Plant species richness increases resistance to invasion by non-resident plant species during grassland restoration

Abstract

Questions

During restoration of bare subsoil, are planted grassland communities with low species richness more susceptible to invasion by non-residents than communities augmented by additional species? What are the mechanisms of invasion resistance in early succession?

Location

Lexington, Virginia, USA (37.8°N, −79.4°W).

Methods

We planted 62 3 × 3 m plots on compacted clay subsoil with 20–21 replicates of three, 12 and 24 native species. Plots were sampled non-destructively using a stratified random point-intercept method each summer for 5 yr to determine number of species and percentage cover per species, which were classified as residents (planted), internal non-residents (planted in other plots) or external non-residents.

Results

A negative relationship between planted species richness and invasion developed by the fourth year after planting and strengthened thereafter. Plots consisting of a single, highly dominant, resident C₃ grass species were vulnerable to invasion. With the exception of one species, which seems to have overcome resistance with a high seed rain, external non-resident species were less diverse and less abundant in higher diversity plots.

Conclusions

Increased planted species richness was correlated with increased resistance to invasion by non-residents, and we attribute this effect to a combination of the sampling effect and species complementarity, potentially augmented by high species dominance/low evenness in the low species richness plots.

The effect of planted species richness on invasion resistance was tested during grassland community establishment from bare subsoil. In less than 5 yrs, a seed mix of 24 functionally complementary native species developed into a community capable of maintaining a low abundance of external invasive species. Our methods were inexpensive and well suited to site restoration after heavy construction.