Can putative indicator species predict habitat quality for American ginseng?

Abstract

American ginseng, Panax quinquefolius L., is a long-lived medicinal understory herb, which has been heavily harvested since the 1700s. Because of the economic value of the root, and the increasing rarity of this plant, P. quinquefolius is often reintroduced across its range. Land managers and hobby growers recommend using ‘associate species’ as a way to determine ideal site conditions for reintroduction. However, the accuracy of these putative indicator species in identifying sites that will maximize growth of this rare herb has not been tested. Using a long-term ecological dataset of 26 populations, we evaluated if 20 putative indicators (herbs, shrubs, and trees) could predict P. quinquefolius performance, as measured by the relative growth rate of the leaf area, at the population and microsite level. Of the indicators, only one tree species was able to predict positive performance. If a P. quinquefolius was within 10m of a Liriodendron tulipifera L., the plant would have increased growth, in terms of leaf area, as compared to plants that were not within 10m of this tree. Surprisingly, the presence of most putative indicator species was found to be unreliable as a site quality measure. At the population level, four putative indicators, Aralia nudicaulis L., Acer rubrum L., Betula lenta L., and Lindera benzoin (L.) Blume, were actually contra-indicators, as their presence at a site implied lower P. quinquefolius performance. If Podophyllum peltatum L. was absent from a site, but B. lenta present, P. quinquefolius had reduced growth as compared to plants present in other combinations of P. peltatum and B. lenta. The results from this study have important implications for in situ conservation strategies of this rare medicinal plant. Planting P. quinquefolius in sites that increase performance can help ensure that reintroduction projects likely have a greater chance of success, effectively reducing the waste of time, money, and resources spent on projects that have lower levels of success.