Multi-decadal vegetation change in dune vegetation of the south-eastern Cape Floristic Region: Is thicket expansion without fire inevitable?

Abstract

It is hypothesized that in the Holocene dune fynbos-thicket mosaics of the Cape, decades-long absence of fire results in closed-canopy thicket replacing more open fynbos shrublands. We tested this hypothesis by analyzing repeat photographs from 16 sites at Cape St Francis in the south-eastern Cape Floristic Region. The period of monitoring ranged from 55 to 17 year and most sites had been last burnt between 30 and 50 years ago. We divided the sites into coastal (salt wind-exposed) and inland zones and computed, for each pair of photographs, the rate of change of four plant types, namely thicket shrubs, fynbos woody, fynbos herbaceous and grass. Coastal zone sites showed a significant increase in thicket shrub cover and a significant decline in both grass and fynbos herbaceous cover. Fynbos woody cover declined in the face of thicket invasion, but increased in grassy sites, most likely due to a decline in herbivory intensity. No significant changes in plant types were observed in the inland sites, although thicket shrub cover did increase marginally at the expense of fynbos woody plants. However, thicket invasion was restricted to swales and lower dune slopes; the dune fynbos of the upper slopes and crests remains uninvaded by thicket and surprisingly vigorous after 30–50 year without fire. This is consistent with the notion that fynbos species occupy drier sites than thicket in these dune landscapes and regular fire is not a prerequisite for its persistence. Overall, we showed a large increase in woody plant cover throughout the study area. While this pattern is consistent with predictions for vegetation change in response to atmospheric [CO2] pollution, it can also be explained by land use changes experienced in the study area, starting in the early 1960s, involving a shift from subsistence agriculture to holiday resort development and tourism.