Refinement of weed risk assessments for biofuels using Camelina sativa as a model species

1. Biofuel production has the potential of reducing CO₂ emissions while decreasing global dependence on fossil fuels. However, concerns have been raised on the invasiveness of biofuel feedstocks. Estimating invasion potential remains a challenge because of inconsistencies and inherent limitations of using first-tier qualitative weed risk assessment (WRA) protocols singularly. 2. We evaluated the usefulness of second-tier quantitative WRA methods using a recently introduced oilseed crop, Camelina sativa, as a model species. First, we subjected C. sativa to the qualitative Australian WRA and found that it should not be allowed entry. We then used demographic models fit with field-estimated parameters as a second-tier approach to quantitatively evaluate its invasion potential. Data on disturbance (two herbicides, mechanical, none) and seeding season (autumn, spring) relative to C. sativa demography were obtained over 2 years in two rangeland ecosystems in Montana, USA. Population growth (λ) was forecast by developing population dynamics models using field data. 3. Emergence rates were greatest when C. sativa was spring-seeded; all survivors to maturity occurred only in mechanically disturbed plots. Population growth rate never exceeded 0·03, and the maximum time to extinction was 6 years. Perturbation analyses indicated that consistent propagule pressure and biologically improbable rates of seed survival are necessary to sustain C. sativa populations, indicating that the risk of invasion by this species in the studied ecosystems is low. 4. Synthesis and applications. Although more site-years of demographic data would strengthen our conclusions about the invasion potential of C. sativa, we contend that the methods developed provide a useful contribution to WRA. If applied to proposed plant biofuel species, our second-tier quantitative refinements will elucidate important population dynamics often overlooked by qualitative WRAs and, in turn, may reduce the frequency of invasions or rejection of potentially useful species.