub covertended to be less associated with animal activity.It is concluded that the correct management regime could effectively control theexcessive invasion of alien grass species into the remnant renosterveld patches.Grazing (including all animal activity and small-scale soil disturbances) was the mainfacilitator of alien grass establishment - even dominating the prominent consequences of edge effects by facilitating alien grass establishment much deeper into the remnantpatches. It could be assumed that a precise balance of low-pressure grazing and firemanagement should be established to enable the effective control of alien grass specieswhile maintaining optimum biodiversity.Keywords: renosterveld, alien grass, grazing, fire, management, small-scale soildisturbances

[发布机构] Stellenbosch University