Theoretical basis for restoration of humid grasslands

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Kevin Kirkman

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Humid grasslands in South Africa comprise diverse assemblages of grass and forb species typically growing on relatively infertile soils and are maintained by regular defoliation. Temporal species turnover appears low in undisturbed grassland, with plant longevity strongly influencing both grass and forb species dynamics. Recruitment of new seedlings is rare in the face of severe above- and belowground competition for resources. Long-term research focused on defoliation and soil nutrient addition has revealed that increased soil nutrient levels severely impact species richness, while reduction in defoliation frequency has a similar effect. Nitrogen addition, on its own and in conjunction with phosphorus, increases productivity at the expense of richness. Likewise, reduction in fire frequency and/or defoliation in the growing season reduces richness in the presence of increased aboveground biomass. Species loss in the presence of increased soil nutrients and/or reduced fire and defoliation frequencies can be rapid (2-5 years). Studies and observation of abandoned crop lands and revegetated opencast mines reveal an extremely low recruitment of indigenous grasses and forbs, with secondary succession commonly stalling at a stage dominated by grazing resistant tall grasses that resist invasion by other species. Contrary to common perceptions, many indigenous grasses exhibit high seed viability, raising the possibility of reseeding to accelerate the secondary succession process, provided post-seeding management is geared towards facilitating species richness, including optimum (low) soil nutrient status and optimum defoliation frequencies (fire and/or mowing and/or non-selective grazing). Management should be geared towards avoiding dominance by grazing resistant species that resist invasion.

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Audio/Video, Conference Presentation, SER2019

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Society for Ecological Restoration