Resource pre‐emption, rather than extending the growing season of native grass assemblages, reduces invasion by exotic species

Abstract

Questions In Mediterranean‐type systems, invasive C3 annual grasses appear early in the season and can pre‐empt resources and attain a competitive dominance over native perennial grasses. Here, we investigated whether planting C3 and C4 native grasses (a) combined, so that resources are extracted over a longer period, or (b) at higher density would make planted communities more competitive against invasive species. Location Para Woodlands Reserve, near Adelaide, South Australia. Methods In 72 experimental plots, native grasses were planted in combinations of seasonal patterns (three levels; single‐season assemblages with either C3 or C4 and extended season with both C3 and C4) and planting density (two levels; high = 44 plants/m2 and low = 20 plants/m2). Data were collected on native plant survival and biomass, invasive biomass and soil properties. Results Overall, C3 native grasses were superior competitors against both invasive C3 grasses and native C4 grasses. We found no interaction between the combination of C3 and C4 grasses planted together and density of planting. Assemblages with higher densities were successful at reducing exotic plant biomass; however, there was a trade‐off with reduced individual performance among the native plants. Even though individual plants were larger in the low‐density treatment, total biomass was lower in these plots suggesting that density limits the growth of native communities as a whole. The C3 native plants were planted earlier than the C4 native plants because of differences in phenology and therefore likely pre‐empted resources and gained a size advantage, making them the superior competitor. Conclusion Native perennial grasses can outcompete exotic plants for resources if planted earlier in the season. This resource pre‐emption appears to be more important than resource use over a longer period with C3 and C4 plants together and could be an effective restoration strategy.