Shifting flowering seasons is a global effect of climate change that can have important long-term evolutionary and demographic effects on plant communities. Life history optimization theory can be a valuable tool to assert the adaptive value and fitness effects of observed phenological shifts, but takes plant-plant competition rarely into account. Here we combine energy allocation models with evolutionary game theory to assess how size-asymmetric competition for light can influence phenological adaptations and fitness responses to a changing climate – here represented as changes of the start, end and intensity of the growing season. We focus on annual plants which, due to their short generation times, are particularly likely to exhibit rapid demographic and evolutionary responses to environmental change. We find that while light competition favors late flowering times, it does not affect the direction of selection in the climate changes scenarios considered here. We predict, however, that plants adapted to light competition face more detrimental fitness consequences if the growing season advances, becomes shorter or less intense. We also show that adaptation to changing growing seasons under light competition can favor increased investment in vegetative growth with the counterintuitive side effect that seed production is reduced at the same time. In sum, our study highlights several effects of light competition that may help to interpret phenological trends and idiosyncratic fitness effects of climate change in wild plant communities.
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