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

Abstract

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) and toward the range edges (New Brunswick and North Carolina). 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 New Brunswick and North Carolina, 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 New Brunswick and North Carolina, we found garden‐by‐latitude (and garden‐by‐precipitation) interactions, indicating plasticity in genetically based trait clines. In the two gardens with substantial herbivory (New York and North Carolina), 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 (New Brunswick and North Carolina) and higher toward 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.

Tests of the coupled expression of latex and cardenolide plant defense in common milkweed (Asclepias syriaca)

“…Analyses were conducted on log xþ1 values and included treatment as a blocking factor in the statistical model. v www.esajournals.org grown in a common environment, we also found no evidence for correlated expression of latex and cardenolides (n ¼ 22 population means, r ¼ 0.151, p ¼ 0.504; data from Woods et al 2012). This finding is consistent with Koricheva et al's (2004) analysis, which reported very little evidence for trade-offs between defense traits in plants.…”

“…An additional population level analysis was conducted based on cardenolide and latex data in Woods et al (2012), which represents 22 populations of A. syriaca across the entire latitudinal range (11 degrees of latitude, from New Brunswick, Canada, to North Carolina, USA). Approximately five individuals of each population were grown in a field common garden in Ithaca, NY, USA and mean population values for cardenolides and latex exudation from the second year of growth were correlated here.…”

Constraints to and conservation implications for climate change adaptation in plants

“…Etterson 2004;Maron et al 2004;Uribe-Salas et al 2008;Bresson et al 2011;Montesinos-Navarro et al 2011;Chen et al 2012;Woods et al 2012). Reciprocal transplant and common garden experiments have been used to determine the degree of plastic versus heritable variation in clinal traits.…”

Population variation affects interactions between two California salt marsh plant species more than precipitation

“…Adaptation to local environmental conditions is common among plant species that occur across large environmental gradients (e.g., Pennings et al 2003;Woods et al 2012;Pratt and Mooney 2013;Vergeer and Kunin 2013). Local adaptation can improve plant fitness by affecting traits such as phenology, reproduction and growth, ensuring, for example, that plants flower at a time during which the environment Abstract Species that occur along broad environmental gradients often vary in phenotypic traits that make them better adapted to local conditions.…”

“…is most favorable for successful pollination (Linhart and Grant 1996;Woods et al 2012). Because adaptive variation along environmental gradients affects so many aspects of plant fitness, it is also likely to affect interactions among plants.…”