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Questions

What are the effects of root competition from mature plants and soil type on the survival and growth of dominant grass and succulent shrub seedlings at an ecotonal site between arid grassland and succulent shrubland? Do these factors explain the occurrence of separate grass-dominated and shrub-dominated communities along the ecotone?

Location

Ecotone between Bushmanland arid grassland and Namaqualand succulent shrublands in the Karoo, South Africa.

Methods

Seedlings of Stipagrostis brevifolia, a perennial C-4 grass, and Ruschia robusta, a leaf-succulent shrub were transplanted into naturally occurring openings within separate Stipagrostis brevifolia and Ruschia robusta communities. The effects of root competition were tested by exposing seedlings to roots of neighbouring adult plants or by excluding the influence of neighbouring roots using metal partitions. The influence of soil on the survival and growth response of seedlings was tested by comparing the response of each species grown in soil of their own community with their response when grown in soil of the other community.

Results

All S.brevifolia seedlings died within the first 3mo of the experiment, irrespective of competition exclusion or whether they were grown in grassland or shrubland soils. Root competition from mature plants reduced the survival of R.robusta seedlings in the grass community, but not in the shrub community. When seedlings of R.robusta were not exposed to root competition, their survival and biomass in the grass community soil type did not differ from that in the shrub community soil type.

Conclusions

Excluding root competition in the grass community provided favourable conditions for R.robusta seedlings to establish, suggesting that root competition from the grass is more important in maintaining separate grass communities, and that any differences in soil properties are less important in influencing such processes. Our study presents an example where a species’ fundamental niche may be differerent from its realized niche, as a result of competitive interactions.