Functional traits differ across an invasive tree species' native, introduced, and invasive populations

It is often speculated that non-native invasive species undergo rapid changes in their phenotypic properties (i.e., traits) that provide adaptive advantage in their new environment. However, few studies have directly compared traits of invasive non-native species with their native counterparts to reveal whether such phenotypic changes occur, and which stages of initial introduction and subsequent invasion contribute to these shifts. We studied trait variation of an invasive tree, Pinus contorta, which is native to northwestern North America and invasive in the Patagonia region of South America (i.e., Argentina and Chile). Commercial plantations of P. contorta were introduced extensively in Patagonia from the 1970s onward, from an unknown seed origin within the Pacific Northwest, USA, where three sub-species are found, including subsp. contorta, latifolia, and murrayana. We employed a home-versus-away study approach, where we compared mean growth, defense, and reproduction trait values, and mean within-stand trait variation (Coefficient of Variation, CV) of Patagonia plantations, with the three native sub-species. We further compared mean traits, and trait CVs between invasive P. contorta and the Patagonia plantations from which they escaped. Patagonia plantations shared the most similar mean trait values with subsp. latifolia and murrayana, suggesting possible source populations. However, both mean trait values and trait CVs of Patagonia plantations differed from all three native sub-species, indicating potential founder effects, population bottlenecks, and/or plastic responses to their new environment that occurred during or after introduction. We also found evidence for selective change during invasion; however, these differences did not suggest growth traits were prioritized over defense traits, which was inconsistent with hypotheses that invaders exhibit an evolutionary trade-off between defense traits and growth traits. Our study highlights that processes occurring both at first introduction and establishment, as well as the subsequent invasion phase can influence the phenotype of successful invaders.