In the Greater Cape Floristic Region (GCFR) of South Africa, afrotemperate forest islands persist within a broader landscape of Mediterranean-type fynbos shrubland. The co-existence of these contrasting vegetation types in the same climate space suggests interactions between broad-scale climatic parameters and localised variables (notably local disturbance regimes and catchment hydrology). In this study, palaeoecological data from a fynbos-forest boundary were used to assess the effects of interactions between changes in climate, fire and land use on vegetation dynamics and biome resilience at millennial timescales. Fossil pollen, spores and charcoal were extracted from radiocarbon-dated sediment cores to provide proxies for vegetation, hydrology, large herbivore abundance and fire. Constrained hierarchical clustering (CONISS) was applied to the fossil data to identify distinct vegetation assemblages in the palaeoecological record, and to further elucidate ecosystem trajectories through time. Regional palaeoenvironmental data are also referred to in inferring local and regional environmental changes. We expected to find shifts between fynbos and forest alternate stable states associated with changes in fire and rainfall. More specifically, we anticipated fynbos expansion during drier periods and/or those with more fire, and forest expansion during wetter and/or less fire prone periods. However, the fossil pollen data reveal remarkable stability of the fynbos-forest ecotone over the past 3000 years, despite significant changes in climate, fire and land-use. We found resilience of fynbos was enhanced through internal dynamics, namely, a shift from grassy, fire-prone fynbos in warmer, less seasonal time periods with less summer drought stress, to proteoid fynbos with less frequent fire during times of greater summer drought stress. The emergent properties of the respective fynbos types offset physiological advantages to forest species afforded by changes in the fire-free interval and abundant abiotic resource supply, inhibiting invasion of the former biome by the latter. We suggest these mechanisms require further investigation.

The mega-diverse, Mediterranean-type fynbos biome may be vulnerable to future changes in climate and associated fire regimes, in particular to increasing summer-drought intensity and associated potential expansion of adjacent semi-arid vegetation types. Studying Holocene vegetation dynamics at the fynbos-succulent karoo boundary may provide insights into the resilience or sensitivity of fynbos to climate change. In this study, fossil pollen, nonpollen palynomorphs and charcoal data spanning ~5500 to −50 cal. yr BP were generated from an accelerator mass spectrometer (AMS) radiocarbondated sediment core extracted directly at the present-day fynbos-succulent karoo biome boundary at Groenkloof, a site in the Kamiesberg Mountains of Namaqualand, South Africa. Contrary to expectations, during the Mid-Holocene Altithermal from 5480 to 4025 cal. yr BP, fynbos and fire thrived through summer moisture subsidies associated with enhanced sub-tropical easterly flow. Subsequent cooling from 4025 to 2005 cal. yr BP resulted in enhanced summer drought and overall fynbos biome contraction, though woody fynbos shrubs persisted through physiological adaptations to drought. Desert succulents typical of the succulent karoo, such as those of Aizoaceae and Crassulaceae, failed to colonise the emergent niche space, resulting in dominance of an ambiguous grassy, asteraceous fynbos. More recent wetting associated with the 'Little Ice Age' Holocene temperature minima from 695 to 100 cal. yr BP prompted a resurgence in fynbos abundance, but frequent fire driven by pastoralists appears to have reduced the fynbos community's functional diversity. Palaeoecological data from the Kamiesberg suggest that both climatic buffering of mountain refugia and high physiological resistance among certain fynbos taxa have contributed to the biome's long-term resilience. Summer rainfall associated with the sub-tropical easterlies has been key in maintaining eastern fynbos refugia in past interglacial temperature maxima. The data also suggest that pre-historic land use and resulting fire-regime manipulations have resulted in the development of a taxonomically and functionally simplified alternative fynbos ecosystem state.

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