Sorghum halepense is a perennial invasive weed causing great harm worldwide, including some regions on Hainan island. In this study, we compared the performance between plants from outbreaking (dense stands covering large areas) and those from non-outbreaking (less dense stands covering smaller areas) populations. We also tested the hypothesis that plants with greater intraspecific competitive ability will have a lower stand biomass when grown under high-density conditions.

We grew plants of S. halepense individually, with an intraspecific competitor, with two interspecific competitors and with three or six plants from the same population per pot, and tested whether outbreaking and non-outbreaking populations differed in their performance. We also tested whether stand biomass (i.e. total biomass when three or six plants were grown together) was related to total biomass of individuals grown alone, and intra- or interspecific competitive ability.

Outbreaking and non-outbreaking populations of S. halepense differed in their biomass when individuals were grown alone, but not when grown in competition or when three or six plants from the same population were grown together. Across populations, biomass of individuals grown alone was negatively correlated with intra- and interspecific competitive ability, indicating that there is a trade-off between individual growth and competitive ability. Stand-level biomass was not related to total biomass of individuals grown alone, and intra- or interspecific competitive ability, indicating that low biomass when grown alone and high competitive ability may not reduce the performance of S. halepense when grown in dense monocultural stands.