Characterizing the recruitment nature of weed species

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The core goal of my research program is to determine how and why weeds emerge when they do. The timing of seedling emergence is a prime determinant of plant species success and for weeds, the transition from seed to seedling represents a transition from the least to the most vulnerable life cycle stage. The relative timing (versus crop plants) and the success of weed seedling emergence also determines the competitiveness of weed infestations, the potential for weeds to escape management, and the quantity and quality of weed seeds left to form persistent seed banks. In addition, emergence timing is a key determinant of the geographic range of species and their invasive potential. Finally, time of emergence greatly impacts the ability of weed species (including crop plants that have escaped farmed fields) to survive in wild or semi-wild habitats including roadsides. The latter has implications for gene flow at the landscape scale and the effective segregation of genetically modified (GM) from non-GM crops. Despite its fundamental importance in these respects, our understanding of the mechanism of emergence timing remains poorly understood for most weed species. To-date in my program we have made some significant contributions in this area including clear demonstration that seed depth in soil is a fundamentally important driver of emergence timing, and that the suitability of emergence conditions can be determined to a great extent by seed depth. We have also shown that timing of flowering and seed shed can play a very significant role in determining what conditions seeds experience (including whether and to what extent they are buried) and that this, in turn, can determine timing of emergence. Most recently, using Canada fleabane, a facultative winter annual weed (able to emerge at an time of the year), we were the first to document an interaction of the effects of these factors (flowering and seed shed timing, and seed depth) on emergence timing. What is not clear is the extent to which these observations represent generalizations and to what extent the effects feed forward to impact population performance (and weediness) in future generations, and how this may influence our ability to manage weeds. Our approach over the next 5 years will be to look forward in the life cycle of weeds (flowering timing and seed shed) in order to gain insight looking backward (to seedling emergence and emergence timing) using facultative winter annual weeds as test species. This research will lead to a better understanding of the mechanisms driving weed emergence timing which in turn will provide insights to aid weed management, weed risk assessments and the effective segregation of GM and non-GM crops.