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Summary Two South African mountain fynbos sites, similar in drainage, elevation, slope angle, slope aspect and soil type but with differing fire histories, were studied to measure how the effect of high densities of overstorey proteas in one fire cycle affects the α‐diversity levels of the plant community in the following fire‐cycle, how their repeated absence due to several short fire‐cycles affects their species richness and finally, at what spatial scale such patterns are most appropriately measured. High prefire canopy cover percentages and densities of overstorey proteas increase the postfire α‐diversity of understorey species. In addition, the increase in species richness observed occurred for all higher plant life history types present. At sites where one or more short fire cycles resulted in the repeated absence of overstorey proteas, the number of plant species present in the understorey was lower than at a site where overstorey proteas persisted. These results are dependent on the spatial scale at which the α‐diversity of understorey species is measured. At small quadrat sizes (< 5 m2), overstorey proteas decrease the number of understorey species present, while at larger quadrat sizes (100 m2) higher species richness is observed. The contradiction in conclusions when α‐diversity is measured at different spatial scales can be attributed to the patchiness of fynbos communities. Overstorey proteas play an important role in maintaining the patchiness component of fynbos communities by diminishing the effect of understorey resprouting species, making available regeneration niches for the maintenance of plant species richness. Where small quadrats are used, the effect of patchiness on the dynamics of the mountain fynbos community is lost. Thus, it is the fire history prior to the last fire and how it affects overstorey proteas that is important in the determination of α‐diversity levels in mountain fynbos plant communities.

Abstract. Two South African mountain fynbos sites were studied to determine the effect of short fire cycles on the cover and density of understorey sprouting species and their subsequent effect on plant‐species richness. Frequent fires (4–6 years between burns) increased the cover of sprouting species by 32% when compared to an adjacent site where the penultimate fire was 28 years previously. There was little or no effect of fire frequency on the densities of understorey sprouters; however, individuals were larger at sites with short fire cycles. The response of individual species of sprouters was variable with one species, Hypodiscus striatus, showing no response to fire frequency. The impact of sprouting species on the species richness of the plant community was great. The mean number of species recorded in quadrats with a high cover of sprouters was 60% lower in comparison to quadrats with low covers or under the burned skeletons of overstorey proteas. The effect of sprouters was consistent for all functional groups of species (i.e. sprouters, non‐sprouters, short‐lived and long‐lived species), in each case reducing the number of species present.

Previous studies in the mountain fynbos of South Africa have demonstrated that short fire cycles favour the establishment of dense covers of understorey sprouters while longer fire intervals enable the establishment from seed of overstorey proteas and the formation of a overstorey. One consequence of these differences between fire cycle lengths is the effect that understorey sprouters and an overstorey protea canopy have on species richness. In the case of short fire intervals, species richness is decreased while longer intervals between fires allow species richness to decrease or increase depending on the patchiness of the overstorey canopy. Such results are suggestive of competitive effects between understorey sprouters and overstorey canopy proteas. In this study, data were collected from several pyric successional stages in mountain fynbos to study the effect of overstorey proteas on the growth and flowering of understorey sprouters since the last fire. Data were also collected to determine the effect that understorey sprouters had on the establishment and fecundity of overstorey protea species. Competitive interactions between overstorey proteas and sprouting understorey species were evident at all the sites studied. The vegetative growth and seed production of understorey sprouters, which grew under a canopy of overstorey proteas during the current interfire period, were significantly lower than that for plants growing in the open. In addition, the postfire growth and seed production of understorey sprouters were significantly lower for individuals, which grew under an overstorey protea canopy during the previous fire cycle, than for those individuals which grew in the open. The fecundity of overstorey proteas, which grew near understorey sprouters, was lower than that of plants which grew in the open. This effect was evident for up to the first 15 years after a fire. However, not all understorey sprouters affected the overstorey proteas equally. Also, seedlings of overstorey proteas established significantly less successfully in close proximity to understorey sprouters after a fire than in the open or under proteas. Finally, the results demonstrate that complex species-specific, understorey -overstorey interactions are important in mountain fynbos. For example, some overstorey species depend on trophically similar species to reduce potential competition from understorey sprouters for their successful establishment at a site.

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