Data from: Adaptive geographical clines in the growth and defense of a native plant

Broad-scale geographical gradients in the abiotic environment and interspecific interactions should select for clinal adaptation. How trait clines evolve has recently received increased attention because of anticipated climate change and the importance of rapid evolution in invasive species. This issue is particularly relevant for clines in growth and defense of plants, because both sets of traits are closely tied to fitness and because such sessile organisms experience strong local selection. Yet, despite widespread recognition that growth and defense traits are intertwined, the general issue of their joint clinal evolution is not well-resolved. To address heritable clinal variation and adaptation of growth and defense traits of common milkweed (Asclepias syriaca), we planted seed from 22 populations encompassing the species' latitudinal range in common gardens near the range center (New York [NY]) and towards the range edges (New Brunswick [NB] and North Carolina [NC]). Populations were differentiated in 13 traits, and six traits showed genetically-based latitudinal clines. Higher latitude populations had earlier phenology, lower shoot biomass, more root buds and clonal growth, higher root-to-shoot ratio, and greater latex production. The cline in shoot biomass was consistent in all three locations. Selection on phenology was reversed in NB and NC, with early genotypes favored in the north but not the south. We found no clines in foliar trichomes or toxic cardenolides. Annual precipitation of source populations explained variation in phenology, clonal growth, root-to-shoot ratio, and latex. Across four traits measured in NB and NC, we found garden-by-latitude (and garden-by-precipitation) interactions, indicating plasticity in genetically-based trait clines. In the two gardens with substantial herbivory (NY and NC), northern populations showed higher resistance to insects. Resistance to aphids was driven by trichomes and water content, while resistance to monarch caterpillars was driven by latex. However, surveys of natural populations indicated that leaf damage and insect diversity on milkweed are low at the geographical extremes (NB and NC) and higher towards the range center. We speculate that milkweed plants evolved clines in growth traits in response to climate, and that this set the template for tolerance to herbivory, which subsequently shaped the evolution of defensive traits