Print PageInvasive plant species are a major threat to global biodiversity. Invasive plants out-compete native plant species and disrupt native mutualisms, lead to local extinctions, and pose a threat to native plant and animal communities. It has proven very difficult, however, to predict which species will become invasive. The role of plant phenology (i.e., timing of growth and flowering) may be essential to understanding the success of invasive plant species and warrants further study. We designed a two-year target-neighbor field study to investigate the effects of differential growth and flowering phenology on competition between native and invasive plant species. Targets and neighbors were co-established with five neighbor treatments. Treatments included an early-flowering native functional group, a late-flowering native functional group, and a mixed native functional group, consisting of both early and late native species. Additionally, there were monocultures with the same number of planted individuals as the native species treatments, and solo plant (no competitors) treatments for both of the invasive species - the early-flowering Hesperis matronalis and later flowering Nepeta cataria.
Results/Conclusions
In year 1 (the year all species were planted) above-ground vegetative biomass in both the early invasive (H. matronalis) and late invasive (N. cataria) was significantly less in monoculture compared to plants grown with native competitors. In year 2, neighbor treatments had little influence on invasives’ final size, with some target individuals as large as solo plants. Across both years, the annual vegetative biomass of H. matronalis was generally less than N. cataria. Seed mass and flowering phenology were also affected by year and neighbor treatment, though the relationship between these variables and biomass was plastic for both invasives.
Early-flowering native neighbors showed the greatest above-ground biomass in year 1. In year 2, however, the late natives had considerably more mass than the early natives. Though overall native biomass had increased in year 2, the competitive effect of neighbors on the invasives was actually less than in year 1 because of vigorous invasive growth. Our study illustrates the complex interactions that can occur across treatments and years and emphasizes the importance of multi-season studies to quantify interactions between competitors in the wild. These results have important implications for invasive management, restoration, agricultural systems, and plant interaction experimental designs.
