Importance of high severity in a physical removal treatment for controlling invasive Spartina alterniflora.
Physical removal treatments (such as clipping, pulling, and digging) are widely used to control invasive plants. The total biomass removed by each treatment is the severity of physical removal treatment; therefore, the magnitude of severity depends on the performance of shoots and new shoots that sprout after treatment. Physical removal treatments weaken the growth of invasive plants by reducing the biomass of invasive plants; however, few studies have demonstrated whether a high control efficacy of clipping treatments depends on it having high severity; and how to construct an efficient and cost-effective combination of timing of first clipping, interval between successive clippings, and frequency of clipping according to the performance of shoots and new shoots. Here, 24 clipping treatments (with different timings of first clipping, intervals between successive clippings, and frequency of clipping) were performed for controlling invasive Spartina alterniflora populations in China. The biomass of S. alterniflora in the following year was measured to evaluate control efficacy. The results showed that control efficacy increased with increasing severity. However, a positive relationship between severity and the frequency of clipping was not found, because the biomass removed by each clipping substantially decreased with increased frequency. In contrast, the highest severity was documented when S. alterniflora was firstly clipped at the start of sexual reproduction and second clipping was performed when new shoots stopped growing, high biomass was removed at each clipping, with a 70% decrease in growth the following year. As only two clippings were required to produce this marked reduction in growth, this cost-effective treatment could potentially be used to control other invasive Spartina species. More broadly, this study confirmed the relationships between high severity and high control efficacy of physical removal treatments and between the severity of the treatments and performance of shoots and new shoots of invasive plants, and such relationships can facilitate the construction of cost-effective combination of the timing of first treatment and interval between successive treatments.