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The causes and effects of indigenous C4 grass expansion into a hyper-diverse fynbos shrubland

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Abstract

The cool season rainfall at our study site should favour C3 rather than C4 grasses. There are, however, several locations where C4 grasses have become dominant, suggesting that rainfall seasonality is not a constraint on distribution. Here, we explored the limitations on C4 grass distribution in a fynbos shrubland. Using δ13C values of SOM, we determined when these grasses had established. We also looked at the role of roads as conduits for establishment and asked what impact these grasses may have on fynbos species richness. We then conducted a field experiment designed to examine the extent to which soil moisture, nutrient availability, and competition with fynbos for resources influence the establishment and growth of the grasses. Finally using aerial photography, we explored the role of changes in land use on distribution. Our results showed that the establishment is recent, that roads may be acting as conduits, and that with establishment, there is a reduction in fynbos species richness and diversity. Disturbance and removing below-ground competition for resources open the system to establishment in wetter areas. This study is the first to look at the potential for C4 grasses expanding into cool season rainfall shrublands such as in Western Australia and South Africa. What is interesting about these results is that C4 grasses can establish and dominate in a cool season rainfall regime. Rather than temperature of the growing season, it is competition for resources from fynbos that prevents these grasses from encroaching.

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Data availability statement

The data used in this study are available on the University of Cape Town open access database at https://doi.org/10.25375/uct.12647057.

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Acknowledgements

We would like to thank Cape Nature for permission to work in the De Hoop Nature Reserve; Masindi Tina Raselabe, and Thembani Namba for assistance in the field; and Joel Lewis for statistical assistance. Thomas Slingsby and Nicholas Lindenberg of the UCT GIS support unit helped with the aerial photography and mapping. NM acknowledges financial support from the South African Environmental Observation Network. This work was funded by the Andrew W. Mellon Foundation.

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Author notes

  1. N. M. Munyai

    Present address: Scientific Services, Mountain Zebra National Park, Private Bag X66, Cradock, 5880, South Africa

  2. C. P. Tucker

    Present address: Rand Girls School, corner Melle St and Empire Rd, Parktown,, Johannesburg, 2000, South Africa

Authors and Affiliations

  1. Department Biological Sciences, University of Cape Town, Private Bag, Rondebosch, Cape Town, 7701, South Africa

    E. C. February, N. M. Munyai, C. P. Tucker & W. J. Bond

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  1. E. C. February
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Contributions

ECF, NMM, and WJB conceived and designed the experiments. All authors performed the experiments. ECF and NMM analysed the data and wrote the first draft of the manuscript. All authors contributed to subsequent drafts and provided editorial advice.

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Correspondence to E. C. February.

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Communicated by Susanne Schwinning.

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February, E.C., Munyai, N.M., Tucker, C.P. et al. The causes and effects of indigenous C4 grass expansion into a hyper-diverse fynbos shrubland. Oecologia 195, 421–433 (2021). https://doi.org/10.1007/s00442-020-04842-2

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