Variable effects of timber harvest on the survival, growth, and reproduction of American ginseng (Panax quinquefolius L.)
Timber harvest is a recurrent, widespread canopy disturbance that has long-affected the eastern deciduous forest. Multi-dimensional shifts in forest dynamics occur following timbering, and while a great deal of research on tree response to timbering has been performed, far less focus has been placed on herbaceous understory response to timbering. American ginseng (Panax quinquefolius L.) is a long-lived, economically valuable medicinal herb believed to be adapted to the dynamic light conditions of old-growth forests. The purpose of this study was to determine if timber harvests of varying intensities had negative effects on the survival, growth, and reproduction of wild American ginseng. Hemispherical images were used to calculate the canopy openness above ginseng plants. Population census data were used to obtain measurements of survival, relative growth rate, seed production, seed production efficiency, and new seedling emergence. Changes in survival and seed production in response to timbering were analyzed using log-likelihood analyses, and the responses of relative growth rate and seed production efficiency to timbering were analyzed using nested two-way ANOVAs.
Overall, ginseng survival decreased, and differences in survival were seen among sites with lowest survival at the more intensely timbered site. Relative growth rate of individuals increased following timbering, with relative growth rate being greatest at sites where more tree biomass was removed. Seed production decreased the year directly following timbering, but increased the two years thereafter. Short- and long-term effects of timbering on seed production efficiency differed among populations. Although seed production increased following timbering, this stimulation may not be realized due to sub-optimal germination conditions.
Timbering can negatively impact ginseng performance due to decreases in survival and to unlikely continued increases in growth and seed production as the canopy fills in. Silvicultural strategies that mimic old-growth forest structure are better suited for the conservation of ginseng and other similar understory species. Some current silvicultural strategies such as single-tree selection and group selection mimic old-growth forest structure; environments to which American ginseng and perhaps other understory shade-species are adapted. These silvicultural strategies are perhaps a good fit for land managers who have multi-dimensional goals involving continued production of valuable timber, maintenance of forest biodiversity, and conservation and stewardship of ecologically and economically valuable understory herbs such as American ginseng.