Are forest-shrubland mosaics of the Cape Floristic Region an example of alternate stable states?

Abstract

The idea of alternate stable states (ASS) has been used to explain the juxtaposition of distinct vegetation types within the same climate regime. ASS may explain the co-existence of relatively inflammable closed-canopy Afrotemperate forest patches (‘Forest’) within fire-prone open-canopy Fynbos in the Cape Floristic Region (CFR) on sandstone-derived soils. We evaluated the hypothesis that although fire and local topography and hydrology likely determined the paleogeographic boundaries of Forest, present-day boundaries are additionally imposed by emergent edaphic properties and disturbance histories. We studied vegetation and edaphic properties of Forest-Transition-Fynbos vegetation at two sites within the CFR on sandstone-derived soils and tracked historical change using aerial photography. Whereas Forest and Fynbos have changed little in extent or density since 1945, transition vegetation increased into areas formerly occupied by Fynbos. Forest soils were ubiquitously more nutrient-rich than Fynbos soils, with transition soils being intermediate. These edaphic differences are not due to geological differences, but instead appear to have emerged as a consequence of different nutrient cycling within the different ecosystems. Soil nutrients are now so different that a switch from Fynbos to Forest is unlikely, in the short term (i.e. decades). Floristically and nutritionally, transitional vegetation is more similar to Fynbos than Forest and may be less resilient to changes in exogenous drivers (e.g. fire). Our findings are consistent with the idea that geologically Forest and Fynbos are largely fire-derived long-term ASS, with the stability of each state reinforced by marked soil nutrient differences. In contrast, the intermediate transitional vegetation that might switch states is unlikely to be stable.