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

The cool season rainfall at our study site should favour C-3 rather than C-4 grasses. There are, however, several locations where C-4 grasses have become dominant, suggesting that rainfall seasonality is not a constraint on distribution. Here, we explored the limitations on C-4 grass distribution in a fynbos shrubland. Using delta C-13 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 C-4 grasses expanding into cool season rainfall shrublands such as in Western Australia and South Africa. What is interesting about these results is that C-4 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.

Automated summary

This study found that C-3 grasses are favored over C-4 grasses in cool season rainfall conditions. However, despite this, there are locations where C-4 grasses have become dominant, suggesting that rainfall seasonality does not constrain their distribution. The research reveals that the establishment of C-4 grasses in fynbos shrublands is recent, with roads potentially acting as conduits, impacting fynbos species richness.