Renewable natural resources that have open access and biological constraints on reproduction may be subjected to intense harvest pressure that limits regeneration. In this case, total economic profits will be lower than what they would be if less total harvest pressure were exerted. This has been found to be true for some marine fish species, and USDA Forest Service scientists at the agency's Southern Research Station hypothesized that it could occur with some non-timber forest products if the products were small, easily concealed, and sparsely distributed over a large geographic range. They tested this theory in the case of wild American ginseng (Panax quinquefoliu), which has been subjected to harvest pressure since the 1700s. American ginseng roots are highly valued in traditional Asian medicine. They found evidence that quantities supplied are negatively related to price in the long run, which is contrary to normal supply curves for most goods. This finding indicates that harvest pressure potentially constrains stocks of harvestable wild ginseng. The scientists also found that a federal regulation banning exports of roots from plants under five years old coincided with a reduction of supply. The regulation has been in effect since 1999, and the reduction in supply is potentially because of the naturally slow rate of American ginseng growth and reproduction.