Climate change drives greater investment in outcrossing by reducing climatic stress for a common mixed mating species

Background/Question/Methods

The balance between cross- and self-fertilization is driven by environmental stress. Under stressful conditions, selfing is employed at higher rates compared to favorable conditions, which promote outcrossing. Yet no long-term study has documented whether anthropogenic climate change is affecting reproductive strategy investment in species with mixed mating systems. To answer this question, we tested whether the common blue violet (Viola sororia; Violaceae) has altered investment in outcrossing across the 20th century in response to temporal climate changes. Using herbarium records spanning 1875 to 2015 from the central United States, we quantified production of obligately selfing cleistogamous (CL) flowers and facultatively outcrossing chasmogamous (CH) flowers by V. sororia, coupled these records with historic precipitation and temperature data, and tested whether changes to the proportion of CL flowers across the past century are explained by temporal climate trends. We also used these data to test whether the timing of CL and CH flowers has shifted across the past century.

Results/Conclusions

We find that climate change has increased outcrossing rates in V. sororia. Specifically, V. sororia plants progressively produced lower proportions of obligately selfing CL flowers, and higher proportions of facultatively outcrossing CH flowers, across the past century and in environments with higher total annual precipitation and lower mean annual temperature (all P < 0.05). We also find that both CL and CH flowers have advanced phenology by ~1.2 days decade-1 across this time period (both P < 0.05). Our results suggest that V. sororia has responded to better growing conditions induced by climate change by shifting investment away from selfing and toward outcrossing. By revealing that CL flowers occur in environments with low water availability and high temperature, our results support obligate selfing as a bet-hedging strategy in stressful environments. This provides the first long term study of how climate change has affected reproductive strategy investment in species with mixed mating systems. As outcrossing can facilitate future adaptation, while selfing can lead to inbreeding depression, understanding the effects of climate change on reproductive strategy investment is crucial for a comprehensive assessment of the biological impacts of global change.