Threats to Wild American Ginseng
Habitat Alteration
The alteration of forest habitat through both natural and man-made disturbances can elicit a diverse, complex suite of responses from understory species depending on disturbance the type, intensity, duration, and scale.
Timber Harvest Canopy Disturbance
Timber Harvest
Timber harvest is a common, cyclical anthropogenic disturbance that affects all forest strata, including the herbaceous layer. Large-scale commercial timber harvest has been common in eastern North America ever since widespread clearcuts removed the vast majority of old growth forests in the late 1800’s and early 1900’s.(1) Additionally, because of the consistent global demand for wood products, timber harvest is one predominant disturbance that is predicted to continue and increase in some areas of the United States as second growth forests reach merchantable size (2,3).
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Timber harvest strategies vary in extent, severity, and intensity, and can range from small, individual tree gaps caused by single-tree selection to stand replacing disturbances such as clearcutting. Harvest activities can have profound effects on the abiotic characteristics of a site, including light availability, air and soil temperature, relative humidity, soil surface moisture, and soil compaction and disturbance. Patches of direct light reaching the understory increase in intensity, duration, and oftentimes temporal and spatial heterogeneity following timber harvest. While increases in light availability can enhance productivity in light-limited understory plants, the response of American ginseng to logging disturbance was relatively unexplored until recently. Two studies evaluated the response of American ginseng to varying types and intensities of timber harvest to determine:
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If canopy disturbances caused by at least one type of timber harvest are physiological stressors to American ginseng, or whether increases in light following harvest provide physiological benefits, particularly in those traits that directly relate to carbon accumulation (4) , and
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If varying intensities of timber harvest common throughout the eastern United States negatively affect the survival, growth, and reproduction of American ginseng. (5)
American ginseng survival tends to decrease after timber harvest, with the lowest survival occurring on sites where the soil profile was disrupted or severely compacted through the physical extraction of timber (5). Of those individuals that survived, symptoms of physiological stress, including increased respiration, thick reddened leaves, and early senescence were evident the summer directly after timber harvest (4). However, signs of stress were not evident two summers following harvest; instead, physiological stimulation was observed, and photosynthesis increased by 67% two summers after harvest compared to pre-harvest levels (4). Additionally, of those individuals that remained directly following harvest, individual growth rate and the proportion of individuals producing seeds increased substantially (5).
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American ginseng may be aptly classified as a “slow opportunist.” Delayed acclimation to higher light is a consequence of American ginseng having determinate growth, which prevents individuals from acclimating within a single growing season. By the time acclimation occurs, the understory and mid-story are already closing in, and as a result, acclimation consistently lags behind canopy changes. Nevertheless, American ginseng is an opportunist whose physiology and growth benefit from rapid, discrete increases in light availability (4). These studies suggest that ecological forestry techniques that mimic natural disturbance regimes will likely promote American ginseng growth.
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References
(1) Wyatt JL, Silman M. (2014) Long-term effects of clearcutting in the southern Appalachians.
(2) Fajvan MA, Grushecky ST, Hassler CC (1998) The effects of harvesting practices on West Virginia’s wood supply.
(3) Wear DN, Greis JG (2002) Southern forest resource assessment: Summary of findings.
(4) Chandler, J.L. 2017. A slow opportunist: Physiological and growth responses of an obligate understory plant to patch cut harvesting.
(5) Chandler and McGraw. 2015. Variable effects of timber harvest on the survival, growth, and reproduction of American ginseng (Panax quinquefolius L.).
Canopy Disturbance
Human activities are indirectly altering natural disturbance regimes that affect temperate forests. Fossil fuel combustion has contributed to the rise in global temperature, causing significant climate shift (1,2). Climate shifts subsequently alter the frequency and intensity of natural disturbance regimes, the most common of which are insect defoliation, windthrow, ice storms, and lightning (3,4,5,6). These types of disturbances typically involve the periodic defoliation, limb loss, and mortality of single trees and small groups of trees (3,7), however, climate shifts are predicted to increase the intensity and frequency of these disturbances (8). Previous research suggested that American ginseng is best adapted to the dynamic, but relatively low light environments typical of mature and old growth forests (9). Less was known about American ginseng’s response to the types of natural canopy disturbances whose frequency and intensity are likely to increase as a function of climate change. To explore this unknown, one study modeled whether the population growth American ginseng increases from additional inputs of light caused by canopy disturbance, or alternatively, declines due to long-term selection under relatively low light conditions (10). The same study explored whether or not the population-level response of American ginseng was consistent, regardless of the type, severity, and duration of natural canopy disturbance (10).Three unique, intermediate intensity disturbance events were analyzed:​
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A 2006 forest tent caterpillar defoliation event that resulted in the loss of over 50% of the deciduous canopy cover in an upstate New York population for several weeks during mid-growing season;
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A 2009 freezing rain event that deposited approximate 5cm of ice in forests of western Kentucky, leading to loss of tree limbs and whole crowns and resulting in long-lasting canopy gaps; and
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A 2012 lightning strike event that killed two dominant trees in an eastern Kentucky population, resulting in canopy gaps above portions of the population.
This work suggested that American ginseng can exploit moderate increases in light availability (10). In each
case, the rate at which population sizes increased was higher as a result of the canopy disturbance, indicating that American ginseng may benefit from the additional light in the understory provided by these intermediate-intensity canopy disturbances (10). The same pattern of enhanced population growth may not exist in American ginseng populations exposed to far more intense or longer-lasting natural canopy disturbances, though this assertion has not been tested directly (10). It is suspected that American ginseng population growth is optimized at some intermediate range of canopy disturbance – at a level that increases light to the understory without substantially damaging the soil profile or opening the canopy up so much that weedy, shade-intolerant species thrive and create deep-shade conditions for other understory plants, including American ginseng.
References
(1) IPCC (2007a) Climate Change 2007: Impacts, Adaptation and Vulnerability.
(2) IPCC (2007b) Climate Change 2007: The physical science basis.
(3) Sousa WP (1984) The role of disturbance in natural communities.
(4) Dale VH, Joyce LA, McNulty S, et al (2001) Climate change and forest disturbances.
(5) Dukes JS, Pontius J, Orwig D, et al (2009) Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?
(6) Roberts MR, Gilliam FS (2014) Response of the Herbaceous Layer to Disturbance in Eastern Forests.
(7) Roberts MR (2004) Response of the herbaceous layer to natural disturbance in North American forests.
(8) Ryan MG, Archer SR, Birdsey R, et al (2008) Land Resources: Forest and Arid Lands.
(9) Wagner, A. and J. B. McGraw. 2013. Sunfleck effects on physiology, growth, and local demography of American ginseng (Panax quinquefolius L.).
(10) Chandler, J.L. and J. B. McGraw. 2017. Demographic stimulation of the obligate understorey herb, Panax quinquefolius L., in response to natural forest canopy disturbances.