Consequences of local adaptation on extinction risk of American ginseng
Under stable climatic conditions, local adaptation enhances fitness of individuals in their home-site relative to non-local individuals. In a directionally changing climate, however, specialization to local climatic conditions may result in range-wide fitness decline. In this study, we use long-term demographic data, as well as a reciprocal transplant approach, to examine local climatic adaptation in the U.S.’s premier wild-harvested medicinal plant, American ginseng (Panax quinquefolius L.). Using these data we asked, ‘Are populations of ginseng adapted to local climate regime?’ and if so, ‘What are the consequences of local adaptation on long-term viability of ginseng?’
Patterns of demographic response to inter-annual temperature variation suggested local adaptation to mean growing season temperature. These findings were supported by reciprocal transplant experiments, in which fitness-related traits were maximized at home-site climatic conditions. Models of population viability response to directional temperature change (1°C / 70 years) indicated that increasing temperature increases extinction risk from 0 to a 6% at a median population size (n =140 individuals). Interestingly, wild-harvest of ginseng exacerbated the negative effect of climate change on population viability. In model simulations that included harvest and climate change, extinction risk at the median population size was 65%, far exceeding the additive effects of either factor alone (extinction risk = 8%, 6% for harvest and climate change, respectively). Taken together, our findings indicate that local adaptation increases sensitivity of ginseng populations to directional temperature change, particularly in the presence of additional stressors (e.g., harvest).