Demographic response of American ginseng to interannual climate variation

Background/Question/Methods American ginseng (Panax quinquefoliusL.) is an ecologically and economically important understory species in the eastern United States. Like most plants, ginseng has the potential to be strongly affected by global climate change, with the demographic response being critical to understanding the net effect. As a first step toward projecting response to climate change, we quantified demographic responses to inter-annual variation in temperature (minimum and maximum) and precipitation, summarized by month and segments of the growing season. A single large natural population (n=350 +/-50) of ginseng was censused over a ten year span (1998-2007) in order to parameterize transition probability matrices and estimate the stable population growth rate (λ). Life table response experiments (LTREs) were conducted for pairs of transition matrices with high and low growth rates in order to determine which demographic parameters contributed most to the variation in lambda. Particular attention was paid to climatic effects on those parameters.

Results/Conclusions Annual population growth rate was stable in the long term, but varied substantially among years (average λ =1.002, 0.8747< λ <1.1024). Higher temperatures (both maxima and minima) in the middle of the growing season negatively affected λ. The underlying cause of this effect appeared to be negative effects on the rate of stasis in juveniles and large adults, as well as reduced rates of growth from small to large adults. These three parameters were identified by the LTRE analyses as key explanatory variables for the annual variation in λ. Effects of precipitation variation on λ were less pronounced, although several significant effects of precipitation patterns at various times of year were observed on underlying demographic transitions. Several demographic parameters exhibited curvilinear relationships to precipitation suggesting local optima. Given the observed patterns of demographic response to annual climate variation, ginseng populations may slowly decline in response to a warmer, and possibly wetter, climate predicted for the next century. This prediction of a slow decline could be modified, however, by an understanding of responses to extreme weather events that may increase in frequency along with overall climatic warming.