| Summary: | Scorzoneroides autumnalis is a highly polymorphic perennial with several described infraspecific taxa, largely differing in involucre colour and indumentum intensity. Here, we examine the role of ecotypic divergence and phenotypic plasticity in shaping large‐scale geographical variation in these characters. We collected phenotypic data from herbarium specimens and garden‐grown plants of S. autumnalis , representing several habitats throughout Scandinavia and Iceland, and subjected progenies from controlled crosses within a subset of the common garden material to different temperature regimes to assess patterns of phenotypic plasticity. Our results strongly suggest that colour and indumentum of involucral bracts, as well as the size of capitula (measured by ligule length), are environmentally plastic and much affected by temperature. Reduced temperature resulted in significantly larger capitula, with both thicker and darker involucre indumentum. Since dark colouration, dense indumentum and large floral structures have been shown to facilitate heat retention and insect visitation in other plant species growing in cold climate, we hypothesize that plants of S. autumnalis benefit from possessing these features under cool conditions, and that much of the geographical variation in capitulum characters reflects adaptive phenotypic plasticity rather than ecotypic divergence. For this reason, we deem these characters to have a low taxonomic value for distinguishing infraspecific taxa within S. autumnalis .
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