Differences in germination characteristics for two biotypes of giant ragweed

Background/Question/Methods

Giant ragweed (Ambrosia trifida) is a native annual weed that is highly competitive in corn and soybean crops. Habitat-based differences in giant ragweed seedling emergence phenology often distinguish crop and non-crop populations, with emergence delayed in crop populations. Throughout the western part of its weedy range, where it is a more recent crop pest than in the east, these habitat-based differences appear to be less pronounced. We hypothesized that habitat-based differences in emergence timing represent local adaptation in response to habitat-specific environmental conditions during spring: the bare ground in crop sites should lead to cooler temperatures and higher magnitude day/night temperature fluctuations relative to crop margin sites insulated by intact vegetation and the corresponding duff layer. We therefore conducted a germination chamber experiment where seeds from non-crop and crop sites collected across a longitudinal gradient were grown in a combination of high and low magnitude day/night fluctuations and high and low mean temperatures to assess whether we could detect signals of local adaptation. Data were collected on emergence phenology and seed fate over twenty days.

Results/Conclusions

Percent germination was 19% higher in non-crop populations than crop populations and increased with higher magnitude temperature fluctuations. The percent of giant ragweed seeds remaining dormant for the duration of our study was marginally higher in crop populations than non-crop populations. Only crop populations from the west experienced expected dormancy reductions in response to higher magnitude temperature fluctuations. The emergence period was shorter with warmer temperatures, but only in western populations. The combination of cooler mean temperatures and lower magnitude temperature fluctuations led to prolonged emergence periods and delays in initial emergence, relative to either warmer mean temperatures or higher magnitude fluctuations. Emergence period was less impacted by temperature fluctuations in eastern populations compared to western populations. Conversely, warmer temperatures result in quicker time to first emergence in eastern populations only, whereas western populations emerged quickly regardless of mean temperature. Although we did not find any clear indications of local adaptation in crop populations of giant ragweed, the fact that geographical patterns were more pronounced in crop versus non-crop populations does indicate that habitat-based differences in seed dormancy and germination vary substantially across this species’ range. Our findings contribute to a large body of research on the evolution of weediness.