Palaeoecological dynamics at the fynbos-forest ecotone: a multiproxy study from Jonkershoek, South Africa

Oden, Glory Nicholas

Palaeoecological dynamics at the fynbos-forest ecotone: a multiproxy study from Jonkershoek, South Africa

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Climate change and land modifications have increased global concerns over ecosystem vulnerability and biodiversity loss. Mediterranean-type ecosystems (MTEs) are expected to be especially sensitive to change due to altering fire regimes, variations in plant available moisture, and invasive species. Jonkershoek Nature Reserve (JNR), in the Cape Floristic Region, South Africa lies within the fynbos biome, a biodiversity hotspot which supports mosaics of forests and Mediterranean-type heathlands. Jonkershoek has been a hub for ecological monitoring and research for almost 9 decades since the hydrological catchment program of the 20th century. Instrumentation records for rainfall, stream flow, and for monitoring the effects of introducing non-native tree species on the landscape during this period are well captured in the literature. However, until now, the longer-term context for these data was lacking. To understand the longterm ecological dynamics, two wetland cores were collected from the Fynbos-Forest Ecotones at the Eerste River Valley Bottom and Swartboskloof areas of Jonkershoek Nature Reserve. This was done to provide information on the vegetation's response to both natural and anthropogenic drivers of change at the wettest end of the winter rainfall gradient. The cores were C14 dated and analysed to reconstruct the vegetation, climate, land use and water quality over the past 8000 years, using fossil pollen, dung spores, charcoal, diatoms, major and trace elements, and stable isotopes of Carbon and Nitrogen. From the results, the Fynbos has shown a high degree of resilience and stability at least since the mid-Holocene. At the Eerste River Valley Bottom site (ERVB), grassy fynbos mixed with Restioid, Asteraceous and Ericoid fynbos shrubs occurred between 8000 cal BP and 4000 cal BP under cool and moist climatic conditions alongside low fire and herbivore activity. A shift to grassy, Restioid-Asteraceous fynbos occurred between 3200 – 2300 cal BP together with an increase in regional fires, herbivore activity and warmer climatic conditions. A transition to a cooler climate, which was typified by increased herbivory and fires between 2100 - 900 cal BP, resulted in the expansion of Restioid - Cliffortia fynbos elements. As humidity increased and conditions ameliorated, grasses declined rapidly. Since 350 cal BP, alien taxa have become a prominent feature of the landscape, coinciding with European occupation at the site. This recent change in land use has resulted in intense herbivory, soil erosion, high levels of weathering, and unprecedented fires. Despite these challenges, however, fynbos vegetation has remained dominant. The 750 years captured in the diatom analysis, reveal that the wetland has remained a predominantly freshwater ecosystem. However, it also suggests a recent increase in pollutant-resistant species (Nitzschia brevissima, N. heufleriana, N. nana, N. palea, N. paleacea), which coincides with periods of agricultural intensification and suggests eutrophication at the site. At Swartboskloof (SWBK), on the adjacent mountainside, the Restioid-Asteraceous fynbos type was stable from 5400- 4300 cal BP under a relatively cool climate and moderate fire and grazing regime. With a shift to a warmer climate and a decline in humidity, a change in fynbos composition in favour of grassier fynbos occurred from 3550 cal BP. Following moist conditions in the late Holocene, a recovery of Restioid fynbos occurred by 600 cal BP, with the occupation of the area by indigenous pastoralists. In the last 200 years, the expansion of fynbos has continued alongside increased fires, grazing, soil erosion, and the introduction of invasive species such as Pinus, Eucalyptus, Acacia, Quercus by the 17th-century European occupation, while the climatic conditions reveal warmer climatic conditions. Overall, the Fynbos has remained the dominant plant type at both sites, demonstrating stability despite differences in climate and land use change, through processes of resistance and resilience. Throughout the study, forest taxa have not shown a significant increase during periods of low fires or increased humidity, providing support for the idea that Fynbos vegetation is relatively resilient to forest encroachment. This may be attributed to fire-vegetation feed-backs, which exclude fire-sensitive forest taxa from fynbos areas and rather result in a form of reorganisation in the fynbos biome structure between Restioid dominant and grassy fynbos types. Furthermore, the results show that the more accessible ERVB site was more intensively utilised over time when compared to SWBK, resulting in a different response and resilience strategy. The high rainfall received at Jonkershoek further buffers fynbos from change and increases resilience, as moisture has not been a limiting factor. However, the fire regimes in the past 400 years are atypical when compared with the longer-term history of the site, in terms of frequency and intensity. This can be linked to anthropogenic influence and fuel load increase from invasive taxa. An understanding of the trajectory of recent changes in MTEs is important for designing effective restoration strategies and requires long-term monitoring and historical knowledge that predates the more recent intensive human impact. The projected warmer climate is likely to be accompanied by increasing fires and severe fire events. This is especially so at Jonkershoek, where high rainfall is often associated with high fire frequency and severity. Though fynbos vegetation appears remarkably resilient at this site, and ecological thresholds may not yet have been crossed, proactive management is essential to mitigate fire frequencies and reduce biomass to preserve ecosystem integrity. This study emphasises the critical role of palaeoecological research, as these results give a good basis for comparing the impact of land management practices pre- and post-colonial occupation at the site. Long-term studies also explore mechanisms of resilience and provide a context for interpreting decadal scale and present-day assemblages. They also provide insights into possible future trajectories of change. The results suggest that control of fire and aliens will be essential in preserving fynbos resilience in the future.

Keywords

ecology

climate change

Reference:

Collections

PhD / Doctoral

Full item page