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Abstract
Resource uptake by neighboring plants can be an important driver of natural selection in a changing environment. As climate and resource conditions are altered, genotypes that dominate within mixed populations today may differ markedly from those in future landscapes. We tested whether and how the dominance of different genotypes of the allergenic plant, common ragweed, may change in response to projected atmospheric CO2 conditions. We grew twelve maternal lines in experimental stands at either ambient or twice-ambient levels of CO2. We then constructed a model that combines classical quantitative genetics theory with a set of a priori predictions about the relative performance of genotypes in the two treatments. Our findings show a complete reversal in the genotypic size hierarchy of ragweed plants in response to projected atmospheric CO2 conditions. Genotypes that are competitively suppressed in size at ambient levels become dominant under experimental doubling of CO2. Subordinated plants, in turn, boost their reproductive allocation to that of dominants, shrinking the fitness gap among all genotypes in high CO2. Extending our model to a contextual analysis framework, we further show that natural selection on size is reduced at elevated CO2, because an individual's position within the size hierarchy becomes less important for reproduction than it is in ambient conditions. Our work points to potential future ecological and evolutionary changes in this widespread allergenic plant.
Highlights
Competition among neighboring plants determines resource allocation to growth and reproduction (Grime 1977, Tilman 1982), and is likely to be an important driver of natural selection and determinant of genetic structure in populations (Bennington and Stratton 1998, Donohue et al 2000)
Contextual analysis To evaluate our results in an evolutionary context, we extended the functional relationship model for log reproductive allocation (LRA) to a contextual framework (Heisler and Damuth 1987, Weinig et al 2007) by introducing two covariates measured at harvest time, log average stand biomass (LAv_T) and log relative size of an individual within the stand size hierarchy (LRatio 1⁄4 log(T/Av_T))
A result of b, 0 in log vegetative biomass (LV) further indicated complete dominance reversal in plant size: subordinate genotypes in ambient CO2 became dominant in elevated CO2, and the dominant genotypes became subordinate (Table 1; Fig. 2A)
Summary
Competition among neighboring plants determines resource allocation to growth and reproduction (Grime 1977, Tilman 1982), and is likely to be an important driver of natural selection and determinant of genetic structure in populations (Bennington and Stratton 1998, Donohue et al 2000). In stands of interacting plants, competitive size hierarchies become established as dominant individuals pre-empt the resource uptake of smaller, subordinate individuals, and can be determined by both genetic and environmental factors (Weiner 1990, Thomas and Bazzaz 1993). When dominant individuals receive the greatest physiological boost from supplemental CO2, competition increases and size differences among individuals become exacerbated (Thomas and Bazzaz 1993, Poorter and Navas 2003), i.e., ‘‘the rich get richer’’. When subordinates experience a disproportionate growth boost from high CO2, size and reproductive variation are reduced and smaller individuals ‘‘catch-up’’ to dominants (Wayne and Bazzaz 1997, Stinson and Bazzaz 2006)
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