CHAPTER 6 COMPETITIVE EFFECTS AND RESPONSES OF SELECTED SPECIES IN
6.4 Discussion
6.4.2 Competition, disturbance and resource availability
Few ecologists today doubt that competition is an important structuring factor in plant communities, but researchers disagree on the circumstances where it is most intense, and on which traits can be considered to contribute to competitive ability for different species. The distinction between a species' effect on resources and its response to reduced resource levels might help to solve these questions. Clipping reduced the growth of all species to varying degrees, while fertilization enhanced it (Table 6. 3).
This demonstrates the fact that irrespective of size plants respond to resource availability as well as disturbance regimes. However, the interaction between these
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two factors gives more insight than their main effects. When plants were grown in resource poor soil (0 %), they were greatly suppressed, and clipping significantly reduced their biomass production except for the medium-sized Themeda triandra, which exhibited the highest performance (Table 6.4). This indicates that the species, which is arguably the most economically important veld grass species in South Africa (O‘Connor 1996), is able to recover from defoliation even at low levels of resource availability. This indeed supports the notion that as a Decreaser species, T. triandra thrives under constant but moderate defoliation (Snyman 2004).
In general, larger or broad-leaved species (e.g. Digitaria eriantha, Eragrostis curvula, Panicum. maximum and Sporobolus fimbriatus) were suppressed by clipping to a higher magnitude under low soil fertility than the shorter T. triandra, C. plurinodis, M.
decumbens and the slower-growing shrub Acacia karroo (Table 6.4). For instance, clipping reduced the growth of E. curvula by about 78 %, while it reduced Melica decumbens by only 42% in the 0 % fertility treatment. Similar results were reported in eastern Ontario by Keddy (et al. 2002), who found out that smaller, narrow-leaved herbaceous species performed relatively better in stress treatment pots than broad- leaved plants, while the broad-leaved species performed better in less intense stress or in the absence of stress.
Increases in soil fertility (50 % and 100 %) conveyed some shifts in the species‘competitive response hierarchy: All the species performed better with no clipping, and the taller grasses gained momentum and produced higher biomasses than the smaller grasses. This illustrated directional change in the species‘
performances across gradients of resource availability and disturbance. The results appear to support the resource-ratio hypothesis of plant succession, which attempts to explain the role of competition for resources by common species in spatially heterogeneous habitats and along spatial gradients (Tilman 1982). The theory also suggests that succession results from a gradient in the relative availabilities of limiting resources over time, thereby implying that changes in resource supply can lead to compositional change or dominance of a particular plant community (Tillman 1985).
Trends in this study showed that the species‘ competitive ability were inversely related if the species differed in size (i.e. either in height or size of leaves). The taller grasses (D. eriantha, E. curvula and P. maximum) had a competitive edge over the
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shorter, narrow-leaved grasses (especially C. plurinodis and M. decumbens) and the invasive Acacia karroo, particularly in high fertility. When the plants were clipped, the shorter species such as T. triandra become more competitive than when unclipped or when grown on fertile soil. These shifts in competitive abilities are an indication of trait trade-offs among the species. Similar results were also observed in a study by Fynn (et al. 2005), who observed that the broad-leaved P. maximum was the most competitive in fertile soil, while the tall, narrow-leaved Hyparrhenia hirta was most competitive in poor soil. These results could be attributed to the fact that when nutrients are more abundant, the taller grasses grow faster and build enough biomass and subsequently usurp the nutrients at the expense of the smaller species. When nutrients become depleted, due to their body size, the smaller grasses are able to persist and perform comparatively better than the taller grass species. Clipping (which is a simulation of grazing intensity), also improved the performances of taller grasses because it reduces shading and improves light interception, so that the species are kept in a more productive, actively-growing state.
Goldberg and Werner (1983) suggested that competitive effect ability is a consistent trait of a species, linked to specific plant traits. Given the fairly consistent hierarchy of competitive effects and responses, what traits determine the species position in a competitive hierarchy? Competitive effects should be related to the plant‘s ability to deplete resources so they become unavailable to others (Goldberg 1990), hence greater depletion rates should result in greater competitive effects. In this study, the larger-sized plants had higher competitive effects than smaller plants, it therefore appears that plant size was related to the resource-depletion ability. Some differences were however found between competitive species in competitive effect (e.g. C.
plurinodis vs T. triandra), implying that plant size is not the only determinant of competitive effect or the ability to deplete resources. This hierarchy of competitive effects was to some extent determined by relative growth rate, consistent with the hypothesis that all else being equal, the effects of relative growth rate on competitive effect are confounded with growth form (Goldberg and Landa 1991). Hierarchies in competitive responses were slightly more variable than in competitive effects, especially when the phytometers were clipped and fertility level varied. Therefore competitive response does not appear to be related to any noticeable plant trait.
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