How do extreme weather events affect native plant populations? A case study of spring frost damage in American ginseng

Background/Question/Methods

Below freezing temperatures constitute a strong selective force in natural plant populations. In particular, early spring frost may interact with phenological development, damaging or killing plants that emerge and grow during early warm periods. Increased weather variability and warmer winter temperatures predicted by global climate models may increase the frequency of damaging frosts. Such an event occurred between April 5^th and 10^th of 2007, and its effects were documented during the spring census of 30 populations of American ginseng (Panax quinquefoliusL.). The objectives of this study were to examine the ramifications of this frost on ginseng population demography, and to explain the differential effect of below-freezing temperatures within and among populations. The censused ginseng populations were located in 7 states in the north-central portion of ginseng's range, and occurred at a variety of elevations and aspects. The census of these populations took place in late May and early June, and morphological characteristics, such as plant height and leaf area, were measured for individually tagged ginseng plants. Visual evidence of frost damage was noted by unusual leaf deformities and necrosis and confirmed during the August census when information on ginseng reproduction and growing season survival was collected. Results/Conclusions Forty percent of ginseng populations were affected by frost. The percentage of individuals damaged by frost ranged from 0.5 to 30 percent among populations. Frost damage differentially affected ginseng stage classes. In general, larger plants tended to be damaged more frequently than smaller plants. Plants damaged by frost were more likely to senesce before the end of the growing season, potentially having a negative effect on rate of population increase. The extent of damage incurred by a population due to frost depended on the cumulative minimum temperature before the freezing event; warmer pre-frost temperatures increased the number of plants damaged by frost. Warmer winter temperatures may stimulate premature emergence in some plant species, lengthening the amount of time populations are vulnerable to freezing episodes. Early senescence of frost-damaged plants demonstrates that extreme weather events associated with climate warming, such as sudden late-season frosts, may contribute to population decline.