Soil and climate contribute to maintenance of a flower color polymorphism

Premise of the Study: Floral pigments such as anthocyanins are well-known
to influence pollinator attraction, yet they also confer tolerance to
abiotic stressors such as harsh soils, extreme temperatures, low
precipitation, or UV radiation. In such cases, environmental variation in
abiotic stressors over space or time could lead to the maintenance of
flower color variation within species. Under this scenario, flower color
in natural populations should covary with environmental stressors.
Methods: Using a comparative approach, we test whether abiotic variables
predict flower color in Leptosiphon parviflorus, a species with pink and
white flower color morphs. We conduct in-depth field studies to assess
morph frequency, soil chemistry, and climate. We then employ community
scientist-powered iNaturalist observations to examine patterns across even
larger spatial scales. Key Results: Across 21 field sites we find that L.
parviflorus has a higher frequency of pink morphs in sites with serpentine
soil, higher average annual temperatures, and higher average climatic
water deficit (a proxy for drought stress). iNaturalist observations
support this finding—the probability of flowers being pink is greater in
locations with serpentine-derived soil, especially when the local average
UV radiation and climatic water deficit are higher. Conclusions: Spatial
variation in abiotic stressors may contribute to the maintenance of flower
color variation across the geographic range of L. parviflorus and other
clade members.  Future studies will examine mechanisms by which
flower color affects stress tolerance and will assess whether fitness
tradeoffs in contrasting habitats are associated with flower color.