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.

Does a tradeoff between trait plasticity and resource conservatism contribute to the maintenance of alternative stable states?

“…Furthermore, closed‐canopy forests limit the intrusion of fire from open shrublands (van Wilgen et al ., ) and, consequently, the pyromineralization of soil nutrients (Stock & Lewis, ; Neary et al ., ). The relative build‐up of nutrients compared with shrublands reinforces the emergent differences in soil nutrition and stability of these alternative states (Cramer et al ., ). Plasticity, however, may enable state instability by supporting the colonization of open habitats by some forest species in the absence of fire.…”

“…10–200 m in width). Fire has been excluded from shrubland vegetation in close proximity to forest since 1972, a period in which there has been a significant increase of forest tree cover in shrubland vegetation (Cramer et al ., ). The geology consists of basement granite at the bottom of the valley, topped by Table Mountain group sandstones with occasional intrusions of Table Mountain group shales (McKenzie et al ., ).…”

Biome: evolution of a crucial ecological and biogeographical concept

“…These feedbacks, assisted by disturbance factors such as fire and mega‐herbivore grazing, create the potential for the emergence of multiple biome states that are stable over long time series (multiple stable states (MSS; Charles‐Dominique et al ., ; Moncrieff et al ., ) or alternate/alternative stable states (e.g. Warman & Moles, ; Cramer et al ., )). The existence of MSS implies that a system, when disturbed from one state to another, does not return to its original state once the cause is removed; however, a second factor takes over and holds the system in the new state for a long time (Dublin et al ., ).…”

New regionally modelled soil layers improve prediction of vegetation type relative to that based on global soil models

“…There is thus a feedback between soil and vegetation characteristics. For example, small forest patches in the GCFR commonly occur on the same geological parent material as neighbouring Fynbos, but the forest soils have higher pH, total N, P, K, Ca, Mg, and organic matter (Cramer et al, ). These differences are attributable to the fire‐susceptibility of Fynbos, which results in nutrient losses, and the limited flammability of forests which fosters nutrient accumulation.…”