Threats to Wild American Ginseng
Like many plant species, ginseng populations are adapted to their local conditions under which they evolved. For ginseng, this is particularly evident with temperature, so that in growing seasons that are warmer or cooler than long-term average for a site, ginseng population growth declines. When the climate changes in a certain direction – for example, warming, - this means that populations will more often experience those “poor years”, and the net result can be eventual extirpation. Population declines consistent with excessive warming have already been seen in natural “wild” ginseng populations.
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, Souther and McGraw (2011b) 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(1)).
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.
Figure 1: Response of ginseng population growth to variation above or below the mean temperature for the site. Population growth rates less than 1 indicate a declining population.
To learn more about local climatic adaptation in ginseng, Souther and McGraw (2012) transplanted populations from the wild to growth chambers where they could precisely control climatic and other environmental conditions. Elevated temperatures increased early yellowing of leaves, increased respiration rates, and depressed growth, reproduction and photosynthesis – all responses consistent with a lower performance in warm years like what was seen in the wild.
Climate change affects both the “average” conditions of a site, as well as extremes. An unusual example was observed in 2007, when an unusually warm spring triggered early ginseng emergence. A few weeks later, temperatures suddenly fell below freezing, and frost damage caused increased mortality, decreased growth, and reduced reproduction, even in years after the frost. (Souther and McGraw 2011a).
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. Souther and McGraw (2014) 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. Results of a formal population viability analysis in which probability of extinction over the next 70 y is plotted vs. initial population size. Most ginseng populations are small, and in the range where the effect of warming and harvest combined is greater than the additive effect of each.
Figure 2 below shows how extinction risk is elevated by the combination of harvest and warming.
Figure 2: Results of a formal population viability analysis in which probability of extinction over the next 70 y is plotted vs. initial population size. Most ginseng populations are small, and in the range where the effect of warming and harvest combined is greater than the additive effect of each.
The predicted negative effects of climate warming go beyond the genetic “overspecialization” mechanism described above. Souther and colleagues also examined the effects of maturing seeds under warmer conditions (a ‘maternal effect’) in another field study (Souther et al. 2022). They found that seed germination declined 30% when mother plants experienced warmer conditions, relative to optimal conditions, further contributing to population decline with warming.
(1) Souther and McGraw 2011b. Evidence of local adaptation in the demographic response of American ginseng to interannual temperature variation.
(2) Souther et al. 2012. Experimental test for adaptive differentiation of ginseng populations reveals complex response to temperature.
(3) Souther and McGraw 2011a. Vulnerability of wild American ginseng to an extreme early spring temperature fluctuation.
(4) Souther, S., J. B. McGraw, J. D. Souther, and D. M. Waller. 2022. Effects of altered climate on American ginseng population dynamics. Population Ecology 64: 47-63.
In other words...
Climate change is a serious looming threat to ginseng’s long-term prospects for survival. The longer solutions to climate change are delayed, the worse this threat will become as ginseng is effectively displaced from the environment to which it is adapted. The climate – ginseng story shows the many ways in which something that appears to be one threat can, in fact, be multi-faceted, making effective solutions even more challenging.
Mature ginseng with canopy shadow.
Credit: batcavebotanicals, 2022