Climate models project global temperatures could increase as much as 6°C (10.8°F) over the next century. Will changes in climate affect ginseng? To answer this question, we examined ginseng’s response to year-to-year variation in climate. Ginseng responds to relatively small, inter-annual changes in temperature in an interesting way. Deviation from mean conditions at each population’s site elicits a decrease in population growth – a pattern that suggests local adaptation to climate (see Figure 1 below; published in Souther and McGraw 2011). By optimizing performance at mean local conditions ginseng populations may gain a competitive edge over other species. However, specialization also makes ginseng populations vulnerable if conditions change. For instance, ginseng occurs across a 10°C (18°F) temperature gradient from southern Canada to northern Georgia, but because populations are locally climatically adapted, small changes – as little as a 1°C (1.8°F)- may trigger range-wide decline in abundance.
To learn more about local climatic adaptation in ginseng, we transplanted populations from the wild to growth chambers where we could precisely control climatic and other environmental conditions. Elevated temperature increased early senescence and respiration rates, and depressed growth, reproduction, and photosynthesis – all responses that could explain the reduction of population growth in warm years that we observed in wild populations.2 Using growth chambers, we were able to examine how change in mean temperature affects ginseng populations. However, increased climatic variability, including increased frequency and magnitude of extreme events, maybe an even more important determinant of long-term viability than mean temperature increase. For example, in 2007 an unusually warm spring across much of the range triggered ginseng emergence, then suddenly temperatures fell below freezing. For the plants that emerged, the frost increased mortality, decreased growth, and reduced reproduction even years after the frost.3
Climate change is occurring as a backdrop to many forms of global change and perturbations that affect ginseng populations. Some of these factors, like harvest, we can regulate as climate changes, while others, such as disease outbreaks, will be much harder to control. We modeled the individual and combined effects of harvest and climate change on extinction risk.4 As expected, warming of only 1°C (1.8°F) increased extinction risk. However, when warming and harvest were examined together, the combined effect was much greater than either factor alone, signaling an urgent need to re-examine current harvest regulations in the context of changing climate. Figure 2 below shows how extinction risk is elevated by the combination of harvest and warming.
Species imperiled by climate change may persist if they track the climate conditions – either by shifting range or by adapting to novel conditions. Ginseng is a sedentary species, characterized by slow growth and low rates of reproduction. These traits suggest a low likelihood that ginseng will keep pace as favorable climate conditions move northward and upward. In an effort to prevent climate change driven extinction of ginseng, current research is examining human-assisted colonization of ginseng and similar species. Ginseng and other herbaceous species that have declined in abundance over the past 50 years were transplanted northward. By comparing performance of relocated individuals versus performance of individuals transplanted back to their home-site, we can determine whether relocating these species is a net gain or cost in terms of plant growth and reproduction. In a second experiment, pollen was moved from a population at a low elevation site to a population at a high elevation – plucking thousands of tiny anthers (male parts) from ginseng plants in one population, and dusting pollen on the stigmatic lobes (female parts) of plants from other population. Now survival and growth of the ginseng seedlings that resulted from these crosses is being measured. By doing this, we can see if “warm-adapted” genes enhance performance of subsequent generations experiencing warmer climatic conditions, or conversely, decrease performance by disrupting genetic complexes responsible for home-site success.