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University of Cape Town (2011)

Some consequences of woody plant encroachment in a mesic South African savanna

Gray, Emma Fiona

Titre : Some consequences of woody plant encroachment in a mesic South African savanna

Auteur : Gray, Emma Fiona

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) 2011

Résumé partiel
The vegetation in Hluhluwe Game Reserve is a patch mosaic of closed, forested vegetation and more open grasslands and savannas. Originally thought to be due to anthropogenic tree felling during the Iron Age, many of those habitats containing a continuous C4 grassy layer are now understood to be ancient systems that predate many of the forests in the area. Over the last century woody plants have encroached into the open savannas and grasslands in the system, a trend that has been documented in open systems throughout the world. A large scale shift in land cover is likely to alter ecosystem functioning significantly, but few studies have explicitly investigated these consequences. This study investigates the diversity present in the patch mosaic of vegetation, and also investigates how woody plant encroachment is affecting ecosystem services in Hluhluwe Game Reserve, with a focus on biodiversity, carbon storage and recreation and tourism. Savannas, grasslands and forests each contributed significantly to overall species diversity in the park. Savannas and grasslands were dominated by fire tolerant, shade intolerant species while forests were dominated by shade tolerant, fire sensitive species. Grassy systems shared few species with established forests, refuting the idea that they are seral to forests. Encroached savannas (thickets) shared many species with both savannas and forests, with older thickets more compositionally similar to forests. I therefore suggest that woody plant encroachment may eventually lead to a complete biome switch to forests, and will result in large scale savanna and grassland species losses and cascading ecosystem consequences. Established forests contained more carbon in their soils than other vegetation types. Encroaching thickets contained only 7% more soil carbon than savannas, suggesting that afforestation in this area is unlikely to provide a major terrestrial carbon sink. I suggest the amount of soil carbon stored will vary depending on climate, and that in more arid areas woody plant encroachment may prove to be a more useful carbon sink. The main carbon gain after encroachment in my study area was due to above ground carbon, an inherently unstable sink.


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