From America to the Holy Land: disentangling plant traits of the invasive Heterotheca subaxillaris (Lam.) Britton & Rusby

Understanding which plant traits confer invasiveness has been a central question in biological invasions research. Heterotheca subaxillaris (camphor-weed), an American plant, is an important invader of coastal sand dunes in Israel. Nevertheless, there has been no comprehensive comparative analysis of H. subaxillaris plant traits from native versus introduced habitats that sheds light on the invasion success of this species in Israel. I studied H. subaxillaris from native (US) versus introduced (Israel) populations to identify functional traits that accorded this species invasion success in Israel. Plant traits considered were shoot and root biomass production, root–shoot ratio, shoot height, root length, number of inflorescences, achene number and mass, and life span. Achenes (seeds) of all populations were germinated under common growing conditions to produce F1 achenes. F1 seedlings were grown in a large-scale common garden aeroponic system until flowering and then harvested. Introduced populations exhibited marked differences in measured parameters than native populations. Notably, root length of introduced populations exceeded 5 m, almost fourfold greater than that of native populations, allowing access to soil moisture and nutrients from deep sand layers and late-summer flowering. Life span of introduced populations almost doubled that of American populations. To the best of my knowledge, this is the first documentation of adaptive micro-evolutionary change favoring deep root allocation and phenological change in an invasive species in sand dunes. Seemingly, a rapid evolutionary change favoring root resource allocation occurred within introduced populations, allowing establishment, expansion, and successful invasion in the harsh ecosystem of Israel’s coastal sand dunes.